7.1 Flat roofs and balconies
Appendix 7.1-A
Commonly used flat roofs
Three types of flat roof are shown here to illustrate the principles of their design:
WARM DECK DESIGN (insulation on top of deck) | |
timber deck | concrete deck |
|
|
INVERTED ROOF DESIGN (insulation on top of weatherproofing) | |
timber deck | concrete deck |
|
|
COLD DECK DESIGN (insulation at ceiling level) | |
timber deck | concrete deck |
| not applicable |
WARM DECK (timber)
Reflective surface
Solar reflective treatment consisting of stone chippings or tiles of concrete or fibre cement (see Appendix 7.1-B for built-up felt and asphalt).
Weatherproofing
Weatherproofing should be one of the following:
- mastic asphalt (BS 6925), 20mm thick on the flat, laid on sheathing felt (Class 4 in BS 747)
- sheet metal roof complying with BS 6915 for lead or the relevant parts of CP 143 for other sheet materials
- bitumen roofing felt to BS 747 from the following table:
Type of roofing felt | Insulation material | Method of fixing first layer | ||
First layer | Second layer | Cap sheet | ||
Type 3B | Type 5U | Type 3B or 3E | glass fibreboards rock fibreboards corkboards | full bond |
Type 5U | - | Type 5B or 5E | ||
Type 3G | Type 3B | Type 5B or 5E | Polyurethane and polyisocyanurate boards | partial bond |
Type 3G | Type 5U | Type 5B or 5E |
Cap sheets with a 'B' suffix require a separate stone chipping finish. Normally, a separate stone chipping finish is required for fire regulation purposes.
Insulation
The following rigid insulation boards are suitable:
- Polyurethane and polyisocyanurate
- Glass fibre or rock fibre
- Corkboard.
Vapour control layer
Vapour control layer must consist of at least one layer of bitumen roofing felt Type 3B partially bonded to the structural deck: all laps must be sealed with bitumen.
Preservative treatment
All roof timbers, joists, wall plates, blocking, strutting, battens, firrings and noggings to be preservative treated, unless naturally durable. Chapter 2.3 'Timber preservation (natural solid timber)' gives full details of preservative treatments.
Deck
Timber or timber-based decks should be one of the following:
Material | Minimum board thickness [mm] for joist centres of | ||
400mm | 450mm | 600mm | |
Pre-treated timber boarding (tongued and grooved) | 16 | 16 | 19 |
Pre-treated plywood, WBP grade | 12 | 12 | 15 |
Marine plywood, WBP grade | 12 | 12 | 15 |
Wood chipboard, Type P5 | 18 | 18 | 22 |
Woodwool slabs, Type SB | 51 | 51 | 51 |
Oriented Strand board Type OSB3 | 16 | 16 | 19 |
Reference should be made to Sitework clause 7.1 - S7 for fixing of the deck to joists.
Joists and firrings
For sizes and spacing, reference should be made to tables prepared to recognised standards.
Detailing
Typical details are shown in Appendix 7.1-C
WARM DECK (concrete)
Reflective surface
Solar reflective treatment consisting of stone chippings or tiles of concrete or fibre cement (see Appendix 7.1-B for built-up felt and asphalt).
Weatherproofing
Weatherproofing should be one of the following:
- mastic asphalt (BS 6925), 20mm thick on the flat, laid on sheathing felt (Class 4 in BS 747)
- sheet metal roof complying with BS 6915 for lead or the relevant parts of CP 143 for other sheet materials
- bitumen roofing felt to BS 747 from the following table:
Type of roofing felt | Insulation material | Method of fixing first layer | ||
First layer | Second layer | Cap sheet | ||
Type 3B | Type 5U | Type 3B or 3E | glass fibreboards rock fibreboards corkboards | full bond |
Type 5U | - | Type 5B or 5E | ||
Type 3G | Type 3B | Type 5B or 5E | Polyurethane and polyisocyanurate boards | partial bond |
Type 3G | Type 5U | Type 5B or 5E |
Cap sheets with a 'B' suffix require a separate stone chipping finish. Normally, a separate stone chipping finish is required for fire regulation purposes.
Insulation
The following rigid insulation boards are suitable:
- Polyurethane and polyisocyanurate
- Glass fibre or rock fibre
- Corkboard.
Vapour control layer
Vapour control layer must consist of at least one layer of bitumen roofing felt Type 3B partially bonded to the structural deck: all laps must be sealed with bitumen.
Concrete deck and screeds
Concrete roof deck, with dense screed topping to achieve the falls. The screed should be a minimum of 40mm in thickness.
Adequate time for drying out of the slab should be allowed prior to plastering/dry lining.
For in-situ construction, allowance should be made for draining away excess construction moisture.
Detailing
Typical details are shown in Appendix 7.1-C
INVERTED ROOF (timber)
(NOT suitable for slopes greater than 10°)
Ballast
Ballast may consist of paving slabs or a 50mm thickness of rounded pebbles of minimum diameter 19mm.
