7.1.11 Waterproofing 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:
- installation of waterproofing layer
- waterproofing systems
- surface treatments
- fire protection.
7.1.11.1 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 5°C or less, unless otherwise agreed with the manufacturer
- self-adhesive bitumen membranes should not be installed or handled below 5°C, 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 AVCL, 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 impede drainage ie, 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 50mm 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.
Note(s)
The depth of ballast required to resist flotation may exceed the minimum depths needed to meet the fire protection.
7.1.11.2 Waterproofing systems
Table 10: Waterproofing layer materials
Material | Standard | Code of practice | Other guidance |
---|---|---|---|
Reinforced bitumen membrane (RBM) | BS 8747 BS EN 13707 | BS 8217 BS 6229 | |
APP and SBS modified bitumen roofing systems | BS EN 13707 evaluation of conformity | BS 8217 BS 6229 | Standards as listed in BS EN 13707 Normative references |
Mastic asphalt | BS 6925 | BS 8218 | Mastic Asphalt Roofing TECHNICAL GUIDE Mastic Asphalt Council (MAC) |
Plastic and rubber sheets | BS EN 13956 | SPRA S01-2020 SINGLE PLY ROOFING DESIGN GUIDE 2020 Single Ply Roofing Association | |
Liquid applied waterproofing (hot or cold applied) | LRWA DESIGN GUIDE issue 1 Liquid Applied Waterproofing Systems for Roofs and Balconies 2020 ETAG 005 Liquid applied roof waterproofing kits Part 1: General GUIDELINE FOR EUROPEAN TECHNICAL APPROVAL Rev. March 2004 |
Reinforced bitumen membranes (RBM) 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. Definitions and characteristics and reinforced with polyester reinforcement. SBS (elastomeric) modified bitumen membranes offer increased extensibility and flexibility, especially at low temperatures. APP (plastomeric) modified bitumen membrane products offer resistance to UV degradation, have high softening points and 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 Safe2Torch guidance.
Table 11: Modified bitumen membranes used in warm roof construction
Table 12: Modified bitumen membranes used in an inverted warm roof construction
Deck | Preparation | First/preparatory layer (S2P3) | Final layer/cap sheet(2) |
---|---|---|---|
Concrete or screed | Smooth surface and prime | Applied in accordance with the membrance manufacturer's instructions | Applied in accordance with the membrane manufacturer’s instructions |
Note(s)
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, eg rock fibre, perlite or foam insulation boards with a torch receivable facing.
2. Classification by performance should be in accordance with BS 8747, eg 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 ie, 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.
Waterproofing systems
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 upstands to walls on 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, 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 Design Guide
- assessed in accordance with Technical Requirement R3.
Liquid applied membranes
Liquid applied membranes, including hot and cold applied systems, should be:
- selected and applied in accordance with the LRWA DESIGN GUIDE issue 1 Liquid Applied Waterproofing Systems for Roofs and Balconies 2020
- 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 and 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 and technical guidance documents published by the Metal Cladding & Roofing Manufacturers Association (MCRMA)
- to avoid damage from condensation
- to follow the supply manufacturer’s recommendations (including design for thermal movement and wind uplift)
- for roofs with maintenance access only.
Table 13: Fully supported flat sheet hard metal roof coverings
Metal type | British Standard |
---|---|
Aluminium (Al) | BS EN 485-1 |
Copper (Cu) | BS EN 1172 |
Galvanised steel (Gst) | BS EN 10346 |
Stainless steel (Sst) | BS EN ISO 9445 Parts 1 and 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-off 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 surface of metal roof coverings. This trapped moisture will increase the risk of corrosion developing in key parts of the construction containing metals such as Zinc, Aluminium and galvanised steel. Prevention of interstitial condensation should be achieved by a ventilated design for either of the cold or warm roof types — with each having 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.
Note(s)
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 ie, 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.
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 ultimate guide to best practice as published by the Lead Contractors Association or other manufacturers recommendations
- installed to include allowance for thermal movement and uplift
- treated with patination oil directly after installation where rainwater run-off may cause staining to adjoining surfaces.
Table 14: Lead materials for roofing
Metal type | British Standard | Code of practice |
---|---|---|
Lead (rolled) | BS EN 12588 | BS 6915 |
Lead (machine cast) | BS EN 14783 |
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 three 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, eg EPDM or foil-faced bitumen membranes, may be checked by visual 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.
7.1.11.3 Surface treatments
Surface treatments should be in accordance with Table 15.
Table 15: Surface treatments for flat roofs
Note(s)
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 must meet the fire protection requirements for the overall roof construction and should be securely fastened to a loadbearing support structure.
7.1.11.4 Fire protection
The surface protection provided to the waterproofing layer must satisfy the fire protection requirements as set out in the 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. Sustainable roof types, including Green Roofs and Brown Roofs, should be designed for resistance to fire spread in accordance with guidance in The Green Roof Organisation — GRO fire risk guidance document and in full compliance with Building Regulations.
Last updated: 2nd January 2025