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
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.
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.
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
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.
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
Downloads
Longitudinal bracing
Other, Size 0 kbOpenings in trussed rafter roofs
Other, Size 0 kbTraditionally framed roof
Other, Size 0 kb
Postscript
This article is a summary and not necessarily a comprehensive review of the subject matter. The views expressed in this article are those of the author only and not necessarily those of Zurich Insurance Company.
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