NHBC Standards 2006

7.1 Flat roofs and balconies

7.1 - Appendix 7.1A
   Commonly used flat roofs
   Warm deck (timber)
   Warm deck (concrete)
   Inverted roof (timber)
   Inverted roof (concrete)
   Cold deck
7.1 - Appendix 7.1B
   Surface treatments acceptable to NHBC
7.1 - Appendix 7.1C
   Construction details of 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

 Timber deck

 Concrete deck


INVERTED ROOF DESIGN (insulation on top of weatherproofing)
timber deck concrete deck

 Timber deck

 Concrete deck


COLD DECK DESIGN (insulation at ceiling level)
timber deck concrete deck

 Timber deck

 not applicable

WARM DECK (timber)

Warm dec (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 3GType 3BType 5B or 5E Polyurethane and polyisocyanurate boards partial bond
Type 3GType 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:

MaterialMinimum board thickness [mm]
for joist centres of
400mm450mm600mm
Pre-treated timber boarding (tongued and grooved) 161619
Pre-treated plywood, WBP grade 121215
Marine plywood, WBP grade 121215
Wood chipboard, Type P5 181822
Woodwool slabs, Type SB 515151
Oriented Strand board Type OSB3 161619

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)

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 3GType 3BType 5B or 5E Polyurethane and polyisocyanurate boards partial bond
Type 3GType 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°)

Inverted roof (timber)

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 5UType 3B or 3E timber boarding
nailing
Type 3BType 5UType 5B or 5E
Type 3GType 3BType 5B or 5Ecement screed
pre-felted surfaces
plywood
chipboard
partial bond
Type 3GType 5UType 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:

MaterialMinimum board thickness [mm]
for joist centres of
400mm450mm600mm
Pre-treated timber boarding (tongued and grooved) 161619
Pre-treated plywood, WBP grade 121215
Marine plywood, WBP grade 121215
Wood chipboard, Type P5 181822
Woodwool slabs, Type SB 515151
Oriented Strand board Type OSB3 161619

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°)

Inverted roof (concrete)

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 3GType 3B Type 5B or 5E

concrete
concrete screed

partial bond
Type 3GType 5UType 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.

Cold deck

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 5UType 3B or 3E timber boarding
nailing
Type 3B Type 5UType 5B or 5E
Type 3GType 3BType 5B or 5E cement screed
pre-felted surfaces
plywood or chipboard
partial bond
Type 3GType 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:

MaterialMinimum board thickness [mm]
for joist centres of
400mm450mm600mm
Pre-treated timber boarding (tongued and grooved) 161619
Pre-treated plywood, WBP grade 121215
Marine plywood, WBP grade 121215
Wood chipboard, Type P5 181822
Oriented Strand board Type OSB3 161619

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 - 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

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

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

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 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

Verge detail - timber deck

  • similar details apply to inverted deck.

CONCRETE DECK

Twin-kerb expansion joint - concrete roof

Twin-kerb expansion joint - concrete roof

  • expansion joint is similar for both warm and inverted concrete roofs.

Handrail fixing

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

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

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.