7.1 Flat roofs and balconies
SCOPE
DESIGN STANDARDS
Design that follows the guidance below will be acceptable for both flat roofs and balconies.
For the purposes of this Chapter:
- a flat roof should be regarded as having a maximum slope of 10° from the horizontal
- "decking" or "deck" is the substrate of a flat roof or the upper surface of a balcony.
STATUTORY REQUIREMENTS
relevant statutory requirements
Designs should be in accordance with relevant Building Regulations and other statutory requirements.
LOADBEARING STRUCTURE
Structural design shall be undertaken in accordance with a recognised standard.
Items to be taken into account include:
(a) dead and imposed loads
Dead and imposed loads should be calculated in accordance with BS 6399.
Appropriate provision should be made where a flat roof is to act as a roof garden or car parking area.
(b) wind loads
Wind loads appropriate to the site should be calculated in accordance with BS 6399. The design should resist uplift from wind forces either by anchorage to the main structure or by being of sufficient weight to prevent lifting. Holding down straps to flat roofs should be provided at 1.2m centres (maximum) (see Materials clause 7.1 - M1, Sitework clause 7.1 - S4(d) and BS 8103).
(c) durability
Technical Requirement R3 states that the structure shall, unless specifically agreed otherwise in writing with NHBC, have a life of at least 60 years.
The use of timber in balconies should be limited to secondary elements which in turn are supported by materials other than timber.
Timber can be used in the following situations provided it has the appropriate durability, see Chapter 2.3 'Timber preservation (natural solid timber)':
- cantilevered solid timber joist balconies with a waterproof membrane above the joist
- open balcony constructions with timber decking. The decking may be supported on solid timber joists which in turn are supported by materials or components other than timber
- balustrading.
Timber should not be used for:
- gallows brackets supporting a balcony
- posts or columns supporting a balcony
- guard rails or their support.
Items to be taken into account include:
(a) timber construction (where appropriate)
Structural design should be in accordance with one of the following:
- BS 5268 and BS 8103
- appropriate load/span tables published by TRADA in conjunction with Building Regulations and associated documents
- tables produced to a recognised standard.
See Clause 7.1 - D3 for guidance on the use of timber in balcony construction.
Reference should be made to Materials clause 7.1 - M1 and Chapter 2.3 'Timber preservation (natural solid timber)' (Design) for timbers requiring preservative treatment.
(b) in-situ reinforced concrete construction
In-situ reinforced concrete construction should be designed in accordance with BS 8110 and, where appropriate, Chapter 2.1 'Concrete and its reinforcement'. A concrete mix with low shrinkage characteristics should be specified.
(c) precast concrete construction
Precast concrete construction should be designed in accordance with BS 8110.
The design of concrete elements should allow for the following:
- continuity or anti-crack reinforcement
- allowance for movement at about 15m intervals and at abutments.
(d) structural steelwork
Structural steelwork should be designed in accordance with BS 5950 and Technical Requirement R5.
(e) differential movement
Allowance should be made for movement in larger roofs (eg roofs to blocks of flats), particularly where the span of the roof deck changes, e.g. in L-shaped buildings. Joints should be continuous through the vertical upstands, walls and edges of the building. Details are shown in Appendix 7.1-C.
(f) lateral restraint
Where walls require lateral restraint, this may be provided by flat roof joists and concrete roof elements. The bearings for concrete elements and timber joists where they are built in should be at least 90mm.
Where joists or concrete beams are parallel to walls, restraint straps at 2m centres (maximum) should be provided in accordance with BS 8103.
PRINCIPLES OF DESIGN
Appendix 7.1-A shows the three flat roof constructions acceptable to NHBC with variations for timber and concrete structural support.
They are:
| warm deck | - timber - concrete |
| warm deck | - inverted timber - inverted concrete |
| cold deck | - timber. |
Note
Cold deck roofs are not acceptable in Scotland.
Cold deck roofs may only be used in the rest of the UK 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.
FLAT ROOF DECKING
Decking materials suitable for the different types of roof design are given in Appendix 7.1-A. Other materials are subject to prior acceptance by NHBC.
