Friday, December 15, 2017

Asphalt Shingles as Roofing Material and its Types

Asphalt Shingles as Roofing Material and its Types

The manufacture of asphalt roofing materials has been undergone from the 1890s. Today asphalt shingles cover 70 to 80 percent of the roofs in united states. It has now spread to different part of the world.

The asphalt roof shingles are very attractive, versatile and provide high fire and wind resistance. The shingles cost is economical which make it relatively inexpensive in terms of aesthetics and the durability it provides to the building.

The asphalt shingles can be very easily installed and requires little maintenance. The normal life expectancy of the asphalt roof shingles is in the range of 15 to 20 years. The fiberglass shingles provides a life expectancy of 20 to 30 years.

Asphalt Roof
Asphalt Roof Shingles

Types of Shingles for Roof

Organic Shingles

The organic based asphalt shingles consist of a base mat composed of cellulose fibers made from recycled paper or the wood chips and cotton or the wool rags. But presently it is made of asphalt -saturated roofing felt. It is coated with asphalt on the both sides as shown in the figure below.

The strength for the shingles is provided by the base mat. The base material is in a saturated state. It is covered with a flexible asphalt that has a high melting point which is called as the saturant.

Ground limestone, slate, trap rock or other inert materials are used to reinforce the saturant which is mineral stabilizers. The coarse mineral granules are pressed on the exposed phase of the asphalt coating which provides color for the asphalt. This exposed phase helps the shingle to resist the weather and the fire.

The materials that used as coarse mineral surfacing are natural colored slate or ceramic coated rock granules. Each shingle’s back side is covered with a talc, mica or sand. This will prevent the shingle to prevent sticking to each other.

Fiberglass Shingles

The first entry of fiberglass was in the late 1950s. By 70s they have grown up as traditional asphalt shingles.

The fiberglass asphalt shingle has a base mat that is saturated and covered by means of flexible asphalt. It is also surfaced with mineral granules.

When compared with cellulose fiber mat, the fiberglass shingles have a very less weight and thickness. Compared with organic asphalt shingles, the fiberglass shingles consist of more asphalt.

The organic shingles have chances to soak the water up from the underneath. This result in the curling up of the bottom. This made the demand of organic-based shingles to decrease.

Deck Requirements for Asphalt Shingles

The asphalt shingles are installed over a solid roof deck. In general, the slope necessary to install asphalt shingles is in the range from 4 in 12 through 21 in 12 by employing standard installation method.

The asphalt shingles can also be used in slopes in the range of 2 in 12 which are flat and 21 in 12 that are steeper in nature. But special application procedures apply.

Types of Asphalt Shingles

Two types of asphalt shingles are mainly used in roof construction. One is three-tab shingles. This has two slots, that divide the exposed part of the shingle into thirds. The second one is the laminated shingles or architectural and dimensional shingles.

These are presently used in a wide range. The laminated shingles consist of two layers that make it more thicker and wind resistant.

Repair of Asphalt Shingles

Shingles are available from the market in the form of strips. They are laid over the concrete decks or boards laid over the roof area and pressed by means of nails.

Proper alignment of the strips helps in filling up the roof by shingles in a systematic manner, leaving no gaps and spaces.

Properly aligning the shingle strip over the end of the roof and pressing by nails to the boards
Properly aligning the shingle strip over the end of the roof and pressing by nails to the boards

Each Strip is properly aligned and pressed to avoid gaps
Each Strip is properly aligned and pressed to avoid gaps

Sometimes the reason of missing shingle may result in leaks or worn outs. Or there are situations where we have to disturb the shingles to install a roof vent or plumbing vent.
Such removal is advised to be done when the shingles are in a cooled state. This will make the adhesive beneath the layer to be brittle and easier for removal. Removal under hot conditions will tear the roof shingle.

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Sunday, December 3, 2017

Attic Construction Using Modern Engineered Timber Components

Attic Construction Using Modern Engineered Timber Components

The considerable growth in the use of attic construction to make the maximum use of the building envelope, coupled with the lack of skilled building labour, has resulted in the rapid development of engineered timber components for use not only in floors, but also in the construction of the roof element itself. This chapter addresses the developments, many of which, like trussed rafters, are trade-named products developed by specialist timber engineering companies. For that reason some of the illustrations in this chapter, like those in Chapter 6 which deals with trussed rafter systems, are drawn from the various manufacturers’ technical and trade literature.