Insulation
Insulation must be of a type unaffected by exposure to the weather. The following materials are suitable:
- Extruded polystyrene
- Extruded polystyrene with ballast surface
- Compressed boards of glass fibre/rock fibre
Weatherproofing
Weatherproofing should be one of the following:
- mastic asphalt (BS 6925), 20mm thick on the flat, laid on sheathing felt (Class 4 in BS 747)
- bitumen roofing felt to BS 747 from the following table: Pre-felting of the deck material cannot be counted as part of the weatherproofing.
Type of roofing felt | Deck material | Method of fixing first layer | ||
First layer | Second layer | Cap sheet | ||
Type 3B | Type 5U | Type 3B or 3E | timber boarding | nailing |
Type 3B | Type 5U | Type 5B or 5E | ||
Type 3G | Type 3B | Type 5B or 5E | cement screed pre-felted surfaces plywood chipboard | partial bond |
Type 3G | Type 5U | Type 5B |
Preservative treatment
All roof timbers, joists, wall plates, blocking, strutting, battens, firrings and noggings to be preservative treated, unless naturally durable. Chapter 2.3 'Timber preservation (natural solid timber)' gives full details of preservative treatments.
Deck
Timber or timber-based decks should be one of the following:
Material | Minimum board thickness [mm] for joist centres of | ||
400mm | 450mm | 600mm | |
Pre-treated timber boarding (tongued and grooved) | 16 | 16 | 19 |
Pre-treated plywood, WBP grade | 12 | 12 | 15 |
Marine plywood, WBP grade | 12 | 12 | 15 |
Wood chipboard, Type P5 | 18 | 18 | 22 |
Woodwool slabs, Type SB | 51 | 51 | 51 |
Oriented Strand board Type OSB3 | 16 | 16 | 19 |
Reference should be made to Sitework clause 7.1 - S7 for fixing of the deck to joists.
Joists and firrings
The use of ballast may affect the timber sizing: for correct sizes and spacing, reference should be made to tables prepared to recognised standards.
The weight of a 50mm thickness of pebbles is approximately 80kg/m2. The highest dead load figure (0.75kN/m2 to 1.00kN/m2) should be used to determine joist sizing.
INVERTED ROOF (concrete)
(NOT suitable for slopes greater than 10°)
Ballast
Ballast may consist of paving slabs or a 50mm thickness of rounded pebbles of minimum diameter 19mm.
Insulation
Insulation must be of a type unaffected by exposure to the weather. The following materials are suitable:
- Extruded polystyrene
- Extruded polystyrene with ballast surface
- Compressed boards of glass fibre/rock fibre
Weatherproofing
Weatherproofing should be one of the following:
- mastic asphalt (BS 6925), 20mm thick on the flat, laid on sheathing felt (Class 4 in BS 747)
- bitumen roofing felt to BS 747 from the following table: Pre-felting of the deck material cannot be counted as part of the weatherproofing.
Type of roofing felt | Deck material | Method of fixing first layer | ||
First layer | Second layer | Cap sheet | ||
Type 3G | Type 3B | Type 5B or 5E | concrete | partial bond |
Type 3G | Type 5U | Type 5B |
Concrete deck and screeds
Concrete roof deck, with dense screed topping to achieve the falls. The screed should be a minimum of 40mm in thickness.
Adequate time for drying out of the slab should be allowed prior to plastering/dry lining.
For in-situ construction, allowance should be made for draining away excess construction moisture.
Detailing
Typical details are shown in Appendix 7.1-C.
COLD DECK
(NOT permitted in Scotland - NOT recommended in the rest of the UK)
Cold deck roofs may be used in the UK (excluding Scotland) only where:
- the required level of ventilation can be achieved
- ventilation paths are not blocked by structural or other members
- a ventilation space of 50mm can be maintained.
Reflective surface
Solar reflective treatment consisting of stone chippings or tiles of concrete or fibre cement (see Appendix 7.1-B for built-up felt and asphalt).
Weatherproofing
Weatherproofing should be one of the following:
- mastic asphalt (BS 6925), 20mm thick on the flat, laid on sheathing felt (Class 4 in BS 747)
- sheet metal roof complying with BS 6915 for lead or the relevant parts of CP 143 for other sheet materials
- bitumen roofing felt to BS 747 from the following table:
Type of roofing felt | Deck material | Method of fixing first layer | ||
First layer | Second layer | Cap sheet | ||
Type 3B | Type 5U | Type 3B or 3E | timber boarding | nailing |
Type 3B | Type 5U | Type 5B or 5E | ||
Type 3G | Type 3B | Type 5B or 5E | cement screed pre-felted surfaces plywood or chipboard | partial bond |
Type 3G | Type 5U | Type 5B or 5E |
Cap sheets with a 'B' suffix require a separate stone chipping finish. Normally, a separate stone chipping finish is required for fire regulation purposes. In areas of high wind exposure, a first layer of Type 5U should be used in place of Type 3B. Pre-felting of the deck material cannot be counted as part of the weatherproofing.
