Flat roofs, terraces and balconies

7.1.12Waterproofing layer and surface treatments

Flat roofs, and balconies forming roofs, shall adequately resist the passage of water to the inside of the building. Issues to be considered include:

  1. Installation of waterproofing layer
  2. Waterproofing systems
  3. Surface treatments
  4. Fire protection.

Installation of waterproofing layer

Prior to the waterproofing layer being installed:

  • The surface should be even and dry and any nails should be punched, or screws countersunk, below the surface.
  • any penetrations for drainage, services, rooflights etc, should be formed before the waterproofing layer is applied.
  • The manufacturer’s recommendations for the preparation, including priming upstands, roof outlets, etc. should be followed to achieve a satisfactory bond with the waterproofing layer
  • The manufacturer’s recommendations for conditioning, and unrolling membranes in advance of laying, should be followed.

Environmental conditions should be suitable for installing the waterproofing layer. Issues to be considered include the following:

  • membranes should not be installed or handled when the product temperature and the air temperature is 5C or less unless otherwise agreed with the manufacturer.
  • self-adhesive bitumen membranes should not be installed or handled below 5C, unless otherwise permitted by the manufacturer
  • membranes should not be installed on damp or frosted surfaces or when any rain, sleet or snow is falling.

Waterproofing layer should be:

  • secured in a manner that resists the wind uplift.
  • Secured in a manner that allows expansion of metal decks.
  • installed in accordance with the design and the manufacturer’s recommendations
  • installed by a specialist roofing contractor approved by the manufacturer, where a proprietary system is used
  • installed by the same contractor who installs the air and vapour control layer, insulation and surface finish
  • checked by the contractor to ensure that the deck and insulation boards are waterproofed at the end of each day with ‘night joints’ and before inclement weather
  • installed so that membrane laps near outlets do not impeded drainage i.e. outlets should be recessed to avoid forming a raised lip with the waterproofing layer.
  • installed so that successive layers do not trap water.

Inverted roofs should:

  • not be used for slopes greater than 10 degrees
  • be laid on a deck with no back falls or ponding
  • be designed to support the loads, particularly from ballast needed to retain insulation
  • be ballasted to the depth specified in the design to resist wind uplift, flotation and provide fire protection, using minimum 40mm paving slabs or 20-40mm diameter rounded shingle ballast minimum 50 mm deep, subject to any other requirements to meet fire protection measures
  • installed with a separating layer between the waterproofing layer and inverted warm roof insulation where required by the membrane manufacturer.

Table 10: Waterproofing layer materials

MaterialStandardCode of practiceOther guidance
Reinforced bitumen membrane (RBM)BS 8747
BS EN 13707
BS 8217
BS 6229
APP & SBS modified bitumen roofing systemsIndependent Third-Party Assessment
BS EN 13707
BS 8217
BS 6229
Mastic asphaltBS 6925BS8218MAC Technical Guide
Plastic and rubber sheetsBS EN 13956SPRA Design Guide
Liquid applied waterproofing (hot or cold applied)LRWA Design Guide ETAG 005

Reinforced bitumen membranes and modified bitumen roofing systems

Bitumen membranes should be high performance to BS EN 13707 ‘Flexible sheets for waterproofing – reinforced bitumen sheets for roof waterproofing – definition and characteristics’ and reinforced with polyester reinforcement. SBS (elastomeric) modified bitumen membranes offer increased extensibility and flexibility, especially at low temperatures, whilst APP (plastomeric) modified bitumen membranes offer resistance to UV degradation and high softening points, and both provide a longer service life than traditional RBMs.

Built-up bitumen membrane roofs should be complete systems provided by the membrane manufacturer and installed in accordance with BS 8217 ‘Reinforced bitumen membranes for roofing – Code of practice’ and the membrane manufacturer’s instructions.

The installation methods should consider the safe use and application of any hot applied materials including such guidance as the NFRC’s ‘Safe to torch’.

Table 11: Modified bitumen membranes used in warm roof construction

DeckPreparationAVCL RBM Minimum (S2P3) or Modified bitumen membranesInsulationFirst /preparatory layer (S2P3)Final layer /cap sheet ² with integral surface protection ³
Concrete or screedSmooth surface, and primeApplied in accordance with the membrane manufacturer’s instructionsSee Note 1 Stitch side laps, prime crowns (or timber decking where provided)Applied in accordance
with the membrane manufacturer’s instructions
Plywood or OSB Prime deckApplied in accordance with the membrane manufacturer’s instructionsSee Note 1 Stitch side laps, prime crowns (or timber decking where provided)Applied in accordance
with the membrane manufacturer’s instructions
Profiled metal e.g. steel, aluminium Stitch side laps, prime crowns (or timber decking where provided)Applied in accordance with the membrane manufacturer’s instructionsSee Note 1 Stitch side laps, prime crowns (or timber decking where provided)Applied in accordance
with the membrane manufacturer’s instructions

Table 12: Modified bitumen membranes used in an inverted warm roof construction

DeckPreparationFirst/preparatory layer (S2P3)Final layer/cap sheet ²
ConcreteSmooth surface and primeApplied in accordance with the membrance manufacturer's instructionsApplied in accordance with the membrane manufacturer’s
instructions

Notes

Notes to Tables 11 and 12:

1     Insulation to be mechanically or adhesive fixed to deck in accordance with the design of the roof build-up. Torching on to insulation boards and timber deck is not acceptable, apart from suitable insulation boards e.g. rock fibre, perlite or foam insulation boards with a torch receivable facing.