Engineered timber in the various forms illustrated in Fig. 3.13 has existed for some considerable time. However, the demand from the house building industry in general for a product of better quality than solid timber coupled with the need for faster installation and also a stiffer floor without creaks and squeaks, has lead to an explosion of engineered timber ‘joists’. A recently reported statistic indicates that in 2005 some 50% of all new homes now use such products in their floor construction, accounting for around 65 000 houses in the UK alone. Bearing in mind that an engineered timber joist clearly costs considerably more than a simple soft wood joist, it is worth considering the advantages of the product before looking at the various types available and the construction methods used.

(1) Better quality floor construction

A stiffer floor giving better ‘feel’ to the user, and a quieter construction avoiding the shrinkage so often associated with conventional timber. Most engineered joist systems strongly recommend that the floor deck is screwed and/or glued to the surface to prevent floorboard joint movement.

(2) Faster construction

Claims of 66% reduction in time to install the floor, i.e. typically half a day to install an engineered floor compared to one and a half days with traditional construction. With an engineered floor the floor joist system is delivered as a pack of premanufactured components to precise length, including trimmers and blockings etc., where as with traditional construction, soft wood joists to the nearest standard available length would be delivered which then have to be cut, notched and trimmed as necessary before installation.

(3) Reduced cost

No wastage – every joist and trimmer is engineered to fit, and the better quality floor means no remedial costs in correcting shrinkage problems for the house builder.

Engineered timber, being a manufactured product, often carries a proprietary name such as Truss Joist, Parallam, Posi Joist, BCI Joist, Finn Joist etc. but they fall into three main categories:

 ‘I’ beams.

 Laminated solid timber.

Fabricated timber using metal connector plates.

All of these fabrications seek to engineer the natural defects of solid soft wood out of the product, i.e. knots, splits, variable slope of grain and density, to provide a stronger product of higher overall performance than that of its individual components, generally resulting in a better span to depth ratio than solid natural timber, without the associated shrinkage and distortion which occurs even with dry timbers. The following is a review of the different types listed above.

‘I’ beams

These concentrate the forces imposed on the component when being used as a joist, beam, purlin or rafter into the top and bottom flanges resulting in the ‘I’ shape so familiar with steel beams. A clear advantage is that they are lighter to handle compared to a solid timber beam of similar performance. The flange, i.e. the top and bottom member, can either be in solid conventional timber, or a further piece of engineered timber similar to the laminated timber described below. The web is again constructed of a man-made timber product, which could be highly compressed timber fibres commonly known as hardboard, or OSB (oriented strand board) or plywood. The flanges and web are usually joined by high performance gluing in the factory. When being made to specific length, the ends will usually be solid blocked to carry the stresses at the load bearing point (see Fig: 1.1).
Timber I
Fig: 1.1 Timber I beam

Laminated solid timber

Glulam is of course laminated solid timber and can be seen in Fig. 3.13. However, most of the laminated timbers used as joists, beams and purlins etc. now use much thinner laminates and are more akin to plywood construction in the thickness of the veneers (see Fig: 1.2), than the traditional 30 mm or 40 mm thick laminates used with conventional glulam. However, unlike conventional plywood where alternative veneers have timber grain laid at right angles to one another, most of the products in this category have laminates parallel to one another bonded by high performance adhesives. The trade name of one such beam, Parallam, describes its construction. This particular product is of course solid, but unlike a piece of solid timber is extremely stable, and again unlike a piece of solid timber, has all of the major strength reducing features engineered out thus enabling it to develop the strength of an almost perfect piece of timber, giving an even higher performance. Such timbers are often used as trimmers and purlins and in other areas of high stress for that reason. Typically then, this type of product could be found as a trimmer supporting the ‘I’ beam described above.