Preservative treatment
All roof timbers, joists, wall plates, blocking, strutting, battens, firrings and noggings to be preservative treated, unless naturally durable. Chapter 2.3 'Timber Preservation (natural solid timber)' gives full details of preservative treatments.
Deck
Timber or timber-based decks should be one of the following:
Material | Minimum board thickness [mm] for joist centres of | ||
400mm | 450mm | 600mm | |
Pre-treated timber boarding (tongued and grooved) | 16 | 16 | 19 |
Pre-treated plywood, WBP grade | 12 | 12 | 15 |
Marine plywood, WBP grade | 12 | 12 | 15 |
Wood chipboard, Type P5 | 18 | 18 | 22 |
Oriented Strand board Type OSB3 | 16 | 16 | 19 |
Reference should be made to Sitework clause 7.1 - S7 for fixing of the deck to joists.
Joists and firrings
For sizes and spacing, reference should be made to tables prepared to recognised standards.
Ventilation
Ventilate every void with a totally unobstructed air flow to two opposite sides of the roof. The area of ventilation openings should be at least equal to continuous ventilation running the full length of the eaves and 25mm wide. A ventilation space of at least 50mm should be left between the insulation and the decking. Strutting should not obstruct ventilation.
For roof forms other than a single rectangle or roofs with a span exceeding 10m, the eaves ventilation should be increased to 0.6% of the roof plan area. The actual width of ventilation openings will depend on the length of eaves and the shape of the roof.
Insulation
Insulation of rock fibre or glass fibre, directly above the ceiling. The thickness should satisfy the Building Regulations. Joist depths should allow for at least a 50mm air space above the insulation.
Vapour control layer
Vapour control layer of foil-backed or moisture-resistant plasterboard with board joints fixed over joists, or taped; alternatively polyethylene sheeting, minimum 250g with 150mm laps.
Detailing
Typical details are shown in Appendix 7.1-C.
Appendix 7.1-B
Surface treatments acceptable to NHBC
The following surface treatments are suitable. They apply to cold and warm decks only.
Maintenance only for roofs up to 10° | Access roof, walkway or terrace deck | Further information may be obtained from | |
Built-up felt | reflective stone chippings* in dressing compound or ballast laid loose or mineral surfaced capsheets (such as Type 3E or Type 5E) | precast concrete paving slabs or proprietary paving slabs | British Flat Roofing Council |
Mastic asphalt | solar reflective paint as permitted by MACEF or reflective stone chipping1 bedded in a bitumen-based compound | concrete or GRC tiles bedded in bitumen or cement/sand screed2 | Mastic Asphalt Coucil and Employers' Federation |
1 Prevent loose surface finishes being removed by weather and discharged into gutters and drain pipes. Choose chippings of size and shape which will not penetrate the roof covering under maintenance foot traffic.
2 Cement/sand screed should be laid on a waterproof building paper or 1000 gauge polyethylene separating membrane. The pavings must be kept back 75mm at perimeters and a 25mm movement gap incorporated for every 9m2 of pavings laid.
Appendix 7.1-C
Construction details of flat roofs and balconies
This Appendix contains the most common details in flat roofs and balconies. The details are typical and variations for different roofs are indicated beneath each sketch. However, not every aspect which needs to be considered is necessarily shown.
CONCRETE DECK
Upstand - concrete deck
- upstand may be fixed to wall
- upstand to be at least 150mm high above chippings
- similar details apply to inverted roofs with concrete decks.
TIMBER DECK
Independent skirting detail - timber deck
- keep upstands separate from wall - allow for movement
- upstand to be at least 150mm high above chippings
- similar details apply to cold deck timber roofs.
Welted drip to external gutter - timber deck
- similar details apply to cold deck timber roofs
- inverted timber decks need special consideration to avoid insulation being lifted off by wind suction. Use an alternative detail.
CONCRETE DECK
Skirting to metal rooflight or ventilator kerb - concrete deck
- similar details apply to inverted roofs. Allow for thickness of ballast to achieve upstand dimension.
TIMBER DECK
Upstand to ventilator or rooflight kerb
- similar details apply to cold and inverted roofs. Allow for thickness of ballast in inverted roofs to achieve upstand dimension.
Verge detail - timber deck
- similar details apply to inverted deck.
CONCRETE DECK
Twin-kerb expansion joint - concrete roof
- expansion joint is similar for both warm and inverted concrete roofs.
Handrail fixing
- form an upstand in concrete roofs
- grout or rag bolt ends of standards into pockets
- do not penetrate timber roofs. Use alternative methods of fixing.
TIMBER DECK
Pipe passing through roof
- ensure vapour control layer is bonded to weatherproofing
- detailing of upstand and flashing is similar for all roofs
Rainwater outlet - timber roof
- properly trim the opening
- ensure outlet is at lowest point in roof
- similar details apply to concrete roof
- ensure outlet is fixed securely to decking to prevent displacement by thermal expansion of rainwater pipe.