7.1 Flat roofs and balconies
Design that follows the guidance below will be acceptable for flat roofs and balconies.
For the purposes of this Chapter:
- generally, a flat roof has a maximum slope of 10° from the horizontal; however, many flat roof systems can be used at greater pitches, in some cases up to vertical. Specifications for sloping roofs are generally the same as for flat roofs, but will require mechanical fixings to hold the materials in place
- "deck" is the structural substrate of a flat roof
- "decking" is the upper trafficked surface of a balcony (commonly a hardwood assembly laid onto the roof or substantial paving tiles bedded to the surface).
Note: Profile sheeted roofing acting as the waterproofing is outside the scope of this Chapter because it is discontinuous and not continuously supported.
Designs should be in accordance with relevant Building Regulations and other statutory requirements.
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.
Where a flat roof is to act as a roof terrace, roof garden or car parking area, appropriate provision should be made for the additional loadings. Intensive green roofs should only be used in conjunction with concrete decks.
(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, where required, should be provided at 2.0m centres (maximum) (see Sitework clause 7.1 - S5(d)).
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 joists
- 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
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 (where appropriate)
Structural design should be in accordance with one of the following:
- BS 5268
- appropriate load/span tables published by TRADA in support of Building Regulations and associated documents
- I-joists and metal web joists should be specified in accordance with the manufacturer's recommendations, but not used in situations where any part of the joist is exposed to external conditions.
Joist hangers should be the correct size for the timber joists being supported and meet with BS EN 845.
Pre-drilled vertical holding down straps should be at least one metre long, and 30mm x 2.5mm in cross section.
All mild steel straps and fixings should be protected against corrosion in accordance with Tables 1 and 14 of BS 5628-3.
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
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
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) profiled metal
The manufacturer's load/span tables for each profile should be consulted and should include the relevant applied safety factor. Profiled metals used for roof decks should have a profile with crowns of at least 50% of the profile width (not the sheet width) for bonded systems and 45% for mechanically fixed systems, in order to provide adequate support for the remainder of the roof build-up.
Construction loadings, including point loads imposed by foot traffic, storage of materials and loads imposed by following trades should be taken into account. Protection sheets, such as plywood, should be used to prevent damage if such loadings are expected.
Fixing to the structure should be in accordance with the manufacturer's instructions and BS 6399-2. Unless the manufacturer agrees otherwise, the deck should be side stitched to ensure it performs as a continuous plane layer.
The frequency of fixings should meet the manufacturer's recommendations, and be calculated to resist the wind uplift figures derived from BS 6399-2.
(e) structural steelwork
Structural steelwork should be designed in accordance with BS 5950 and Technical Requirement R5. Supporting steelwork and purlins should be square, true and free from twists or sagging.
(f) differential movement
Allowance should be made for movement in larger roofs (e.g. 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.
(g) lateral restraint
Where walls require lateral restraint, this may be provided by 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.
(h) sound transmission
Where the roof is a terrace above another dwelling, care should be taken to ensure that the design is in accordance with relevant Building Regulations.
Appendix 7.1-A shows the three flat roof constructions acceptable to NHBC with variations for timber and concrete structural support.
|warm roof||-||concrete deck|
|-||profiled metal deck|
|inverted warm roof||-||inverted concrete deck|
|*||A green roof should be a complete system from the membrane manufacturer and not individual components or materials. The details for green roofs in Appendix 7.1-A are intended to be a guide and may vary depending on the individual manufacturer's system.|
Cold roofs are not recommended, due to the difficulty of providing:
- an effective vapour control layer at ceiling level
- the required level of ventilation
- an unobstructed ventilation space above the insulation of 50mm
- ventilation at both ends of each joist void.
Deck materials suitable for the different types of roof design are given in Appendix 7.1-A.
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 "Accredited 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 construction. The insulation material for inverted roofs should be suitable for external use and to withstand any anticipated traffic.
Where insulation is mechanically fixed the fixings should be of sufficient length to ensure they have adequate penetration into the supporting structure.
