Prefabricated Roof Truss Construction: 7 Deadly Sins
With an excess number of Roof Trusses (herein after called trussed rafters) there has been, to date, no known failure of a trussed rafter “ex-factory” condition, a remarkable safety record. Kelbrick’s Roof Trusses provides you with the information on how trussed rafters work, their strengths and their limitations. Common misunderstandings are examined that will hopefully costly and obstinate modifications.
Trussed rafters are defined as triangulated timber frameworks positioned at close centres (generally 600mm) to form roof structures. Trusses are engineered to suit the application requested. Roof Truss manufacturers use very sophisticated software (known as the system owners) such as MiTek to design the trusses, produce quotations, manufacture information and produce data.
The advantage of trussed rafters is that it enables 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.
The 7 Deadly Sins
Mistreating trusses can be costly and disruptive to subsequently repair. Below the 7 sins are briefly discussed.
1. Not knowing how trusses function:
A fundamental understanding of how trusses are designed and manufactured will greatly assist the process of specifying and assessing trusses. Timber has an excellent strength to density ratio, making it an ideal structural material.
The most effective option to joint timber to mobilise its full strength is the use of connector plates. The plates are pressed into the timber. Connector plate performance is normally set out in an Agreement Certificate, which will set out the acceptable force per nail and the permissible stress in the plate.
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 i.e. 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.
Notes: As with any process, there are fixed costs associated with truss rafter manufacture. 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.
2. Unauthorised alterations:
The adage ‘think twice, cut once’ has never been truer than when applied to trussed rafters. trusses are much more than the sum of their parts, and any alteration is likely to destroy the framework action of the truss rendering it of insufficient strength to carry the loads applied.
Notes: When inspecting trussed rafter roofs, be on the lookout 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.
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.
3. Insufficient bracing:
Various sources of information provide guidance on the bracing of trussed rafter roofs. All the support statements regarding standard trussed rafter bracing do make clear that the bracing is dependent upon the presence of a plasterboard ceiling. In garages or exposed trusses where there is no ceiling applied to the underside of the trussed rafters, diagonal bracing should be used.
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.
Notes: Properties built during the 1960s and 1970s frequently do not have diagonal bracing and all the necessary longitudinal bracing. Even if the roof shows no signs of any deformation it is important that such bracing is added retrospectively. The advice provided by trussed rafters manufacturers should be carefully adhered to and bracing in non-standard situations should be applied.
4. Overloading / under loading:
10s of thousands of houses carry a heavy tile such as plain tiles but have only been designed for concrete interlocking tiles. 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. The choice of tile covering can be randomly with decisions often being made at site level 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 problem that can arise with trussed rafter roof coverings is where light weight coverings are used such as asbestos and slates. 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.
Notes: Always be on the lookout for existing roofs that seem to have substantial amount or significantly heavy items stored in them. Books and paper create high loadings. It is important to decide what loading the trussed rafter should be designed for and to communicate such information to the truss rafter manufacturer.
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. Trusses are bundled together in the factory to enable them to be lifted as one unit.
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 if the trusses have a firm and level support of the ground.
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.
Notes: Keep an eye out for poor storage procedures and insist that trusses are checked when they appear to have been victim from such situations. 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 and decide whether to construct proper storage facilities or whether to hire a crane and lift the trusses directly onto the wall plates (recommended).
6. Poor manufacture and poor construction:
Not all trussed rafter manufacturers subscribe to a quality assurance scheme for eg. SABS. However, this does not guarantee the trusses are free from fault. The design of truss rafters will normally include a plate positioning tolerance of 5mm.
Very few buildings would have wall plates that are totally and truly parallel to each other and even fewer where the distance over wall plates is exactly as assumed in the design. It is important that such measurements are taken in several locations to determine the widest distance.
A complicated roof may have a special wide web to facilitate hanging a special metal bracket for an incoming load from say a girder or a purlin. In such circumstances, careful planning of the unloading and hoisting to roof level of the trusses needs to be carried out. It is essential that the side on which the wide web is located is identified early on to ensure the trusses are loaded onto the roof in the right orientation.
Notes: Check that major incoming loads to trussed rafter girders have a suitable hanger fixed to a suitable web. Check that valleys are constructed in a way to distribute the load on the underlying trussed rafters and that the underlying trussed rafters have some fixings to their top chord. When writing specifications for trussed rafters, require trusses to be ordered with the specified tolerance at the heel. plan unloading and erecting procedures carefully particularly where asymmetric girders are used properly construct valleys to ensure loads are distributed evenly on the underlying trusses and that there is restraint to the top chords of the trusses.
7. Poor specification:
Trussed rafter technology and more importantly the power of the software to design and draw trussed rafter roofs have increased significantly over the last two or three decades. Specifiers often want eaves details to align with window heads etc.
Often designers and specifiers complain that they cannot finalise the eaves detail until they know the top chord size of the trussed rafter. Top chord sizes for most span / pitch combinations are in fact not decided by design but using tables of tested spans.
Very often the preferred eaves detail will involve the ceiling being placed above the wall plate level, known as a raised tie truss. The trussed rafter industry decided a long time ago that such movement must be limited and generally works on a figure of 6mm at each eave.
Very often to enable a feasible design to be executed the trussed rafter designer will specify a timber supplement to the rafter referred to as a scab. In extreme cases, there may be a scab either side. A trussed rafter manufacturer will provide details of the scabs and details of how they should be nailed.
For the above reasons raised tie type roofs are limited in what can be achieved with trussed rafters where there is say a hip end or a big intervention to the line of trussed rafters by say a large roof light.
Notes: Always be on the lookout for poorly executed loft conversions where the original roof was standard trussed rafters. Always be wary of designs (not supported by a trussed rafter manufacturer’s calculations and details) where excessive raised tie distances or excessive room widths are specified. always be aware of the limitations of attic trusses and the limitations of raised tie trusses.
Where very large room widths are required or a very complicated roof plan results it may well be cheaper to have internal load bearing walls and / or major purlins (glulam or steel).
Kelbrick’s Roof Trusses has been manufacturing roof trusses for 31 years and considers the above mentioned seven sins as significant when designing, manufacturing and constructing roof trusses.
As the first approved SABS and SANS 1900 Roof Truss Manufacturer in South Africa since 2002 (15 years), Kelbrick’s Roof Trusses experiences 7 unexpected inspections from SABS and can boastfully confess that no fault has ever been recorded on our design, manufacturing processes, or the quality of our roof trusses.
Trussed rafters provide a very economic form of roof structure and can generally be built very quickly. Discipline needs to be exercised in the specification of trussed rafters and particularly in their erection. Be always wary of trussed rafter raised tie roofs where the limitations of what can be achieved may make a design unfeasible using factory made trusses.
Building Control inspectors and surveyors always need to be on the lookout for unauthorised alterations to trussed rafter roofs.
Building Inspectors always need to be vigilant for poor storage and poor erection practices. Laying trusses flat on uneven ground is unfortunately a common practice that the author has witnessed many times.
Despite all the above, trussed rafters are still the structural solution of choice for residential properties and for an increasing number of commercial properties. Their use allows elegant economic solutions to be formed.