1. Selection of materials
Built-up roofs can be made up:
- Traditional roofing felts – see BS 747
- High-performance roof felts – see BS 747
- Non-British standard high performance felts
- Atactic polypropylene
- Metal-faced felts
- Pitch polymer sheets.
Specification should be agreed with the manufacturer and supplier to ensure that the finished product is suitable for use with the proposed substrate.
Any bonding compound or solar protective finishes must be agreed with the manufacturer of the finishing layer to ensure compatibility. These require regular maintenance to ensure they remain effective.
2. Drainage and waterproofing
3. Durability and design life
The ability of the roof covering to accommodate movement and fatigue is critical for performance and depends on the choice of material, number of layers, type of insulation, method of attachment to the substrate and quality of workmanship during installation. Insulation used below membranes must be strong in compression as embrittled felts, through cold or age, are vulnerable to fracture if excessive point-loading occurs. These hazards exist on roofs assumed to be subject only to light traffic.
Inverted roof systems or protected membranes reverse these risks. Through this construction the membrane has a much reduced temperature range, shrinkage stresses are minimal, seams are not endangered, point loads are unlikely and oxidation of the bitumen is much reduced.
Vertical upstands should be 150 mm minimum.
4. Wind loading
Most roof decks rely largely on gravity plus strapping down to restrain them. The wind forces on a lightweight deck can be sufficient to cause flex. The roof covering must be restrained and be flexible enough to accommodate the variations in movement. Most failures due to wind pressure occur at the roof perimeter. All edges and upstands must be fully restrained in accordance with the manufacturers' recommendations. Always ensure that all eventualities are considered.
5. Fire protection
The Building Regulations require roofs, in the event of fire, to provide protection to adjacent buildings or upper parts of the same building. Advice should be sought from the manufacturers on how best to achieve fire protection. Check that chosen system has correct fire rating for conformity with Building Regulations.
The insulation materials likely to be used in this type of roof construction are usually fibre based in either slab or sheet form. Fibrous materials, which absorb water from a leak or condensation, will suffer a loss of thermal performance. Vapour barriers should be used with all insulation materials that require it, to reduce condensation. Some insulates have a high coefficient of thermal expansion resulting in movement. If used below the felt covering, special precautions should be taken and a separating layer introduced to prevent damage and distortion of the felt.
7. Health and safety
Falls from height are top of the accident statistics, so risks must be fully assessed before start on site. Care should be taken to provide safe access and working platforms for the installation and maintenance of the roof and any rooftop plant.
It is necessary to provide defined walkways including barrier rails to items on roofs requiring maintenance, to comply with the Health and Safety at Work etc Act 1994.
There must be sufficient working space for installation, repair and maintenance to be completed in accordance to the specification and quality expected by the client.
On completion of the works it is essential that a maintenance program is put in place. Regular inspections should occur and any reported damage repaired immediately to maintain the performance of the roof. When the roof is new more frequent inspection is recommended.
- British Board of Agrément Certificates 97/3431
- BRE Digests 312: 1986; BRE Digest 324: 1987; 419: 1996
- BS747: 2000, BS 8217: 1994, BS 6229: 2003
- British Flat Roofing Council and CIRIA Flat roofing design and good practice 1993
- Flat Roofing Alliance, various
- GLC Bulletin 30, item 8
- Lead Sheet Association Rolled Lead Sheet – The Complete Manual 2003
This and other subject guides are available from Barbour Index as part of their Construction Expert and Specification Expert services. For further information contact Barbour Index on 01344 899280 or visit www.barbour-index.co.uk