Micro laminated timber
Fig: 1.2 Micro laminated timber beam

Metal nail plate and timber
Fig: 1.3 Metal nail plate and timber beam

Fabricated timber using metal nail plate connectors

This product, illustrated in Fig: 1.3, is invariably the product of the trussed rafter manufacturer as it uses the same engineering and manufacturing technology used to produce the now common trussed rafter roof construction assemblies. Unlike the trussed purlin illustrated in Fig. 3.13, the top and bottom timber flanges for this form of engineered joist or beam lie flat rather than vertically. This of course gives an improved bearing area for both the floor decking and the ceiling and increases the bearing area of the joist itself where it is built into the wall or set on a hanger. Posi Joist by Mitek, Eco Joist by Gang Nail, and Wolfs Easi-Joist all use similar construction to that shown in Fig: 1.3. Each, of course, have their own design of ‘V’ shaped metal strut connector system, where as Alpines’ Twin–I Beam uses conventional punched metal rectangular plates with vertical timber struts between the flanges as illustrated in Fig: 1.4, but Alpine revert to timbers being used vertically rather than horizontally with the systems mentioned above, although the timber is generally much thicker than one would find in roof truss construction, again to give the better support for floor and ceiling.

All of the punched metal plate connected types give copious open space for services between the joists, thus avoiding the potential problems of incorrect notching and boring for services which is so often one of the problems with the use of even conventional solid soft wood floor joists. Over notching with the installation of pipes on the upper surface, and electrical installation on the lower surface, can dramatically decrease the joist’s performance. Clearly with the ‘I’ beam and the solid laminated beams, the question of piercing for services has to be addressed and the manufacturers’ literature should be carefully adhered to as to avoid weakening the floor diaphragm being constructed.


When using any of the engineered components mentioned above do not cut, notch or bore holes in any element without checking with the designer or manufacturer. It should be noted that this applies to this particular manufacturer’s product, but similar information is available from all of the manufactured joist and beam companies. It should be carefully noted that no notching of top or bottom flange, i.e. the most highly stressed areas, is allowed.

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Sunday, October 22, 2017

What are the different type of roof?

What are the different type of roof?

Roof is the upper most portion of the building which protects the building from rain, wind and sun.
Various types of roofs used may be divided broadly into three types:

1. Flat roofs
2. Pitched roofs
3. Shells and folded plates.

Flat roofs are used in plains where rainfall is less and climate is moderate. Pitched roofs are preferred wherever rainfall is more. Shells and folded plate roofs are used to cover large column free areas required for auditoriums, factories etc. Brief description of these roofs is presented below:

1. Flat Roofs: These roofs are nearly flat. However slight slope (not more than 10°) is given to drain out the rain water. All types of upper storey floors can serve as flat roofs. Many times top of these roofs are treated with water proofing materials-like mixing water proofing chemicals in concrete, providing coba concrete. With advent of reliable water proofing techniques such roofs are constructed even in areas with heavy rain fall.

The advantages of flat roofs are:

(a) The roof can be used as a terrace for playing and celebrating functions.
(b) At any latter stage the roof can be converted as a floor by adding another storey.
(c) They can suit to any shape of the building.
(d) Over-head water tanks and other services can be located easily.
(e) They can be made fire proof easily compared to pitched roof.

The disadvantages of flat roofs are:

(a) They cannot cover large column free areas.
(b) Leakage problem may occur at latter date also due to development of cracks. Once leakage problem starts, it needs costly treatments.
(c) The dead weight of flat roofs is more.
(d) In places of snow fall flat roofs are to be avoided to reduce snow load.
(e) The initial cost of construction is more.
(f) Speed of construction of flat roofs is less.

Types of Flat Roofs: All the types listed for upper floors can be used as flat roofs.

2. Pitched Roofs: In the areas of heavy rain falls and snow fall sloping roof are used. The slope of roof shall be more than 10°. They may have slopes as much as 45° to 60° also. The sloped roofs are known as pitched roofs. The sloping roofs are preferred in large spanned structures like workshops, factory buildings and ware houses. In all these roofs covering sheets like A.C. sheet, G.I. sheets, tiles, slates etc. are supported on suitable structures. The pitched roofs are classified into

(a) Single roofs
(b) Double or purlin roofs
(c) Trussed roofs

(a) Single Roof: If the span of roof is less than 5 m the following types of single roofs are used.