2    Classification by performance should be in accordance with BS 8747 e.g. a minimum value of S2P3 for ballasted roofs with access for light maintenance only and minimum S5P4 for accessible roofs with paving slabs on supports. S = tensile strength and elongation, P=resistance to puncture both static and dynamic, the higher the number the better the performance.

3    The use of stone chippings is not recommended unless required to enhance fire protection. Solar reflective paint, if used, should not be used as the sole protective finish to the membrane.

Mastic asphalt

Mastic asphalt should be:

  • to BS 6925, type 988 T25, 20mm thick on the flat and installed on black sheathing felt
  • 3 x 10mm layers on horizontal surfaces and 3 layers with a total thickness of 20mm on vertical surfaces, for green roofs.

Polymer modified asphalt should be assessed in accordance with Technical Requirement R3.

Single-ply membrane

Single-ply membranes, including materials such as, PVC (polyvinyl chloride), Ethylene Propylene Diene Terpolymer (EPDM), and TPO (thermoplastic polyolefin), should be:

  • either bonded to the insulation, mechanically fixed to the deck or loose laid, and sealed and ballasted. Fixing methods should be in accordance with the manufacturer’s recommendations
  • welded at laps using either hot air or a specific solvent with or without taped seams
  • designed and installed in accordance with SPRA guide
  • assessed in accordance with Technical Requirement R3

Liquid applied membranes

Liquid applied, include a variety of cold and hot applied membranes, should be:

  • selected and applied in accordance with the Liquid Roofing and Waterproofing Association’s guidance notes.
  • applied in accordance with the manufacturer’s recommendations
  • assessed in accordance with Technical Requirement R3

Fully supported flat sheet hard metal roof

Fully supported flat sheet hard metal roof coverings should be designed and constructed:

  • to the material Standards as shown in Table 13 or assessed in accordance with Technical Requirement R3.
  • in accordance with BS EN 14783 ‘Fully supported metal sheet and strip roofing, external cladding and internal lining. Product specification and requirements’.
  • in accordance with the relevant Codes of Practice, the FTMRC ‘UK guide to good practice’ and the supply manufacturer’s recommendations. Including allowance for thermal movement and wind uplift.
  • to avoid damage from condensation
  • with maintenance access only.

Table 13: Fully supported flat sheet hard metal roof coverings

Metal typeMaterial Standard
Aluminium (Al)BS EN 485-1
Copper (Cu)BS EN 1172
Galvanised Steel (Gst)BS EN 10346
Stainless Steel (SS)BS EN ISO 9445 parts 1-2
Titanium Zinc (Zn)BS EN 988

Direct contact between dissimilar metal roof coverings and metal fixings can cause corrosion, similarly, rainwater from dissimilar metal roofs should not discharge on to one another. Acid run-offs from timber cladding, contact with fresh cement mortar, and acidic cleaners, can also affect metal roofing particularly Zinc, and in marine environments metal roofing can suffer unsightly salt deposits. Various coatings have been developed to protect against corrosive environments and unsightly deposits. Advice on the above aspects should be sought from the metal roofing manufacturer when considering the roof design.

It is essential that interstitial condensation does not build up on the underside of the metal roof coverings which can lead
to detrimental wetting and potential corrosion of the metal or decay of the deck; Zinc, galvanised steel, and aluminium are particularly at risk of corrosion. This should be achieved by designing the roof as a ‘ventilated cold’ or ‘ventilated warm roof’ incorporating a ventilated void beneath the deck supporting the metal covering.

Ventilation needs to be effective in accordance with BS 5250 and BS 6229 with minimum 50mm ventilated void with continuous 25mm edge ventilation gaps and a maximum 5m between ventilation points in the direction of the joists. With verge to verge cross ventilation, and ventilation in the direction of the joists and 5-10m between ventilation points, a ventilation gap of minimum 100mm and continuous edge ventilation gaps of 60mm is recommended. Intermediate mushroom ventilators are ineffective in reducing these ventilation distances.

Notes

Ventilation gaps should have mesh or grilles (nominal 4mm openings) to stop the entry of large insects and birds.

Ventilation also helps form a protective patina to the underside of aluminium, galvanised steel and zinc. Timber sarking boards with 3-5mm gaps between are the preferred deck option, if plywood is used it should have 3-5mm gaps between boards. The metal should be laid on a structural underlay i.e. a breathable underlay with outer layer of bonded polypropylene mesh, which assists air and moisture movement all in accordance with the metal roof manufacturer recommendations.

