Roof timbers and trusses

After external walls, roofs are the second most likely area where defects occur. Examining a truss roof defect requires a basic understanding of how they work.

In the 1980s the truss roof came to dominate the new-build housing sector. By the 1990s it was the major structure used in domestic low-rise construction. The gang nail truss or Fink truss roof was first designed and developed in the 1960s and made its first appearance in the volume house building market in the 1970s.

As pieces of engineering technology they are a superb example of efficient value engineering. The thin yet strong triangulation of struts and tie beams, interlocked by flat galvanised plates, is economic and durable and has ensured the success of the truss roof as a modern construction technique. The engineering principles of these roof members are robust and accurate, and mass production processes and treatment make the gang nail (Fink) roof truss one of the most reliable elements of a domestic structure.

Modern building regulations ensured that the construction principles of roofs designed using a gang nail truss are correct, well-documented and reliable. So why do defects arise in what appears to be a reliable product? Most defects arise when the known details and the regulations are ignored or omitted. Poor workmanship (possibly compromised by poorly targeted incentivised productivity pay) or poor supervision are the main reasons why truss roofs fail. Defects caused by material failure are comparatively rare.

All roof trusses must be tied and braced together, then anchored to the structure to make a unified framework. These roofs rely on the most economic transfer of loads through triangulated frames, and get their lateral stability by acting as a whole unit. If any of these principles are compromised, the roof can deform because of overloading or wind distortion. Every part of the roof has an important role to play, and if major components are missing or removed, the whole roof could be compromised. The capacity of the roof acting as a whole unit is quite considerable, and minor or localised defects (missing nails, or slight bows) may not necessarily compromise the roof.

When examining a truss roof to assess apparent movement or sagging, the key characteristic features to note are that:

• diagonal bracing runs at 45 degrees beneath all slopes for the full length of the roof;
• all of the rafters are connected to each other by longitudinal timbers, the full length of the roof;
• nothing is missing - any cuts, notches, holes or missing components must be investigated thoroughly;
• all nailing is accurate. Where trusses are permitted to be nailed to the top timber wall plate by driving the nail diagonally down into it, the carpenter must be skilled at tosh nailing, otherwise the timbers will split. If there are enough splits, the roof could be compromised. If nails are not long enough, the mechanical fixing may be too slight;
• all water tanks must be fully supported over at least 3 trusses.

Other defects associated with roofs generally include:

• poor detailing of underfelts around vent stacks;
• poor ventilation - with the advent of 'breathable' underfelts it can be difficult to assess if there is sufficient ventilation, but look for signs of condensation or a sense that the roof space is too still;
• inadequate thickness of insulation;
• no insulation on the access hatch;
• insulation blocking air circulation from the eaves;
• underfelt laid without a slight sag between the trusses can be a sign of a builder's shortcut, closely examine the details of all the roof finishes for build quality;
• no flame spread protection between properties.

Tip: Observe the roof space from the access hatch with no lighting on in the loft. This will highlight any holes in the roof, which can then be investigated.

Bolted roof trusses

With steel in short supply after the Second World War, the Timber Research and Development Association (TRADA) designed the bolted Fink truss. This forerunner of the gang nail plate truss was a highly successful engineered element. However, the significance of bracing was not well understood and some roofs have had to be retro-fitted with bracing.

Because of their efficiency, there is a significant drop in capacity if these trusses are cut for any reason. If all of the trusses are interconnected this may not be a problem on one joist. However, if they are not diagonally or longitudinally braced, the roof can be deformed in high winds.

No bracing applied to truss roofs

The Timber Research and Development Association gang nail Fink truss was developed in 1962. This design is still used today and is structurally efficient. However, like the bolt truss before it, the significance of bracing was not well understood, and in 1976 following a major roof collapse, new regulations were introduced to standardise the placing of bracing. Where bracing is found to be missing, it has to be retro-fitted. Other integrity issues are wall plates not strapped to the walls or gables, and gable ladders poorly fixed to rafters. If battens are cut to align on one rafter, then this can also affect the lateral restraint in the truss.

Sagging roof timbers

The development of the traditional cut roof was consolidated in this period. Jack rafters, common rafters and cripple rafters are cut with birdsmouth joints on a wall plate and joined at the ridge by a ridge board. The geometry of the roof improved with the advent of standardised joist sizes supplied by sawmills, and the concept of setting out the roof based on a datum one-third the depth of the joist made the roof frame more structurally predictable.

Purlins on hipped roofs are often supported using struts that rest on internal load-bearing walls. Sometimes these are inadvertently cut away, causing the roof to sag. Replacement of the struts usually resolves the problem.

If new concrete tiles are used to replace slate tiles on traditional cut roofs, the additional weight can cause an overload. Additional purlins or joists set parallel to the originals can help carry the extra loads. Velux-type skylights fitted into stressed roofs can exacerbate the situation. The presence of an attic room should alert the surveyor to potential venting problems and the possibility that struts may have been removed with no compensating structure added, e.g. metal purlins or straps.

The Victorians did not fit underfelt, and the presence of such a material is an indication of a re-roof. These felts should sag slightly between the joists and run into the gutters.

Rotten rafters are commonly found around chimneys, at eaves and at valleys - places where rainwater can penetrate the roof and usually a sign that flashing details require attention.