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 ROOF (insulation on top of deck)
concrete deck	timber deck	profiled metal deck
* mineral surfaced or solar reflective treatment where required

INVERTED WARM ROOF (insulation on top of waterproofing)

concrete deck

GREEN ROOF
intensive (requires regular maintenance. Plants contained within soil)	extensive (requires periodic maintenance. Plants generally contained in the sedum blanket

WARM ROOF (concrete deck)

Surface treatment

See Appendix 7.1-B

Waterproofing and insulation

Waterproofing should be one of the following:

 

Vapour control layer

The vapour control layer should consist of at least one layer of bitumen roofing membrane (S2P3) fully bonded to the structural deck and all laps sealed with bitumen.

Concrete deck and screed

Concrete roof deck, suitably primed, with sand/cement screed topping to achieve the falls. The screed should be in accordance with Clause 7.1-S7(a).

Detailing

Typical details are shown in Appendix 7.1-C.

WARM ROOF (timber deck)

WARM ROOF (profiled metal deck)
 

Surface treatment

See Appendix 7.1-B

Waterproofing and insulation

Waterproofing should be one of the following:

 

Vapour control layer

In bonded systems the vapour control layer should consist of at least one layer of bitumen roofing membrane (S2P3) fully bonded or nailed to the structural deck and all laps sealed with bitumen. In mechanically fixed systems the vapour control layer should consist of suitable polyethylene sheet sealed at all laps.

Preservative treatment

All roof timbers, joists, wall plates, blocking, strutting, battens, firrings, noggings to be preservative treated, unless naturally durable. Chapter 2.3 'Timber preservation (natural solid timber)' gives details of preservative treatments.

Deck

Timber or timber-based decks should be one of the following:

Material	Thickness of deck (mm)
	Joist centres (mm)
	450mm	600mm
Pre-treated plywood, WBP grade	15	18
Marine plywood, WBP grade	15	18
Oriented Strand board Type OSB3	15	18
Pre-treated timber planking - tongue and grooved ('close boarded timber') Max. board width 100mm.	19	19

Reference should be made to Sitework clause 7.1 - S6 for fixing of the deck to joists.

Joists

For sizes and spacing, reference should be made to appropriate load/span tabales published by TRADA in support of Building Regulations and associated documents.

Detailing

Typical details are shown in Appendix 7.1-C.

INVERTED WARM ROOF(concrete deck)

(NOT suitable for slopes greater than 10°)

 

Ballast

Ballast should consist of paving slabs, or of rounded pebbles of minimum diameter 19mm to the depth specified in the design.

Filter layer

Geo-textile layer, laid over insulation boards to prevent fines from reaching the membrane surface.

Insulation

Insulation should be of a type unaffected by exposure to the weather and capable of supporting the weight of the ballast. Only the following materials are suitable:

• extruded polystyrene (XPS)
• extruded polystyrene, with cementitious surface.

Waterproofing

Waterproofing should be one of the following:

 

Concrete deck and screed

Concrete roof deck, suitably primed, with sand/cement screed topping to achieve the falls. The screed should be in accordance with Clause 7.1-S7(a).

Detailing

Typical details are shown in Appendix 7.1-C.

Note:

Inverted roofs should only be used with timber (solid or I-joist) or metal profiled decks if they have been designed to support the loads, particularly from the depth of ballast needed to retain the insulation material.

Green Roof

A green roof, either intensive or extensive, should be a complete system from the membrane manufacturer and not individual components or materials. The details given below are intended to be a guide and may vary depending on the individual manufacturers system.

The following identifies the two types of green roof:

Intensive Green Roof

Extensive Green Roof

Appendix 7.1-B

Surface treatments

	Maintenance only for roofs up to 10°	Access roof, walkway or terrace deck	Further information may be obtained from
Reinforced Bitumen Membranes (RBM)	mineral surfaced capsheets (e.g. Type S5P5) or reflective stone chippings, bedded in a dressing compound, or a suitable thickness of washed, rounded 20 - 40mm shingle ballast laid loose	pre-cast semi porous concrete tiles bedded in bitumen or other approved adhesive, or pre-cast concrete proprietary paving slabs on supports or sand/cement blinding2, or proprietary timber decking systems3	Flat Roofing Alliance http://www.fra.org.uk/
Mastic Asphalt	reflective stone chippings1, bedded in a bitumen based compound, or a solar reflective paint, as approved by the Mastic Aphalt Council	pre-cast semi porous concrete tiles bedded in bitumen or other approved adhesive, or pre-cast concrete proprietary paving slabs on supports or sand/cement blinding2	Mastic Asphalt Council http://www.masticasphaltcouncil.co.uk/
Thermoplastic Single Ply Membranes	products do not require supplementary solar reflective coatings or other finishes where laid loose, membranes can be ballasted with suitable thickness of washed, rounded 20-40mm shingle ballast laid on a non-woven polymeric protection layer	proprietary flexible, non-slip walkway sheets or tiles, compatible with the membrane product pre-cast concrete proprietary paving slabs on adjustable supports or suitable non-woven polymeric protection layer proprietary timber decking systems with bearers set on additional membrane or suitable non-woven polymeric protection layer	Single Ply Roofing Association http://www.spra.co.uk/

Notes

1. Loose surface finishes should be prevented from being removed by weather and discharged into gutters and drain pipes. Chippings should be not less than 12.5mm limestone or white spar, not pea gravel.
2. Cement/sand blinding should be laid on two layers of waterproof building paper or two layers of 1000 gauge polyethylene separating membrane. The slabs should be kept back 75mm at perimeters and a 25mm movement gap incorporated for every 9m² of paving laid.
3. Timber decking systems should only use compatible preservative treatments. The undersides of the bearers should have large, smooth contact areas, with no sharp edges or corners.