THERMAL INSULATION
The BRE Report "Thermal insulation: avoiding risks" discusses aspects of insulation relevant to flat roofs and balconies. In England and Wales account should be taken of Robust Details.
Thermal insulation materials suitable for the different types of roof design are given in Appendix 7.1-A together with materials for vapour control layers and their position in the structure.
WEATHERPROOFING AND SURFACE FINISHES
The roof coverings and surface finishes suitable for different types of roof design are given in Appendices 7.1-A and 7.1-B
Appendix 7.1-A includes typical details of the following:
- built-up roofing felt
- mastic asphalt finish
- details at abutments and where projections pass through the weatherproofing
- surface finishes.
Other flat roof coverings are given in Materials clause M1 and used in accordance with the following:
Zinc sheet CP143-5 Code of Practice for sheet roof and wall coverings
Copper sheet CP143-12 Code of Practice for sheet roof and wall coverings
Lead sheet BS 6915 Design and construction of fully supported lead sheet roof and wall coverings.
RAINWATER DRAINAGE
Items to be taken into account include:
(a) falls
All flat roofs (and balconies functioning as roofs) should be designed with a fall of not less than 1:40. To ensure that flat roofs and balconies have an adequate finished fall of not less than 1:80, twice this figure (1:40) should be used for design purposes unless a detailed analysis of the roof is carried out, including overall and local deflections.
Allowance for deflection should be made in the structural design where falls are achieved by screeds, particularly on large roofs.
On flat dormer roofs, the fall should be away from the intersection with the pitched roof.
Falls to balconies should be away from or be parallel to the dwelling.
Suitably drained decking may be incorporated above the waterproofing but less than 150mm below the sill.
(b) outlets
Outlets should have a recessed mouth to allow the free flow of water. Rainwater drainage design is covered in Chapter 7.2 'Pitched roofs'. Reference should be made to Chapter 5.3 'Drainage below ground', where necessary.
(c) prevention of flooding
Where a flat roof or balcony has an upstand on all sides, an overflow outlet should be provided through parapet walls or perimeter upstands to prevent a build-up of water in the event of other outlets becoming blocked. The size of the overflow should not be less than the size of the outlet (or outlets if there are several outlets).
GUARDING TO BALCONIES
Items to be taken into account include:
(a) provision of guarding
Guarding should be provided to the perimeter of all flat roofs and balconies to which persons have regular access other than for maintenance, unless the drop is 600mm or less. The minimum height of guarding should be 1100mm.
Guarding should be designed as follows:
- the balustrading should not be easily climbed
- no opening in the balustrading should be large enough for a 100mm diameter sphere to pass through
- any glazing in the balustrading should be toughened or laminated glass or glass blocks. Wired glass is not safety glass and should not be used
- balustrading should not be fixed through the weatherproofing unless special precautions are taken.
(b) stability of guarding
Parapet walls and balustrading should be designed to resist horizontal loading as required by the relevant Building Regulations or BS 6399. Particular care is needed when the design incorporates balustrading fixed to parapet walls to ensure stability and prevent overturning. End fixings or returns may be needed to ensure stability.
In balcony walls (especially long balconies) the structural stability should be checked as the dpc at the base of the wall can create a slip plane that can seriously limit the ability of the wall to resist horizontal forces. In such cases, it may be necessary to incorporate a ring beam or other support to ensure stability.
In the design of parapet walls, movement should be allowed for.
ACCESS FOR MAINTENANCE
Where the flat roof is 3 or more storeys above the ground, access should be provided from within the dwelling. In blocks of flats, access should be obtained from the common areas.
PROVISION OF INFORMATION
Clear and fully detailed drawings should be available on site to enable work to be carried out in accordance with the design. The drawings should include:
- amount and direction of falls and position of outlets
- sections through the construction indicating how the falls are formed, means of ventilation, if required, and size, specification and position of all the roof components
- all treatment and protection of materials to achieve the necessary durability
- details of construction at critical junctions
- details of balustrading and method of fixing
- details of fixings for insulation and surfacing.
Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.
