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Commercial flat roofing glossary

Updated 23 October 2026 · SEO Dons Editorial

This glossary defines 80 terms used on commercial flat roofs, in plain English. Each entry is short and links to the guide that covers the subject in depth, and the flat roof systems hub links to each system. Where a definition carries a rule or a figure, the source is listed at the end. Trade bodies named are the Single Ply Roofing Association (SPRA), the Liquid Roofing and Waterproofing Association (LRWA), the National Federation of Roofing Contractors (NFRC), the Metal Cladding and Roofing Manufacturers Association (MCRMA) and the Health and Safety Executive (HSE). Jump to a letter: A, B, C, D, E, F, G, H, I, L, M, O, P, R, S, T, U, V, W, Z.

A

Adhered system. A membrane bonded to the insulation or substrate with adhesive. SPRA names it, with mechanical fastening and ballast, as one of three main attachment options. See fixing methods.

Air and vapour control layer (AVCL). A continuous layer on the warm side of the insulation restricting air and water vapour. LRWA notes one layer often does both jobs in flat roofs. See vapour control.

Approved Document. Government guidance on ways to meet the Building Regulations. Parts B (fire), C (moisture) and L (energy) matter most to roofs. See building regulations for a replacement.

Asbestos cement. Cement sheeting reinforced with asbestos, found on some older roofs. HSE says falls through fragile materials such as asbestos cement sheets cause more roof work deaths than any other single cause. See asbestos cement roof options.

B

Ballast. Loose weight such as washed stone, paving slabs or soil and planting that holds a loose-laid membrane or insulation down. See ballasted systems.

BBA Agrément certificate. An independent certificate from the British Board of Agrément, relating to a specific product and organisation for a particular use. See BBA certificates explained.

Blue roof. A roof that holds rainwater temporarily on the waterproofing and releases it slowly. SPRA gives a maximum stored head of 100 mm and a 24-hour drain-down for single ply blue roofs. See green and blue roofs.

Box gutter. A channel formed within the roof, between slopes or behind a parapet, that carries water to an outlet. See industrial gutter lining.

BROOF(t4). The highest European classification for a roof construction's resistance to external fire, assessed under BS EN 13501-5, with FROOF(t4) the lowest. See BROOF(t4) explained.

BS 5250. BSI's code of practice for managing moisture in buildings: where it comes from, how it moves and where it is deposited. The 2021 edition is current. See condensation and BS 5250.

BS 6229. The code of practice for flat roofs with continuously supported flexible waterproof coverings, up to a 10 degree pitch. The 2025 edition supersedes the 2018 one. See falls and BS 6229.

BS 8217. The code of practice for reinforced bitumen membrane roofing: selecting membranes, combining them into roofing and applying them to flat and sloping roofs. See built-up felt roofing.

BS 8218. The code of practice for mastic asphalt roofing, including trafficked roofs such as rooftop car parks. See mastic asphalt.

Build-up. The sequence of layers from structural deck to covering, such as deck, vapour control layer, insulation and membrane. The order decides where condensation can form. See warm and cold deck build-ups.

Building Regulations. The legal requirements for building work in England. Requirement C2 covers condensation and Part L covers energy. See building control on a re-roof.

C

CDM 2015. The Construction (Design and Management) Regulations 2015, which set health and safety duties for the client, designers and contractors on a project such as a re-roof. See CDM on a re-roof.

Cold deck. A build-up with the insulation below the deck, leaving the deck substantially colder than the building in winter. See warm deck versus cold deck.

CompetentRoofer. A competent person scheme. GOV.UK lists it, with NAPIT, as authorised for replacing roof coverings, and such schemes let installers self-certify certain building work. See accreditations.

Condensation risk analysis. A calculation of whether moisture will build up in a roof over a year. BS EN ISO 13788 gives a simplified method. See the analysis explained.

Construction phase plan. The plan for managing health and safety while the work is done, drawn up under regulation 12 or 15 of CDM 2015. See the construction phase plan.

Core sample. A small cut through the roof showing its real layers, insulation, moisture and deck. LRWA calls core sampling in many cases the most effective survey method. See roof surveys.

Cut-edge corrosion. Rust that starts at the cut edge of coated metal sheets, where bare metal is exposed, and spreads under the coating. See cut-edge corrosion.

D

Deck. The structural base the build-up sits on, such as profiled metal, concrete, timber or woodwool. See roof deck types.

Deflection. How far a deck sags under load. A deflected deck can hold water even where the roof was laid to fall. See ponding and falls.

Dew point. The temperature at which air has cooled enough for its water vapour to condense into droplets. See why metal roofs sweat.

