Condensation in commercial flat roofs: vapour control and BS 5250
Updated 23 October 2026 · SEO Dons Editorial
What is interstitial condensation in a commercial flat roof?
Approved Document C defines it as the deposition of liquid water from a vapour, occurring within or between the layers of the building envelope. SPRA explains the mechanism: condensation in a roof occurs when moist air is cooled below its dew point, and the more moisture the air holds, the higher that dew point. Warm, humid air from inside the building moves up into the build-up, and wherever it reaches a layer colder than its dew point, water forms out of sight.
It matters in law as well as for the roof. Requirement C2 in Schedule 1 to the Building Regulations 2010 requires the roof to protect the building and its users from the harmful effects of interstitial and surface condensation, and Approved Document C says roofs should be built so that their structural and thermal performance is not adversely affected. Both apply in England, and the regional rules guide covers the other nations. Where the insulation sits is in the warm deck versus cold deck guide; this guide covers the moisture controls and the calculation.
What does a vapour control layer do, and is it a seal?
Approved Document C defines a vapour control layer as a material, usually a membrane, that substantially reduces water vapour transfer. The word is "reduces", not "stops". SPRA states that the air and vapour control layer is never totally resistant to vapour or air movement. A small quantity of vapour passes through the membrane or its joints and condenses on the cold underside of the waterproof membrane. The design aim is therefore no accumulation of condensate over a complete annual cycle of winter condensation and summer evaporation.
BSI says BS 6229:2025 aims to close gaps where designers were unaware of building physics issues such as vapour resistivity differences between vapour control layers and roofing membranes. On site, SPRA expects the layer to be sealed at side and end laps and turned up to all vertical surfaces by the thickness of the insulation. Those upstand and penetration details are where a re-roof succeeds or fails, and the warm-deck re-roof page shows where the layer sits in the build-up.
More insulation does not solve the problem by itself. SPRA says thicker insulation reduces surface condensation on ceilings but not interstitial condensation, and may increase it. An archived BRE digest on factory roofs, published in 1958, made the same point: improving insulation does not necessarily reduce condensation risks.
Why do pools, kitchens and wet processes need a higher-performance layer?
Approved Document C singles out the roofs of swimming pools and other buildings where high levels of moisture are generated, and says specialist advice should be sought. SPRA says metal foil laminate greatly increases a layer's water vapour resistance and may be required for high humidity applications, and that special procedures or attachment methods may be needed in places such as swimming pools. One manufacturer says the exceptionally high vapour resistance of its foil-faced self-adhesive layer enables use in sports halls, swimming pools, kitchens and wet industrial processes. That is the maker's description of its own product, not a recommendation.
| Building use | Condensation risk according to the source |
|---|---|
| Kitchens, swimming pools, shower rooms | SPRA: particularly high risk because of high internal humidity. |
| Laundries, some food processing plant, buildings run at an over pressure | MCRMA's archived 2004 guide: higher risk, and over pressure can drive warm moist air into the structure. |
| Classrooms and community centres heated intermittently and poorly ventilated | SPRA: also at significant risk. |
| Warehouses with background heating, offices with air management | SPRA: very low risk. |
A design is only as good as the humidity it assumed. A warehouse that becomes a food-packing unit has moved up the table, so ask for the assumed internal conditions in writing. Scotland's non-domestic handbook says buildings for factory processes with intended high humidity are generally designed by specialists.
What does a condensation risk analysis under BS 5250 and BS EN ISO 13788 show?
Approved Document C (2013 edition) says a roof will meet the requirement if designed and built in accordance with Clause 8.4 of BS 5250:2002 and BS EN ISO 13788:2002. BSI lists BS 5250:2021 as the current edition, shown as under review when checked on 22 October 2026, so ask which edition the analysis used. The 2021 edition covers the sources of moisture in buildings, how it moves and where it is deposited.
BS EN ISO 13788 gives a simplified calculation for the risk of interstitial condensation from water vapour diffusion, and for how long water trapped between two high-resistance layers takes to dry out. LRWA says its Glaser method is generally sufficient for flat roofs, and that the assessment should allow for clear sky radiative cooling in winter as described in BS 6229. SPRA adds that BS 6229 advises maximum levels of annual accumulation, and that all its insulation manufacturer members offer the calculation.