(i) Lean to roofs
(ii) Coupled roofs
(iii) Coupled-close roof
(iv) Collar beam roof

In all these roofs rafters placed at 600 mm to 800 mm spacing are main members
taking load of the roof. Battens run over the rafters to support tiles. Fig: 1.3
shows various types of single roofs.

Fig: 1.3 Single Roofs

(b) Double or Purlin Roofs: If span exceeds, the cost of rafters increase and single roof
becomes uneconomical. For spans more than 5 m double purlin roofs are preferred. The
intermediate support is given to rafters by purlins supported over collar beams. Fig: 1.4
shows a typical double or purlin roof.
Double or purlins
Fig: 1.4 Double or purlins roofs

(c) Trussed Roof: If span is more, a frame work of slender members are used to support sloping roofs. These frames are known as trusses. A number of trusses may be placed lengthwise to get wall free longer halls. Purlins are provided over the trusses which in turn support roof sheets. For spans up to 9 m wooden trusses may be used but for larger spans steel trusses are a must. In case of wooden trusses suitable carpentry joints aremade to connect various members at a joint. Bolts and straps are also used. In case of steel trusses joints are made using gusset plates and by providing bolts or rivets or welding.

Depending upon the span, trusses of different shapes are used. End of trusses are supported on walls or on column. Fig: 1.5 shows different shapes of trusses used. Fig: 1.6 shows a typical wooden truss details and Fig: 1.7 shows the details of a typical steel truss.

Types of
Fig: 1.5 Types of trusses

A typical wooden truss (king past)
Fig: 1.6 A typical wooden truss 

Steel roof
Fig: 1.7 Steel roof truss

3. Shells and Folded Plate Roofs: Shell roof may be defined as a curved surface, the thickness of which is small compared to the other dimensions. In these roofs lot of load is transferred by membrane compression instead of by bending as in the case of conventional slab and beam constructions. Caves are having natural shell roofs. An examination of places of worships built in India, Europe and Islamic nations show that shell structures were in usage for the last 800 to 1000 years. However the shells of middle ages were massive masonry structures but nowadays thin R.C.C. shell roofs are built to cover large column free areas. Fig: 1.8 shows commonly used shell roofs.

Advantages and Disadvantages of Shell Roofs

Advantages of shell roofs are:

(a) Good from aesthetic point of view
(b) Material consumption is quite less
(c) Form work can be removed early
(d) Large column free areas can be covered.

Disadvantages are:

(a) Top surface is curved and hence advantage of terrace is lost.
(b) Form work is costly.
Types of shell
Fig: 1.8 Types of shell roof

Types of folded plate
Fig: 1.9 Types of folded plate roofs

Folded plate roofs may be looked as slab with a number of folds. These roofs are also known as hipped plates, prismatic shells and faltwerke. In these structures also bending is reduced and lot of load gets transferred as membrane compression. However folded plates are not so efficient as shells. Fig: 1.9 shows typical folded plate roofs.

Advantages and Disadvantages of Folded Plate Roofs Over Shell Roofs

Advantages are:

(a) Form work required is relatively simpler.
(b) Movable form work can be employed.
(c) Design involves simpler calculations.

Disadvantages are:

(a) Folded plate consume more material than shells.
(b) Form work can be removed after 7 days while in case of shells it can be little earlier.

Roof Coverings for Pitched Roofs Various types of covering materials are available for pitched roofs and their selection depends upon the climatic conditions, fabrication facility, availability of materials and affordability of the owner. Commonly used pitched roof covering materials are:

(a) Thatch
(b) Shingle
(c) Tiles
(d) Slates
(e) Asbestos cement (A.C.) sheets
( f ) Galvanised iron (G.I.) sheets

(a) Thatch Covering: These coverings are provided for small spans, mainly for residential buildings in villages. Thatch is a roof covering of straw, reeds or similar materials. The thatch is well-soaked in water or fire resisting solution and packed bundles are laid with their butt ends pointing towards eves. Thickness varies from 150 mm to 300mm. They are tied with ropes or twines to supporting structures. The supporting structure consists of round bamboo rafters spaced at 200 mm to 300 mm over which split bamboos laid at right angles at close spacing. It is claimed that reed thatch can last 50 to 60 years while straw thatch may last for 20–25 years.

The advantage of thatch roof is they are cheap and do not need skilled workers to build them.