Appendix 7.1-C

Construction details for flat roofs and balconies

This Appendix contains common details for flat roofs and balconies. The following sketches show examples of typical common construction details and illustrate general principles. Further information on specific waterproofing systems may be obtained from BS 8217 'Reinforced bitumen membranes for roofing - Code of Practice', the Flat Roofing Association, Mastic Asphalt Council or Single Ply Roofing Association.

CONCRETE DECK

Upstand

• upstand may be fixed to wall
• upstand to be at least 150mm high
• similar details apply to inverted roofs with concrete decks 
  
  
  

Handrail fixing

• upstand should be formed in concrete roofs

Skirting to rooflight or ventilator kerb

• similar details apply to inverted roofs. Allow for thickness of ballast to achieve upstand dimension.
   
  
  
• Twin-kerb expansion joint expansion joint is similar for both warm and inverted concrete roofs.
  

TIMBER DECK

Mansard edge

• All elements should be firmly fixed to prevent peelback in high winds

Pitched roof abutment

 

Independent skirting detail

• upstands should be kept separate from wall - allow for movement
• upstand should be at least 150mm high
  
• similar details apply to cold deck timber roofs

Verge detail

• similar details apply to inverted deck

Welted drip to external gutter

• similar details apply to cold deck timber roofs
• inverted timber decks need special consideration to avoid insulation being lifted by wind suction. An alternative detail should be used.

Pipe passing through roof

• vapour control layer should be bonded to waterproofing
• detailing of upstand and flashing is similar for all roofs

Upstand to ventilator or rooflight kerb

• similar details apply to cold and inverted roofs. The thickness of ballast in inverted roofs,  to achieve upstand dimensions, should be allowed for.

Rainwater outlet

• the opening should be properly trimmed
• the outlet should be at the 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.

Appendix 7.1-D

Balcony access,weatherproofing and drainage

This Appendix contains guidance on the principles to adopt for the weatherproofing and drainage of balconies which may then have an accessible threshold. Depending on the design, specific fire, thermal and acoustic issues may also need to be taken into account.

The design and construction of accessible balcony thresholds should incorporate all of the following: 

• a door threshold with an upstand of not more than 15mm

The 15mm threshold upstand is measured at the door position. Additional sloping transition elements, such as a small internal ramp and external sill, may be provided either side of the upstand. The maximum slope on ramps and sills should be 15 degrees.

• a door threshold with a minimum 45mm projecting sill and drip.

The sill should have a minimum 45mm overhang and drip to shed rainwater away from the interface between the waterproofing layer and the sill and to avoid reliance on exposed joint sealants and their limited design life.

• a balcony upstand of minimum 75mm below the underside of the threshold.

The balcony upstand is measured from the balcony drainage layer to the underside of the projecting sill. Note: the drainage layer may not be the waterproofing layer. For example, with an inverted roof the drainage layer would be the top of the insulation and not the waterproofing layer below. This is because drainage between the insulation and waterproofing layer could become silted resulting in the majority of rainwater flowing over the top of the insulation.

• a waterproofing layer designed to prevent ponding and associated stagnant water.

Finished falls should be a minimum 1:80 away from the building to the rainwater outlet(s). Where balconies are designed with falls toward or parallel to the building care must be taken to ensure any blockage of the outlet(s) cannot cause flooding into the building. Waterproofing layers at zero falls will only be accepted if the waterproofing membrane has a third party assessment specifically for that use. The membrane should also be fully protected from direct trafficking, for example by provision of paving slabs or decking, and be UV resistant unless fully protected from daylight. The membrane should be capable of withstanding any point loads from supports to decking or paving.

• an effective drainage system and suitable overflow

The drainage arrangement should ensure that if an outlet or downpipe becomes blocked it will not lead to flooding into the building. This can be achieved by using one outlet and an overflow (not less than the capacity of the outlet) or two outlets connected to independent downpipes. Alternatively, the balcony kerb can be set a minimum 25mm below the level of the door threshold to allow safe spillage in the event of water build up. An outlet chute through perimeter construction into external hoppers can also act as the overflow if it is of an appropriate size to serve both the discharge and overflow capacities.

• drainage gaps between any decking or paving and at balcony perimeters

Drainage gaps between individual lengths of decking or between each paving slab should be a minimum of 10mm. A similar continuous drainage gap should be provided between decking or paving and the threshold sill, perimeter walls and kerbs. Spacers and supports to raised decking or paving should not obstruct the flow of rainwater to outlet(s). The position of drainage outlets beneath decking or paving should be clearly identifiable and accessible for maintenance.

• a minimum 150mm high splash zone above the decking or paving

The design of the wall for minimum 150mm above decking or paving should ensure that any splashing off the decking or paving does not reach any part of the wall that could be adversley affected by the presence of moisture. This may be achieved by the use of an impervious wall finish/cladding or an extension of the balcony waterproofing layer to form an upstand with cover flashings and cavity trays if required.

 Inverted balcony

 

     Warm deck balcony

Cantilevered balcony