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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