The disadvantages are they are very poor fire resistant and harbour rats and other insects.

(b) Shingles: Wood shingles are nothing but the split or sawn thin pieces of wood. Their size varies from 300 mm to 400 mm and length from 60 mm to 250 mm. Their thickness varies from 10 mm at one end to 3 mm at the other end. They are nailed to supporting 
structures. They are commonly used in hilly areas for low cost housing. They have very poor fire and termite resistance.

(c) Tiles: Various clay tiles are manufactured in different localities. They serve as good covering materials. Tiles are supported over battens which are in turn supported by rafters/trusses etc. Allahabad tiles, Mangalore tiles are excellent inter-locking tiles. They give good appearance also.

(d) Slates: A slate is a sedimentary rock. Its colour is gray. It can be easily split into thin sheets. Slates of size 450 mm to 600 mm wide, 300 mm long and 4 to 8 mm thick are used as covering materials of pitched roofs in the areas where slate quarries are nearby. A good slate is hard, tough, durable. They are having rough texture and they give ringing bell like sound when struck. They do not absorb water.
A.C. sheet
Fig: 1.10 A.C. sheet roofing

(e) A.C. Sheets: Asbestos cement is a material which consists of 15 per cent of asbestos fibres evenly distributed and pressed with cement. They are manufactured in sufficiently large size. The width of a A.C. sheet varies from 1.0 to 1.2 m and length from 1.75 to 3.0 m. To get sufficient strength with thin sections they are manufactured with corrugation or with traffords [Fig: 1.10]. They are fixed to the steel purlins using J-bolts. The roofing is quite economical, waterproof. However not very good thermal resistant. They are commonly used as covering materials in ware houses, godowns or for larger halls. In auditorium etc., if these sheets are used, false ceilings are provided to get good thermal resistance.

(f ) G.I. Sheets: Galvanised iron corrugated sheets are manufactured in the sizes 1.0 to 1.2 m wide and 1.65 m length. Galvanisation of iron makes them rust proof. They are fixed to steel purlins using J-bolts and washers. They are durable, fire proof, light in weight and need no maintenance. They are commonly used as covering materials for ware houses, godown, sheds etc. Table 1.11 gives comparison between GI and AC sheets for roof covering.

Comparison between GI and AC
Table 1.11 Comparison between GI and AC sheets

Read more:- What are the different type of roof?

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Tuesday, October 17, 2017

The Coupled Roof, Ceilings and Trusses


Moving away from early roof forms that provided both wall and roof in one unit, the next development showed a true roof built on masonry or timber walls. The simplest form of roof was a coupled roof, consisting of two lengths of timber bearing against each other at the top and resting on a wall plate at their feet. The timbers, called couples, were pegged together at the top with timber dowels and were similarly pegged or spiked to the wall plate. The term ‘couple’ was used until the fifteenth century when the terms ‘spar’ or ‘rafter’ started to be used. The term rafter of course is still used to describe the piece of timber in a roof spanning from the ridge to the wall plate. paced about 400 mm apart tied only by horizontal binders and tile battens. The simple couple was adequate for small span dwellings and steep pitches, but the outward thrusting force at the feet of the rafters caused stability problems with the walls, and excessively long rafters sagged in the middle under the weight of the roof covering. The illustration in Fig: 1.1 shows the required shape in solid line and the deflected shape in dotted line.

To overcome both of these problems the ‘wind beam’ or ‘collar’ was introduced. Whether the collar acts as a tie or a strut for the couples will depend upon the stiffness of the supporting wall below. Assuming however, that the wall is so substantial that it will not be pushed outward by the bottom section of the couples, then the collar will act as a strut. If however, as is more likely with early timber framed buildings, the wall is relatively flexible then in that case the collar would act as a tie holding the couples together. There would still be some outward thrust but this would be limited by the collar to the degree of bending in the lower part of the couple only. It can readily be appreciated that in larger roofs, where the walls are relatively flexible, there is a considerable tying effect in the collar demanding a more sophisticated joint between collar and couple than could be achieved with simple iron nails. The collar was therefore frequently jointed to the couple with a halved dovetail shaped joint, often secured with hardwood pegs.
Simple Coupled
Fig: 1.1 Simple Coupled rafter


The next development was to fit additional members to assist with the stability of the roof in windy conditions and these were called ‘sous-laces’ or braces. On roofs constructed on substantial masonry walls which were also very thick, further struts or ‘ashlars’ were introduced to stiffen the lower section of the couple. Fig: 1.2 illustrates this form of construction, the wall plate being well fixed to the wall with the bottom member of the ashlar halved over it to prevent the roof sliding on the top of the wall.

