Conservative estimates would suggest there is somewhere in excess of 60 million trussed rafters in service in the UK. To date, there has been no known failure of a trussed rafter in its “ex factory” condition, a remarkable safety record. However, experience has shown that trussed rafters are not always well understood by the people who specify them or who erect them. Such misunderstanding can, in some circumstances, lead to poor installation or worse unsuitable alteration.
This paper seeks to provide a basic understanding of how trussed rafters work, theirs strengths and their limitations. It is hoped that by examining common misunderstandings, costly and awkward modifications can be avoided.
Trussed rafters are defined as triangulated timber frameworks positioned at close centres (generally 600mm) to form roof structures. The idea of trussed rafters arrived in this country from the States during the 1960’s and quickly became the standard method of constructing roof structures in domestic construction.
Trussed rafters should not be confused with a system, commonplace in the 1950’s, which used “TDA” trusses at 1.8 to 2.4 metre centres. The TDA trusses were site built using toothed plate connectors based on standard designs published by the Timber Development Association (TDA), the forerunner of TRADA. The trusses supported purlins and binders which in turn supported rafters and ceiling joists.
Trussed Rafter Industry
Trusses are made by some forty four manufacturing companies dotted around the country. Trusses are engineered to suit the particular application requested. The software used by each manufacturer will be supplied by one of three companies who also make the connector plates used to fasten the members together (often referred to as “the system owners”). The major System Owners are
- Mitek Industries
- Wolf Systems Ltd
The system owner’s software is very sophisticated and not only designs the trusses, but also produces quotations, manufacturing information and production data.
Advantages of Trussed Rafters
Trussed rafters enabled Architects the freedom with the layout of the upper floor by avoiding the need for internal load bearing walls. Consequently, the ground floor layout and upper floor layout need no longer harmonise. Kitchens could be bigger than the bathrooms above them, and bedroom layouts could be independent of the lounge and dining room layout.
The Seven Deadly Sins
Perhaps an exaggeration, but mistreating trusses can be costly and disruptive to subsequently repair. The following pages examine each of these “sins” in more detail.
Sin No 1 Not Understanding how Trusses Function
A fundamental understanding of how trusses are designed and manufactured will greatly assist the process of specifying and assessing trusses. Trussed rafters come in all shapes and sizes, from parallel chord types to classic triangles. The most common form by far is the fink, illustrated below. Most trusses will require splices in the bottom chord and many in the top chord as well . Splices are located at points of low stress, usually at about a fifth of the bay length (middle bay bottom chord, upper bay on the top chord)
The webs are set out by considering the overall internal triangle. The intersection of the webs with the top chord will normally be at the quarter points. The intersection of the webs with the bottom chord will normally be at the third points of the internal triangle. For small span or for large span trusses, alternative internal configurations are used
Timber has an excellent strength to density ratio, making it an ideal structural material… if one overlooks the difficulty in forming joints. It is virtually impossible to joint timber so as to mobilise its full strength. By far the most effective option available is the use of connector plates. These comprise 1mm thick steel where the teeth have been punched by a heavy duty press at the system owner’s premises.. The plates are then pressed into the timber using press after the truss has been set out on a jig. The connector plates are only effective if so pressed, and simply bashing them into the timber with a lump hammer will not achieve the same effect.
The diagram shows the comparison between a site nailed joint and a pressed connector plate. The sheer space taken up by the array of nails with a site nailed joint shows why site alterations to a truss can be very challenging.
Connector plate performance in normally set out in an Agreement Certificate, which will set out the acceptable force per nail and the permissible stress in the plate.
Plates are usually manufactured from galvanised steel. However, stainless steel plates are available for use in more challenging environments.
For illustration, the diagrams below show a truss set out on a jig. It is important to note the tables in the jig take time to set up, and that during the set up the jig is not making any trusses. It follows that the “overhead” cost of the set up reduces as more trusses are made from one set up. Set up and transport costs are typically fixed costs in the supply of a roof structure.
Within a trussed rafter roof, variations in profile are normally accommodated by maintaining as much as the main profile as possible. For example, if a truss is to be stubbed, only the nearest web will be moved, Similarly, if a truss is to be cantilevered an extra web will be included. Importance of Frameworks
The most important characteristic of trussed rafters requiring understanding is the action of a framework. Beams carry load by bending, frameworks carry loads by direct force ie stretching or compressing. There is some bending in the top chord and in the bottom chord (due the application of the roof and ceiling loads). The effective span of the top and bottom chords is reduced due to the presence of internal webs.
