7.1.15 Detailing of flat roofs
Flat roofs shall be detailed to ensure satisfactory performance.
Table 17: Materials for flashings
| Flashing material | Guidance |
|---|---|
| Rolled lead sheet | Minimum Code 4. BS EN 12588 |
| Aluminium and aluminium alloys | BS EN 485 and BS EN 573, 0.6-0.9mm thick and protected from contact with mortar by a coating of bituminous paint |
| Zinc alloys | BS EN 988 and 0.7mm thick |
| Copper | BS EN 1172, 0.6mm thick and fully annealed |
| Stainless steel | BS EN ISO 9445 Parts 1 and 2 |
| Galvanised steel | BS EN 10142/3 |
| Proprietary flashing, including plastic and composite | Assessed in accordance with Technical Requirement R3 |
Note(s)
Where two metals are to be joined, they should be compatible and not cause bimetallic corrosion in that environment. Alternatively, they should be isolated from each other.
The following figures are intended as a guide to demonstrate the general principles of the flat roof, detailing commonly used on flat roofs, terraces and balconies. Where indicated, the principles are applicable to other types of roof construction. Further information on specific waterproofing systems may be obtained from BS 6229 and BS 8217.
Concrete decks
Upstands may be fixed to the wall and should be a minimum of 150mm high. Similar details apply to inverted roofs with concrete decks.
Similar details apply to inverted roofs. Allow for thickness of ballast to achieve a minimum 150mm upstand.
Similar details apply to inverted roofs.
Timber decks
Elements should be firmly fixed to prevent peelback in high winds.
The upstand should be a minimum of 150mm high above surface finish. Upstands should be kept separate from wall, and allow for movement. Similar details apply to cold deck timber roofs.
Similar details apply to inverted decks.
Inverted timber decks should be detailed to avoid insulation being lifted by wind suction, and an alternative detail used.
A VCL should be bonded to the waterproofing.
Detailing of upstand and flashing is similar for all roofs.
Similar details apply to cold and inverted roofs. Allow for the thickness of ballast in inverted roofs, to achieve upstand dimensions.
Outlet should be at the lowest point in the roof.
The opening should be properly trimmed.
Ensure that the outlet is fixed securely to decking to prevent displacement by thermal expansion of the rainwater pipe.
Similar details apply to concrete roofs.
Last updated: 2nd January 2025