These now very substantial ‘couples’ began to be spaced further apart and became known as ‘principals’. Between these main members simple couples or ‘rafters’ were placed, but to avoid sag or to accommodate longer rafter length possibly not available in one length of timber, an intermediate support was needed and this was called a ‘purlin’. The purlin is in turn supported by the principal couples, as shown in Fig: 1.3.

Fig: 1.2 Ashlar stiffening

Principal truss and purlin
Fig: 1.3 Principal truss and purlin roof

Tie beam
Fig: 1.4 Tie beam truss

The tendency for the roof to spread was now concentrated in the heavily loaded principals and it became apparent that if spans were to increase this spreading would have to be controlled. The ‘tie beam’ was introduced thus forming the first ‘trussed’ or ‘tied’ roof. Fig: 1.4 illustrates the roof form described.

As development progressed the span of the roof was limited only to the availability of long timbers used for the tie beam, but it is obvious that these long beams themselves would tend to sag under their own weight. To prevent this happening they too had to be supported and this was done with the introduction of ‘struts’ fitted to a corbel built into the wall below, as illustrated in Fig: 1.5.

With this tie beam now becoming a major structural member a different configuration of members evolved becoming more like the truss common today. Having stiffened the tie beam it became apparent that this could be used as a major structural item from which to support the principals. The major support running from the center of the tie beam to the ridge purlin was known as the ‘mountant’, now referred to as a ‘king post’ (see Fig: 1.6). A king post truss is also illustrated in Fig. 9.1, being used as part of the structure of an attic room. With two posts introduced the roofform is known as a ‘queen post’ truss, which in its simplest form is shown in Fig: 1.7. This particular roof form gave the opportunity of providing a limited living space within the roof. It should be remembered that until this stage of development all roof forms and trusses described had no ceiling and were open to the underside of the rafters and roof covering. To use the queen post roof form as an attic, a floor was needed thus creating a ceiling for the room below.

Strutted tie
Fig: 1.5 Strutted tie beam

King post
Fig: 1.6 King post truss

Queen post
Fig: 1.7 Queen post truss


Ceilings were first referred to in descriptions of roofs in the fifteenth century when they were known as ‘bastardroofes’ or ‘false roofs’ and then later as ‘ceiled roofs’, hence ‘ceiling’ as we know it today.

The ceiling supports were known as joists or cross beams again being supported by the hard working tie beam between the principals. The construction is illustrated in Fig. 9.2.

Continuing developments of the roof form itself, and demand for even larger spans and heavier load resulted in some relatively complex principals or trusses being developed. One such form was the ‘hammer beam’ roof, illustrated in Fig: 1.8. Clearly this is not a roof to be ‘ceiled’, being very ornate as well as functional.

Hammer beam
Fig: 1.8 Hammer beam truss
The hammer beam roof is generally to be found supporting the roof over halls in large mansions and of course churches. The roof was framed in such a way as to reduce the lateral thrust without the need for a large and visually obstructing tie beam. The walls onto which such a roof was placed had to be substantial and were often provided with buttresses in line with the principals to contain any lateral thrust that may develop.


Roofs in truss form developed using carpentry joints and some steel strapping, until the latter part of the eighteenth century when bolts, and even glues, started to be used to create large truss forms from lighter timber members. Such truss forms often used softwoods, as distinct from the hardwoods more frequently used in the shapes previously described. The large timber sections in oak particularly were becoming very scarce and of course very expensive. Whilst some significant advances in span were achieved, using the techniques described above, the domestic roof did not require very large spans and changed very little from the collared coupled roof. Indeed many small terraced houses built during the eighteenth and nineteenth century required no principals at all. The dividing walls between the houses were close enough to allow the purlins to rest on these walls, effectively using them as principals. Fig: 1.9 illustrates a typical terraced house roof construction.