Ventilated cold roof design

Ventilated warm roof design

Fully supported lead roof

Fully supported lead roofing should be:

  • laid loose on building paper to BS 1521 Class A for plywood deck or smooth concrete/screed deck, or polyester geotextile felt for timber boards (fitted with penny width air gaps)
  • installed as a ventilated cold or ventilated warm roof system.
  • installed in accordance with BS 6915, The Lead Contractors Association’s, ‘The ultimate guide to best practice’ or manufacturer’s recommendations. Including allowance for thermal movement and wind uplift.
  • Designed with maintenance access only.
  • Treated with patination oil directly after installation where rainwater run-off may cause staining to adjoining surfaces.

Table 14:

Metal typeMaterial StandardCode of Practice
Lead (rolled)BS EN 12588BS 6915
Lead (machine cast) Third party assessment

Integrity testing of waterproofing layer

The waterproofing layer should be inspected for defects after installation. Any defects are to be repaired and retested and left in a satisfactory condition.

Waterproofing layers on flat roofs, terraces and balconies greater than 50m2, or roofs which are difficult to access (such as on buildings over 3 storeys), should be subject to visual inspection and an appropriate integrity test, undertaken by a suitably qualified surveyor.

Waterproofing layers under 50m2 or those unsuitable for electronic testing e.g. EPDM or foil-faced bitumen membranes, may be checked by visually inspection which should include inspection of any seams with suitable probes.

Guidance on electronic test methods and their application can be found on the ‘Roofing And Waterproofing Test Association’ website.

A test report containing the test results and photographic record of the roof should be made available to NHBC.

Surface treatments

Surface treatments should be in accordance with Table 15

Table 15: Surface treatments for flat roofs

MaterialAccess for maintenance onlyAccess roof, walkway or terrace
Reinforced bitumen membranes■ Mineral surfaced cap sheets (e.g. type S5P5).
■ Reflective stone chippings ¹, bedded in a bitumen-based compound
■ A minimum thickness of 50mm washed, rounded 20-40mm shingle ballast laid loose
■ Precast semi-porous concrete tiles bedded in bitumen or approved adhesive.
■ Precast concrete proprietary paving slabs on supports, or sand/cement blinding ².
■ Proprietary decking systems ³
Single-ply membranes■ Supplementary solar reflective coatings or other finishes not required.
■ Where laid loose, membranes can be ballasted with a 50mm minimum thickness of washed, rounded 20-40mm shingle ballast installed on a non-woven polymeric protection layer.
■ Proprietary flexible, non-slip walkway sheets or tiles, compatible with the membrane product.
■ Precast concrete proprietary paving slabs on
adjustable supports or suitable non-woven
polymeric protection layer.
■ Proprietary decking systems with bearers set on
an additional membrane or suitable non-woven polymeric protection layer ³
Cold applied liquid roofing membranes■ Products generally do not require supplementary solar reflective coatings or other finishes.■ Proprietary surface treatments compatible with the membrane product.
■ Proprietary non-slip walkway tiles compatible with the membrane product.
■ Precast concrete proprietary paving slabs on supports on a suitable non-woven polymeric protection layer.
■ Proprietary decking systems with bearers set on additional pads on suitable non-woven polymeric protection layer/filter layer ³.
Hot melt rubberised bitumen systems■ Use in inverted/buried membrane applications or in roof garden/green roofs.
■ Must be protected with a substantial reinforced bitumen membrane protection sheet or protection board.
■ All upstands/details where the membrane becomes exposed need a protective membrane to be applied to prevent UV degradation.
■ Use in inverted/buried membrane applications or in roof garden/green roofs.
■ Must be protected with a substantial reinforced bitumen membrane protection sheet or protection board.
■ All upstands/details where the membrane becomes exposed need a protective membrane to be applied to prevent UV degradation.
Mastic asphalt■ Reflective stone chippings ¹, bedded in a bitumen-based compound
■ Solar reflective paint in accordance with BS 8218.
■ Precast semi-porous concrete tiles bedded in bitumen or approved adhesive.
■ Precast concrete proprietary paving slabs on supports or sand/cement blinding ².

Notes

1     Loose surface finishes should be prevented from being removed by weather and discharged into gutters and drainpipes. Chippings should be a minimum of 12.5mm limestone or white spar, not pea gravel.

2     Cement/sand blinding should be installed on two layers of 1000 gauge polyethylene separating membrane.

3     Decking systems should meet the required fire protection requirements for the overall roof system and should not float in the event of flooding.

Fire protection

The surface protection provided to the waterproofing layer must satisfy the fire protection requirements as set out in the National building regulations.

Account must be taken of the waterproofing detail at abutments with a building and the extent that the waterproofing can be dressed up the wall of the building and the jointing detail between the roof waterproofing and the cavity trays/dpc, this may include the use of non-combustible trays, in the external wall of the building.

Extensive green roof systems should include non-combustible perimeter abutment strips to buildings, roof lights etc and at regular intervals across the roof in accordance with the guidance in the GRO fire risk guidance document and National building regulations.