A poorly constructed roof can have devastating consequences on the home. The effects of repairing or replacing a roof structure can be disruptive to the homeowner and costly to the builder and warranty provider. Here Nick Cuffe, technical manager at Zurich Insurance Building Guarantee, examines three ways to head off trouble by avoiding defects in the first place.
Q
How do you avoid defects associated with trussed-rafter roofing?
A
Causes
- Site damage through inappropriate storage and handling
- Inadequate bracing, lateral restraint and associated fixings
- Incorrect trimming around openings and roof penetrations
- Poorly constructed or positioned water storage tank supports
Solutions
The delivery of trussed rafters should be planned so that the period of on-site storage is as short as possible. Trusses should always be kept clear of the ground and be supported by level bearers under or adjacent to the points of support. Trusses can be stored vertically, preferably in purpose-built racks or horizontally on suitable bearers to avoid twisting. Stored trusses should always be given weatherproof cover, with enough ventilation to prevent condensation. They should be inspected on arrival for damage – particularly to connector plates and truss feet – that may have been caused during transportation.
It is imperative that site handling is adequately planned, not only to avoid damage to roof trusses but to meet safety legislation. Mechanical transportation should be strictly supervised by trained operatives.
Manufacturers' instructions must be followed and made available to site operatives. All bracing is to be fully installed and multiple truss connections completed as set out in manufacturers' details. Roof bracing should be twice nailed every truss. Diagonal bracing should be fixed at an angle of between 35° and 50° and fixed not only to the trusses but also to the wall plate. Overlaps should be carried over at least two trusses. Longitudinal bracing should be positioned tightly to abut separating and gable walls. Chevron bracing should be installed where the span of the roof exceeds 8 m. Chevron bracing can be identified as diagonal bracing on the web members of the roof truss.
Sarking boards such as moisture-resistant plywood or OSB board may provide adequate bracing without the need for additional wind bracing in the roof.
Care should be taken with attic trusses and the positioning of bracing. Again, closely follow the manufacturer's design. Once installed, roof bracing should not be cut or removed by other trades people. Homeowners should be told that neither trusses or bracing should be altered. This is particularly relevant when occupiers are considering extra rooms or storage in roof spaces.
It is critical that openings such as loft accesses and trimming around roof penetrations such as chimneys are adequately formed. Ad hoc designs should be avoided.Seek confirmation from the roof designer that a trussed-rafter roof design can support water storage tanks. The use of bearer beams and supports fixed with moisture-resistant plywood or OSB sheet material used for the platform should form tanks stands. The size of the timbers will depend on the capacity of the water storage vessel and the span of the trusses. The tank bearers should be sited as close as possible to the node points and bear across a minimum of four trusses.Lateral restraint is needed on gable masonry. Lateral restraining straps and associated noggings must be fixed to roof truss rafters and ceiling cords at 2 m centres across three trusses. For cavity masonry, the straps should project and fit tightly on the outside face of the inner leaf of blockwork within the cavity. Q
How do you avoid defects associated with traditional "cut and pitch" roofs?
A
Causes
- Incorrect material specification
- Badly cut joints at the intersections of structural members
- Insufficient restraint
Solutions
Ensure the roof construction is designed by a structural engineer, and that details and drawings of truss rafter construction are available to site operatives. Span tables, for structural timber members used in roof construction, should be used appropriately.All structural timbers should be stress graded and marked KD (kiln dry) or DRY. Timbers must be preservative treated in areas where house longhorn beetle is a problem. Where internal ceilings follow the pitch of the roof timbers, they also need to be treated.
Joints at all intersections should be tightly formed and fixed. Particular care is needed when forming hips and valleys. Unskilled craftsmen may be incapable of forming or cutting complicated angle cuts. Equally, care should be taken when cutting "bird's mouth" joints not to reduce the effective depth of the rafters. A "birdsmouth" should not be deeper than a third of the depth of the rafter timber. Collars, purlins and associated struts provide extra support to the roof structure and help with load transfer. They need to be correctly specified by the structural engineer and positioned and fixed accordingly. This will prevent roof spread and avoid roof sagging.Beware that replacing a roof covering could have serious structural implications as far as changing the loading of the roof. Expert guidance should be sought.
Q
How do you avoid defects attributed to lack of ventilation?
Causes
- Soffit and eaves ventilation inadequate
- Inappropriate use of vapour-permeable roof membranes
- Extraction ducts not connected to atmosphere
Solutions
Adequate ventilation is vital for preventing excessive build-up of condensation in cold, pitched roof spaces. Vent the roof space on opposing faces of a building through the soffit, by the use of a 10 mm continuous ventilation gap. This should be increased to 25 mm for pitches of less than 15°. Roof insulation in the roof space must not block this ventilation. Proprietary eaves fittings can be fixed to help with this. Ceilings, following the pitch of the roof, provide 25 mm of continuous eaves ventilation along opposing faces. Provide additional proprietary ridge or tile ventilation equivalent to 5000 mm2 per metre run. Alternatively, condensation in cold and warm pitched roof spaces can be controlled with vapour-permeable membranes as the underlay in a non-ventilated system. Some systems do not need extra ventilation but must be installed as recommended by the manufacturer and the third party accreditation certification.Extraction ducts should not end in the roof space, but outdoors. This would normally be via proprietary tile, ridge or soffit ventilators. Also, if ventilation pipes to drainage systems are meant to terminate outside the building, this must happen, especially if the property is at the end of a drain run.
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