E

Electronic integrity testing. Checking a membrane for breaches by electrical means. LRWA's hot melt code (version 2.2, August 2026) confirms it as the recognised on-site method, in place of flood testing. See liquid-applied roofing.

EPDM. Ethylene propylene diene monomer, a synthetic rubber single ply membrane, normally jointed by tape bonding with a special primer, or with adhesives, says SPRA. See EPDM, TPO and PVC.

F

F10. The HSE form that notifies a construction project. A project is notifiable if scheduled to last over 30 working days with over 20 workers at once, or to exceed 500 person days. See CDM on a re-roof.

Fall. The slope a roof surface is built to so water drains to the outlets, usually written as a ratio. BS 6229:2025 made major updates to its falls clause. See falls and ponding.

Flashing. A strip of metal or membrane sealing the junction between the roof and an upstand, wall or penetration. See roof details.

Flat roof. In BS 6229, a roof with a pitch of up to 10 degrees. It is built to fall and is not level. See flat versus pitched roofs.

FPO and TPO. Flexible polyolefin and thermoplastic polyolefin, polyolefin single ply membranes. SPRA describes FPO as a heat-weldable family. See EPDM, TPO and PVC.

Fragile roof. A roof that may not bear a person's weight. HSE says treat every roof as fragile until a competent person confirms it is not. See fragile roofs and rooflights.

G

Green roof. A living build-up of growing medium and planting over a waterproofing layer. SPRA says that layer should be independently certified and root-resistant. See green and blue roofs.

GRP. Glass-reinforced plastic: glass fibre in polyester resin, applied cold on site as a liquid system. See liquid-applied and GRP roofing.

H

Hot melt. A liquid system of hot-applied polymer-modified bitumen, usually in two layers with polyester reinforcement. See liquid-applied roofing.

Hot works. Work that can ignite material, such as cutting, grinding, welding, torch-on roofing and bitumen boilers. NFRC says it can be a hazard whether or not flammable material is present. See hot works on roofs.

I

Interstitial condensation. Condensation inside the layers of a building element, usually unseen. Approved Document C defines it as liquid water deposited from vapour within or between the layers. See flat roof condensation.

Inverted roof. A build-up with the insulation above the waterproof covering, which keeps the covering near the temperature inside. See warm deck versus cold deck.

L

Lap. Where one sheet overlaps another, side to side or end to end. Laps must be sealed, welded or taped. See fixings and lap seals.

Liner sheet. The inner profiled sheet of a double skin metal roof. MCRMA says a carefully sealed liner can act as the vapour control layer. See over-sheeting.

Liquid-applied membrane. A waterproofing coat applied wet and cured in place, including GRP, PMMA, polyurethane and bitumen-based products. LRWA recommends specifying only systems with current third-party accreditation. See liquid-applied roofing.

M

Major renovation. Renovation of a building where more than 25% of the surface area of the building envelope is renovated. See building regulations.

Manufacturer's warranty. A manufacturer's promise to compensate the owner if its product or system fails. SPRA says it bears little relation to durability or service life. See warranties explained.

Mastic asphalt. A hot-applied material of graded mineral matter and asphaltic cement that forms a coherent, impermeable mass. See mastic asphalt.

Mechanical fixing. Securing a membrane or insulation with fasteners and plates through to the deck. See fixing methods.

Metal deck. Profiled galvanised steel, coated steel or aluminium sheet used as a lightweight roof deck. See roof deck types.

Moisture survey. Testing for water in a roof build-up, using methods such as thermal imaging and moisture mapping, with core samples to confirm. See roof surveys.

O

Over-sheeting. Fixing a new profiled sheet on spacers over an existing metal roof, with insulation between the two. Also called over-roofing. See over-sheeting.

Overlay. Laying a new covering over the existing roof instead of stripping it. BS 8217 applies to overlays where the existing construction is suitable. See overlay versus strip-and-recover.

P

Parapet. A low wall around the edge of a roof, which the waterproofing is dressed up and over or into. See parapets and upstands.

Penetration. Any pipe, duct, flue or support passing through the roof, each needing its own sealed detail. See roof details.

Planned preventive maintenance (PPM). Scheduled roof inspections and minor repairs that catch faults early. See planned maintenance.

PMMA. Polymethyl methacrylate: a two-component, fast-curing liquid system applied by spray, roller or other means. See PMMA, polyurethane and GRP.

Ponding. Water that stays on the roof instead of draining, usually because of inadequate falls or a deflected deck. See why flat roofs pond.

Pre-construction information (PCI). Information held or reasonably obtainable by the client that is relevant to the construction work, including health and safety hazards. See pre-construction information.