The larger properties where the span of the purlin was too long for one piece of timber, or where hip ends were involved, continued to use the established methods of construction using principals, collars and purlins, but it was common practice to omit the principals and to support the purlins off the walls below with posts or struts.
Purlin and common
Fig: 1.9 Purlin and common roof


In 1934 the Timber Development Association (TDA) was formed, now known as TRADA (Timber Research and Development Association). The Association took up the work already being done at that time by the Royal Aircraft Establishment and progressed work on timber technology alongside the Forest Product Research Laboratories. Although the Royal Aircraft Establishment may sound a strange body to be interested in timber, it must be remembered that many aircraft of that era, and some notable ones after such as the Mosquito, used highly stressed timber structures for the fuselage and wings. Some aircraft hangars were of timber construction and utilised record breaking large span small timber section trusses with bolted joints.

After the Second World War shortages of materials resulted in a licence being required for all new building works, making economy in use of paramount importance. Imported materials such as timber were very much at a premium and TDA was given the task to find ways of economising on the country’s use of timber. Quite correctly they identified the roof structures of buildings as a high volume user of timber and developed a design for a domestic roof using principal trusses constructed of small timber sections connected with bolts and metal connector plates. The roof used purlins and common rafters similar to the systems previously discussed. These trusses became known as ‘TDA’ trusses, and with some minor modifications are still in use today. It appears that some of these designs were available shortly after the Second World War but were first published as a set of standard design sheets around 1950.

The designs were based on existing truss shapes but were not engineered in the sense that structural calculations were prepared for each design. Load testing on full size examples of the truss was used to prove their adequacy and from these tests other designs developed.


The first designs produced were known as ‘A’ and ‘B’ types, dealing with 40° and 35° pitches respectively. They covered spans up to 30 ft (9 m).

House design fashion changed during the later 1950s and early 1960s, demanding lower roof pitches. 1960 saw the introduction of the TDA type ‘C’ range for pitches between 22° and 30°. Spans were also increased up to 32 ft (10.8 m). Around 1965 the types ‘D’, ‘E’ and ‘F’ ranges were published; these later designs using a slightly different truss member layout went down to 15° pitch and up to 40 ft (12 m) span. Further designs used trusses spaced at 6 ft (1.8 m) centres and had some degree of pitch and span flexibility within specified limitations.

A range of designs for trussed rafters (i.e. each couple tied together at ceiling level) was produced also using bolt and connector joints, but these were designed only to carry felt roof coverings and did not prove as popular as the principal truss designs.

Industrial roofs were not neglected, with principal truss designs using the bolt and connector joint techniques for pitches of 22.5° spacing between 11 and 14 ft (3.35– 4.25 m) and up to 66 ft (20.1 m) span.

Whilst roofs are still constructed using these techniques, the TDA designs are no longer available from TRADA.


All of the TDA principal and trussed rafter designs used bolts and connectors at joints where previously mortice and tenon, half lap or straight nailed or pegged joints would have been used. The small timber sections used in the designs of the trusses did not allow the use of conventional carpentry joints and gave insufficient nailing area for an all nailed assembly. The connector allows the forces in the joint to be spread over a large area of the connected timber, the bolt holding the timbers in place thus allowing the connector to transmit the load from one truss member to the other.
Fig: 1.10 illustrates the typical single connector joint.
Toothed plate connector
Fig: 1.10 Toothed plate connector joint
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Roof Shapes and Terminology

Roof Shapes and Terminology

Fig: 1.1 Duo pitched roof. This is the most common roof shape with equal pitches on either
side, i.e. angle A equals angle B.

Fig: 1.2 Asymmetric roof; angle C is not equal to angle D.
Fig: 1.3 Mono pitched roof; angle E equals 90°.

Fig: 1.4 Truncated duo pitched roof; angle F equals angle G. This truss form is often introduced into domestic housing in conjunction with the conventional duo pitched roof to form
an interesting roof line.
Fig: 1.5 Fink truss shape. This is the most common trussed rafter form used on spans of
up to 8 to 9 m.
Fig: 1.6 Fan truss shape. This is used on larger spans and is a common trussed rafter
Fig: 1.7 Double ‘W’ shape. This is used on spans above 14 m and is not often used on
Fig: 1.8 Howe four bay truss. This is often used in trussed rafters in girder form. This could
also be used in six bay confi guration.
Fig: 1.9 Pratt four bay truss. This is occasionally used in trussed rafters in girder form.