What it cannot show matters as much. BS EN ISO 13788 does not allow for changes in material properties with moisture, capillary suction, air movement through gaps or the moisture capacity of materials. SPRA calls the calculation theoretical, and LRWA says it does not excuse poor construction quality. SPRA also says a calculation should always be done when a cold deck is converted to warm deck, and Approved Document L allows a lesser U-value standard where Part C compliance needs it, as the building regulations guide explains.
What signs of trapped moisture turn up during a re-roof?
| Sign found | What the sources say |
|---|---|
| Wet insulation under the membrane | SPRA: check the moisture content and stability of existing materials before laying a new membrane, and avoid overlays to wet roofs. |
| Rusting metal deck or fixings | MCRMA: sufficient condensation could cause corrosion of metal components and wetting of thermal insulation. |
| Soft or rotted timber, chipboard or woodwool deck | LRWA: a full examination is particularly critical for timber and other degradable decks. HSE: woodwool slabs may be weakened by water damage. |
| Vapour control layer missing, torn or unsealed at upstands and penetrations | SPRA: laps sealed, layer turned up vertical surfaces, penetrations sealed. |
| Drips indoors that do not follow rain | MCRMA: condensate could run into the building, staining finishes or dripping onto equipment or processes. |
None of these confirms the cause on its own. Test cuts and a moisture survey do, as the survey guide explains, and the roof deck guide covers each deck type. SPRA lists entrapped moisture from past ingress as a disadvantage of overlaying; the overlay versus strip-and-recover guide compares the routes. Dripping metal industrial roofs are a related case, covered in why metal roofs sweat.
What should a building owner ask for?
- The internal humidity assumed for the building's use, in writing.
- A record of the existing build-up from test cuts.
- The condensation risk analysis for the proposed build-up, naming the standard and edition.
- The vapour control layer product, its certificate and the detail drawings at upstands, penetrations and the perimeter.
- A note of what changes if the building's use or heating changes.
These questions suit any warm-deck re-roof. To compare contractors, send an enquiry through the form on this site.
Sources
- Approved Document C: site preparation and resistance to contaminants and moisture, Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
- The Building Regulations 2010, Schedule 1 (requirement C2), legislation.gov.uk, checked 22 October 2026.
- Approved Document L, Volume 2 (2021 edition incorporating 2023 amendments), Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
- Approved Documents collection, Ministry of Housing, Communities and Local Government, GOV.UK, checked 22 October 2026.
- BS 5250:2021 Management of moisture in buildings. Code of practice, BSI Knowledge, checked 22 October 2026.
- BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings. Code of practice, BSI Knowledge, checked 22 October 2026.
- BS EN ISO 13788:2012 Hygrothermal performance of building components, BSI Knowledge, checked 22 October 2026.
- Single ply roofing: design guide 2020 (S1/2020), Single Ply Roofing Association (SPRA), checked 22 October 2026.
- Design guide for specifiers 2020 (liquid applied waterproofing), Liquid Roofing and Waterproofing Association (LRWA), checked 22 October 2026.
- Technical Paper 6: Profiled metal roofing design guide (2004, archived), Metal Cladding and Roofing Manufacturers Association (MCRMA), checked 22 October 2026.
- Technical handbook 2022 non-domestic, 3.15 Condensation, Scottish Government, gov.scot, checked 22 October 2026.
- Condensation and the design of factory roofs (Digest 117, archived), BRE Group, checked 22 October 2026.
- ProVap S/A vapour control layer, Rubberseal, manufacturer's own product page, checked 22 October 2026.
- Health and safety in roof work (HSG33), fifth edition, Health and Safety Executive, checked 22 October 2026.
Common questions
Can a roof with a vapour control layer still get condensation?
Yes. SPRA says the layer is never totally resistant, so a little vapour always passes. The roof stays dry when that small amount is within what the design allows and is gone again by summer, which depends on the build-up and on how well the laps, upstands and penetrations were sealed.
Does BS 6229 cover cold stores?
BSI says BS 6229:2025 does not apply to certain types of cold stores and high-temperature enclosures. SPRA notes that a cold store can be assessed in a similar way but in reverse, with the external waterproofing also controlling vapour moving in from outside.
Is a condensation risk analysis a legal requirement?
Requirement C2 is the legal requirement. The government says the Approved Documents give guidance on ways to meet the Building Regulations, and Approved Document C says a roof meets the requirement if designed to BS 5250 and BS EN ISO 13788. What evidence is needed on a given job is agreed through the compliance route.