For a given span, and a set load therefore, the pitch makes a huge difference to the member forces. The diagram above shows an example of the triangle of forces that apply at a support. The vertical component is the load passed onto the support (“the reaction”), the sloping line the top chord force, and the horizontal force. As the pitch flattens the forces are greatly magnified.
Notes for Specifiers / Builders
As with any process, there are fixed costs associated with truss rafter manufacture.
Notes for Surveyors / Builders / Building Inspectors / Specifiers
A truss is so much more than the sum of its parts. Any alteration to a truss once manufactured is likely to be involved and costly. Surveyors and Inspectors need to be vigilant in spotting unauthorised alterations.
Sin No 2 Unauthorised alterations
The old adage ‘think twice, cut once’ has never been truer than when applied to trussed rafters. As we saw in the last section, trusses are much more than the sum of their parts, and any alteration is likely to completely destroy the framework action of the truss rendering it of insufficient strength to carry the loads applied. The diagram below illustrates with red and green shading those areas which can be cut on site. Overhangs are generally designed to be cut to suit the site circumstances, either the correct length of trimming back where the support is on a girder truss at 90 degrees to it. So the simple rule with standard trusses do not cut or modify them in any way apart from trimming the overhang.
Attic trusses provide an additional opportunity for alterations to be carried out in appropriately. There is industry guidance concerning cutting and notching of joists for cables and central heating pipes. However, such guidance cannot be used with a framework such as an attic truss. Unfortunately, instances of plumbers notching and drilling the bottom chord of an attic truss are all too commonplace. Any services run within the loft space should be surface fixed in the dead spaces either side of the loft room.
Notes for surveyors inspectors/building inspectors
When inspecting trussed rafter roofs, be on the look out for the removal of any internal webs or any other alteration to the trusses where bolts, nails or screws have been used to fix members. Such an addition to a roof structure would generally indicate than some other member has been removed.
Notes for specifiers and builders.
Trussed rafters need to be ordered correctly to suit the site conditions and no reliance should be made on subsequent site alterations other than those referred to above such as overhangs. Drilling or notching of attic trusses is likely to weaken them and should not be undertaken in any circumstances.
Sin No 3 Insufficient bracing
The British Standard for trussed rafters, The Trussed Rafter Association and the NHBC handbook amongst other sources of information provide guidance on the bracing of trussed rafter roofs. Trussed rafters can almost be considered as a thin playing card in terms of their robustness about their weaker axis. Therefore for their stability and for the stability of the other roof a series of diagonal bracing and longitudinal bracing is required to ensure stability.
The idea of trussed rafters crossed the Atlantic from the United States during the early 60s. Standard construction practice in America was to use plywood sheathing over the whole of the roof thereby giving it rigidity and stability. Unfortunately when the idea of trussed rafters took hold in the UK, where plywood decking is not habitually used, most if not all trussed rafter roofs were built with very little or without any bracing at all. Many properties built during the 60s and 70s therefore have an inherent weakness and it is common to see rafter members bowing sideways within the roof and also leaning slightly to one side. Often this condition manifests itself in gable overhangs also in poor alignment. The diagram below gives an indication of the bracing standards that would normally be used for contemporary buildings. Where properties have been found to be un-braced, it is important that remedial bracing is installed.
There are some special situations which are likely to cause surprises even amongst those familiar with house building. All of the support statements regarding standard trussed rafter bracing do make clear that the bracing is dependant upon the presence of a plasterboard ceiling. In garages where there is no ceiling applied to the underside of the trussed rafters, diagonal bracing should be used. The requirement for this bracing is specifically required in the NHBC handbook and is hinted at in the British Standard for Trussed Rafters and in Approved Document A. Another situation where specific bracing requirements need to be considered is mono pitch trusses. The highest end of the truss would generally not be fixed to any structural item and therefore diagonal bracing along the plane of the end verticals is the requirement for stability. It is also worth noting that the diagonal running down to the corner of the high end carries a similar force to the rafter at the eaves and is likely to require some restraint. Trussed rafter manufacturers would normally provide such bracing information with their trussed rafter supply although it is the author’s experience that such bracing is often omitted. The diagram below illustrates these principles of non standard bracing.