Fig: 1.10 Mono pitch truss two bay. This is a common trussed rafter form often used in
conjunction with trusses in Figs: 1.5 to 1.7.
Fig: 1.11 Attic or ‘room-in-roof’ truss shape. This is a popular shape in trussed rafters: there
are no minimum heights set for h and w, but for h a 2.3m minimum is recommended, with
1.2–1.5 m being the practical minimum height for w.

Fig: 1.12 Mono pitch truss three bay. This is similar to the mono pitch truss two bay (Fig: 1.10), but is suitable for larger spans.
Fig: 1.13 Scissor truss. This is a possible trussed rafter shape occasionally used to create
a feature ceiling in the lounge of a house.
Fig: 1.14 Raised tie truss also used to create feature ceilings. Often Fink-based with rafters
extended down to the wall plate.


Fig: 2.0 Roofing terminology

The reader is referred to Fig: 2.0

A – Wall plate – sawn timber, usually 50 × 100 or 50 × 75 mm bedded in mortar
on top of the inner skin of a cavity wall. Straps must be used to secure the wall plate
to the structure below.

B – Common rafter – sawn timber placed from wall plate to ridge to carry the
loads from tiles, snow and wind. Long rafters may need intermediate supports from

B1 – Jack rafters – sawn timber rafter cut between either a hip or valley rafter.

C – Ceiling joist – sawn timber connecting the feet of the common rafter at plate
level. The ceiling joist can also be slightly raised above the level of the wall plate,
but this would technically then be termed a collar. The ceiling joist supports the
weight of the ceiling finish (normally plasterboard) and insulation. It may in addition
have to carry loft walkways and water storage tanks, in which case it must be specifically designed to do so.

D – Ridge – a term used to describe the uppermost part of the roof. The term is
also used to describe the sawn timber member which connects the upper parts of the
common rafters.

E – Fascia – usually a planed timber member used to close off the ends of the
rafters, to support the soffit M, to support the last row of tiles at the eaves N and
to carry the rainwater gutter support brackets.

F – Hip end – whereas a gable end O is a vertical closing of the roof, the hip is
inclined at an angle usually to match the main roof.

F1 – Hip rafter – sawn timber member at the external intersection of the roof slope
(similar to a roof sloping ridge), used to support the jack rafters forming the hip.

G – Valley – term used to describe the intersection of two roofs creating a ‘valley’
on either side. The illustration has only one main valley, the building being L-shaped
on plan. A further small valley is illustrated on the dormer roof with its junction to
the main roof. Valley jack rafters are fitted either side of a valley rafter.

H – Dormer – the structure used to form a vertical window within a roof slope. This structure gives increased floor area of full ceiling height within an attic roof construction, and is usually fitted
with a window, hence the term ‘dormer window'.

I – Barge board – the piece of planed timber is in fact a sloping fascia. It is often
fitted to gable ends, as illustrated.J
– Dormer cheek – the term used to describe the triangular infill wall area between
dormer roof, main roof and the dormer front.

K – Roof window – sometimes termed roof light, the former being able to be
opened for ventilation hence becoming a true window, the latter being fixed simply
allowing additional light into the attic roof space.

L – Gablet – a small gable over a hip end. It is used as an architectural feature.

M – Soffit – the ply or other sheet material panel used to close off the space between the back of the fascia and the wall of the building.

N – Eaves – term used to describe the extreme lower end of the roof, i.e. the area
around the fascia and soffit.

O – Gable – triangular area of wall used at the end of a roof to close off
beneath the roof slopes. This is usually a continuation of the wall construction

P – Purlin – large section sawn solid structural timber, or fabricated beam, used
to carry the common rafters on larger roof slopes where the commons are not strong
enough or cannot be obtained in one single length, to span between the wall plate
and the ridge.

Roof Shapes and Terminology

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All Theory About Roof Construction; roof structure terminology

Construction, Final Account & Tender

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