Q - How do you prevent roof coverings from being dislodged?
A - Causes
- Poorly specified coverings and fixings for the exposure of the development
- Incorrectly fixed roof tiles, slates or specialised coverings
Solutions
The roof underlay to a pitched roof should be fixed in accordance with the manufacturers' recommendations, with care taken to ensure that water will run off into the gutter – for example, by using a tilting fillet.
Roofing underlay must have sufficient horizontal and vertical laps. Vertical laps should only occur on rafters, to which they need to be securely fixed. The underlay extends into the gutter and the roof covering normally projects to the centreline of the gutter. Particular care needs to be taken in ensuring weathertightness at eaves, verges and valleys.
Tiling must be fixed in accordance with the manufacturer's instructions, due account being taken of the site exposure, height of the building and roof pitch. Many large tile producers have advisory services and will provide site-specific fixing specifications.
Non-ferrous or stainless steel nails and/or clipped fixings should be used. Generally, as a minimum, interlocking tiles are fixed at all perimeters, which include eaves, ridges, verges and tiles adjacent to hips, valleys and roof penetrations such as chimneys. Where the pitch exceeds 45o, all tiles need to be nailed. Where pitches exceed 55o, additional clipping is normally required.
Once again, as a minimum, plain tiles should be twice nailed every fifth course and at perimeters, roof penetrations and areas subject to uplift. Where the pitch exceeds 60o, all plain tiles should be twice nailed.
Imported slates are commonly used, often from Spain, China or Brazil. They should be Class 1 or Grade A, show compliance with BS680 and be relatively free from pyrites. For slates 150 mm wide or greater, twice nail over the entire roof.
Spacings for battens, to both tiles and slates, must ensure adequate laps at each course, depending on pitch and exposure, and provide a neatly aligned and uniform gauge over the whole roof. The gauge may be reduced, but not increased. Battens need to be at least 1200 mm in length and span across a minimum of three supports. Sizes of battens are dependent on rafter spacings and type of covering. Treat battens with preservative, but do not use aluminium nails in conjunction with treatments containing copper chrome arsenic. In coastal areas or other regions with an aggressive atmosphere, corrosion-resistant nails are recommended.
Ridges and verges to tiling and slate should be adequately fixed. Mortar for roofing work should be 1:3 cement:sand and in accordance with BS 5534. In exposed areas, additional mechanical fixing should also be considered.
Proprietary dry verge and ridge systems should be installed in strict compliance with the manufacturer's recommendations and be suitable for the roof's exposure. These types of dry systems can often reduce maintenance costs.
Thatched roofing should be put up by a suitably experienced thatcher and in accordance with specialist recommendations. The depth of thatch should be between 300 mm and 375 mm. The recommended type of thatch is Norfolk reed. Combed wheat straw may be used but is subject to more maintenance. It is recommended that thatch roofs are subject to an annual inspection and that ridges are renewed every five years.
Other specialised roof coverings, which are numerous, should have third-party accreditation such as a British Board of Agrément or British Research Establishment "WIMLAS" certificate. They must be used and fixed in accordance with this third-party accreditation and be suitable for the pitch and exposure of the roof.
Q - How do you prevent a roof from leaking?
A - Causes
- Incorrectly fixed and poorly specified roof coverings
- Poor detailing or specified flashings
Solutions
Well-fixed roof coverings generally will not leak. Much of what has been previously said for preventing coverings from being dislodged is relevant to avert a leaking roof.
Flat roofs should be laid to a minimum fall of 1:40. Weatherproof coverings should be hot-bonded and consist of three layers of high-performance felt. Single layer, weatherproof coverings are available. However, all weatherproof coverings to flat roofs must have third-party accreditation. Chipboard should not be used as a decking material. Non-traditional flat roof coverings include glass-reinforced plastic. They should be laid level or to falls as recommended by the manufacturers.
Valleys should be made with purpose-made tiles or other durable sheet materials so as to provide a continuous channel. Neatly form junctions between the covering and valley with a cement-based mortar or a proprietary system.
When using lead for flashings, fix in accordance with the Lead Sheet Association recommendations.
Movement joints, when required, should extend through the roof covering and be adequately weatherproofed. In timber-frame construction, flashings should allow for up to 30 mm in vertical differential movement between the roof surfaces and abutments.
Roof coverings that are penetrated or abut at intersections with other materials require close scrutiny. Detailing at these points is crucial to maintain the water shedding capabilities of the roof covering.
Flashing at parapets, chimneys, dormer windows, skylights and abutments, and around soil stack and flue intrusions, must be adequately detailed and linked to associated cavity trays.
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UK levels of exposure to wind-driven rain
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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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