Notes for surveyors / building control officers. Properties built during the 1960s and 1970s frequently do not have diagonal bracing and all of the necessary longitudinal bracing. Even if the roof shows no signs of any deformation it is important that such bracing is added retrospectively.
Notes for specifiers and builders. The advice provided by trussed rafters manufacturers should be carefully adhered to and bracing in non standard situations should be applied.
Sin No 4 Overloading / under loading
It would be my guess that there are perhaps 10s of thousands of houses in this country carrying a heavy tile such as plain tiles but have only been designed for concrete interlocking tiles. During the author’s short time in the trussed rafter industry several instances of this circumstance became apparent. Whilst the difference in weight between a plain tile and a concrete interlocking tile is not great, in respect of the actual load carried by the rafters the difference is marked. It never ceases to amaze the author how random the choice of tile covering can be with decisions often being made at site level fairly late in the contract. Manufacturers inevitably will assume that trusses will be carrying concrete interlocking tiles and they normally clearly mark their quotations and specifications to say so.
Another subtle problem that can arise with trussed rafter roof coverings is where light weight coverings are used. Whereas for many instances concrete interlocking tiles provide sufficient dead weight to prevent roof uplift problems, such difficulties are far more serious when lightweight coverings such as asbestos and slates are used. Typically the slates are used for large span low pitch roofs where uplift is particularly significant. The use of truss clips is essential for smallish roofs and double truss clips can be needed where large spans are used.
When the author has inspected roofs there have been some occasions when the householder has to be advised to remove or lessen the amounts of goods stored in the roof for danger of overloading it. Typically the bottom chord of a truss is designed for a superimposed load of 0.25 kN/m2 which the author often refers to as empty cardboard boxes loading.
Notes for surveyors and building control officers.
Always be on the look out for existing roofs that seem to have substantial amount or significantly heavy items stored in them. Books and paper in particular create high loadings.
Notes for specifiers and builders.
It is important to decide what loading the trussed rafter should be designed for and to communicate such information to the truss rafter manufacturer.
Sin No 5 Poor site handling and storage
The precise design and careful manufacture of trussed rafters is often undermined by poor site practice. Furthermore, poorly planned unloading and storage can be dangerous and the author is aware of at least one death resulting from bundled trussed rafters falling onto a site operative. Trusses are bundled together in the factory to enable them to be lifted as one unit. One tip for builders to be aware of if they are not already is that manufacturing tolerances on trusses mean that if two trusses were placed adjacent to each other but one to the opposite hand of its manufacture a dip in the roof could well result due to the variation in profile. Manufacturers generally as a habit run a mark down the rafters on one side to show which side of the rafters should always be orientated towards a particular eaves to avoid this problem.
On large well organised sites, a storage facility is put in place into which trusses can be unloaded and stored in an upright position, and then picked off as and when required. On a well organised site, a small crane is often hired to enable the bundled trusses to be lifted directly from the lorry onto the wall plates of the building.
However in many other instances trusses are simply laid flat upon the floor. Such a practice is satisfactory provided that the trusses have a firm and level support of the ground. However this again would involve some preparation.
A frequent practice on small building sites is for trusses to be manhandled from the ground up over the first floor scaffolding and onto the roof. It is inevitable trusses can sometimes bend under their own self weight over the scaffolding handrails.
Trusses are very susceptible to damage when bending out of plane typically the plate connectors can pull out as the joint acts as a hinge. Whilst there is no doubt that this happens and that the plate is pressed back in, (perhaps with a lump hammer), the joint can never have the same strength again if this practice is used. A similar problem can occur when trusses are stored on their sides on uneven ground the cumulative weight of all the trusses can bend the lower trusses out of shape. Notes for Surveyors, it is very difficult to tell whether any plates have been forced back in or indeed whether trusses have suffered from mishandling during construction.
Notes for Building Inspectors, keep an eye out for poor storage procedures and insist that trusses are checked when they appear to have been victim from such situations.
Notes for specifiers, specifications for trussed rafters should require the builder to pre determine the methods by which trusses will be stored on site or if they are to be hoisted directly onto their bearing points. Notes for builders, plan ahead and decide whether to construct proper storage facilities or whether to hire a crane and lift the trusses directly onto the wall plates (recommended).