Appendix 7.1-A includes typical details suitable for the following:
- built-up Reinforced Bitumen Membrane (RBM) roofing (formerly called "felt")
- mastic asphalt roofing
- single-ply roofing systems
- green roofs (intensive and extensive).
Appendix 7.1-B includes details of surface treatments.
Appendix 7.1-C includes typical details for flat roofs and balconies.
The manufacturer of the membrane system for a green roof should confirm that the overall roof design is compatible with the system. The complete green roof should be installed by a contractor trained and approved by the membrane manufacturer.
Once completed the waterproof membrane should be visually inspected and electronically tested for waterproofing integrity, faults rectified, and the roof re-tested before further layers, such as reservoir or filter layers and the subsoil and topsoil are placed. The tested membrane should be protected from damage until subsequent layers are applied.
The results of the test should be made available to NHBC.
The waterproofing for an intensive roof should be of reinforced bitumen membrane (RBM) or mastic asphalt and the design should include protection of the membrane from possible damage during maintenance of the garden e.g. from weeding/planting. A filter layer should be placed above the reservoir layer in accordance with the manufacturer's recommendations.
OTHER FLAT ROOF COVERINGS
These are given in Materials clause M1 and should be 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'.
Items to be taken into account include:
Other than the exceptions given below, all flat roofs (and balconies functioning as roofs) should be designed with a fall of not less than 1:40. A fall of 1:40 should be used for the design of flat roofs and balconies, unless a detailed analysis of the roof is carried out including overall and local deflection, to ensure the finished fall is not less than 1:80. Falls to balconies acting as flat roofs and slatted balcony decking should drain way from the dwelling.
Allowance for deflection should be made in the structural design where falls are achieved using screeds, particularly on large roofs.
Falls to balconies should be away from or parallel to the dwelling.
Where decking is to be installed above the waterproofing but less than 150mm below the sill, it should be of a type and design that prevents a build-up of standing water. A grating or channel should be used immediately adjacent to the threshold.
TAPERED INSULATION TO ACHIEVE FALLS
Drainage falls to warm-decked roofs using tapered insulation should be designed by the insulation manufacturer, with falls of not less than 1:60. They should be laid directly onto the vapour control layer, with the primary waterproofing above. Cross-falls should be achieved using mitred joints.
METAL SHEET ROOFS
Flat roofs with metal sheet roof coverings should be designed with a fall of not less than 1:30 to ensure a finished fall of not less than 1:60.
Green roofs should be designed to retain some water, to support and nurture the vegetation, and to manage the run-off, but the waterproofing should have falls of not less than 1:60 or in accordance with the system manufacturer's recommendations.
The size and number of outlets should be designed to meet the expected rainfall intensity in accordance with BS EN 12056-3. For flat roofs bounded by parapets at least two outlets (or one outlet plus an overflow) should be provided. 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. The roof design should incorporate rainwater outlets from flat roofs that are accessible for maintenance. For green roofs an accessible and visible inspection hatch should be provided at every outlet.
(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 position and height of the overflow should be such that any build-up of water will not enter the building. The capacity of the overflow should not be less than the size of the outlet (or the aggregated capacity of the outlets, if there are several outlets).
Items to be taken into account include:
(a) provision of guarding
Guarding should be designed as follows:
- the balustrading should not be easily climbed
- any glazing in the balustrading should be toughened or laminated glass or glass blocks
- balustrading should not be fixed through the waterproofing unless special precautions are taken (see Appendix 7.1-C).
(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. Reference should be made to Chapter 6.1. 'External masonry walls' clause D3(g).
Provision should be made for safe future access to flat roofs for the purposes of maintenance.
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:
- the specification for intensive or extensive green roofs
- extent and direction of falls and position of outlets
- sections through the construction indicating how the falls are formed, and means of ventilation, if required
- size, specification and position of all the roof components, including the vapour control layer, insulation and waterproofing layer
- 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 fixing methods and fixings for insulation and surfacing.
Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.