Principal contractor. The contractor appointed under CDM 2015 to perform the duties in regulations 12 to 14. See CDM on a re-roof.

Principal designer. The designer appointed under CDM 2015 to perform the duties in regulations 11 and 12. See CDM on a re-roof.

PVC-P. Plasticised PVC, a single ply membrane that can be heat or solvent welded. SPRA says it is generally unsuitable for direct contact with bitumen unless fleece-backed. See EPDM, TPO and PVC.

R

RAAC. Reinforced autoclaved aerated concrete: a lightweight, bubbly concrete used from the 1950s to the mid-1990s, often as precast roof planks. See RAAC roof panels.

Rainwater outlet. The drain that takes water off the roof to a downpipe or gutter. Its position and capacity affect ponding. See flat roof problems.

RBM. Reinforced bitumen membrane, often called felt: reinforced bitumen sheet laid in one or more layers. See felt roof repair.

Renovation. In the Building Regulations, providing a new layer in a thermal element or replacing an existing layer. See building regulations.

Rooflight. A translucent panel in the roof for daylight. HSE names fragile rooflights among the main causes of roof work death and injury. See rooflight replacement.

S

Self-adhesive membrane. A reinforced bitumen sheet with a self-adhesive underside, laid without a torch. BS 8217 covers it. See torch-on versus self-adhesive.

Single ply. A covering of factory-made polymer sheet in one layer, jointed on site. SPRA classes the polymers by cross-linking, which determines the seam method. See single ply roofing.

Spacer system. The structural components in a twin skin roof that hold the outer sheet above the liner and make room for insulation. See over-sheeting.

Standing seam. A metal roof of flat or lightly ribbed panels joined by raised interlocking seams. MCRMA says over-roofs below 4 degrees need secret-fix or standing seam sheets. See types of flat roof.

Strip and recover. Removing the existing covering and fitting a new one. SPRA says removal gives most scope to correct design faults and upgrade thermal performance. See overlay versus strip-and-recover.

T

Tapered insulation. Insulation boards cut to a slope to build falls into a warm roof. SPRA says falls can be made in the deck or with tapered insulation. See falls and ponding.

Thermal bridge. A part of the structure that conducts heat more readily than the insulation around it, so it runs colder inside and can attract condensation. See warm-deck re-roofing.

Thermal element. In the Building Regulations, a wall, floor or roof, but not windows, doors, roof windows or roof-lights. See building regulations.

Torch-on. Applying reinforced bitumen membrane with a gas torch. NFRC says it is not unsafe in itself but the specification must reflect areas of fire risk. See torch-on versus cold-applied.

U

U-value. How much heat passes through a roof, in watts per square metre per kelvin. A lower figure means better insulation. See warm-deck re-roofing.

Upstand. The vertical part of the waterproofing turned up at a roof edge, wall or kerb. See parapets and upstands.

V

Vapour control layer (VCL). A material, usually a membrane, that substantially reduces water vapour transfer through the roof. See vapour control and BS 5250.

W

Warm deck. A build-up with the principal insulation immediately below the covering, keeping the deck close to the temperature inside. See warm-deck re-roofing.

Wind uplift. The suction wind exerts on a roof. SPRA says attachment must be chosen only after calculating wind uplift to BS EN 1991-1-4. See wind exposure and uplift.

Woodwool. A timber-and-cement slab used as an older roof deck. HSE says it may fracture beneath someone's weight. See roof deck types.

Z

Zero falls. A roof laid with little or no slope. SPRA's 2020 guide, written against BS 6229:2018, defines it as falls between 0 and 1:80. See why flat roofs pond.

If a term in a quotation or specification is not here, ask the contractor to explain it and say which standard or guide it comes from. To compare contractors for a re-roof, send an enquiry through the form on this site, and read the systems hub and the types of flat roof guide first.

Sources

Common questions

Is a flat roof really flat?

No. BS 6229 treats a flat roof as one with a pitch up to 10 degrees, and a roof is built to fall so water drains to the outlets. A roof laid dead level, or one whose deck has sagged, holds water. The ponding and falls guide explains the design.

What is the difference between a standard and a certificate?

A standard such as BS 6229 or BS 8217 describes good practice for a type of work. A certificate speaks for a product. The BBA says its Agrément certificate relates to a specific product and organisation, for a particular use, so check that the certificate matches the roof.

Which terms matter most when comparing contractor quotations?

Ask each quotation to state the deck, the build-up, the covering, the falls and vapour control details, the fire classification and any hot works. If two quotations use these terms differently, ask for a definition. The CDM terms show who holds the project's health and safety duties.

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