Flat Roofing Glossary

Abutment: An abutment is the junction where a flat roof meets a wall that rises above it, such as the house wall behind an extension roof. The waterproofing is turned up the wall as an upstand and covered by a flashing, and a poorly sealed abutment is one of the most common sources of leaks.
Base sheet: A base sheet, also called an underlay, is the first layer of a built-up felt roof, bonded or fixed to the deck or insulation. It provides the first waterproof layer and a sound base for the cap sheet laid on top.
BBA certificate: A BBA certificate is an assessment by the British Board of Agrément of whether a construction product is fit for its intended use, including its expected durability under stated conditions. It assesses the product, not the installation, and it is not a guarantee.
BROOF(t4): BROOF(t4) is the top external fire classification a roof build-up can achieve under test method 4, the method used in the UK. It applies to the whole tested build-up, including the deck, insulation and covering, not to a membrane on its own. It matters most where a roof is close to a boundary.
Building warrant: A building warrant is the approval issued in Scotland by the council's building standards service before certain building work can start. Like-for-like covering repairs can be exempt, but alterations and structural work usually need one.
Cap sheet: A cap sheet is the top layer of a built-up felt roof, usually with a mineral surface that protects the bitumen from sunlight and weathering. This mineral-surfaced layer is what most people mean by mineral felt.
Chippings: Chippings are small stones spread over older felt roofs, or bonded into the surface of a cap sheet, to shade the bitumen from sunlight. Where they have worn or washed away, the bare bitumen ages faster and starts to crack.
Cold roof: A cold roof is a flat roof with the insulation between or below the joists, so the deck above stays cold. It needs ventilation to reduce the risk of condensation, and industry guidance advises avoiding it where possible.
Deck: The deck is the structural layer the roof build-up sits on, such as timber boards, OSB3, plywood, concrete or metal. A soft or spongy deck is usually wet or rotten and needs replacing before a new covering goes on.
Drip trim: A drip trim is an edge trim fixed along the roof edge above the gutter so rainwater drips cleanly into the gutter. It stops water running back under the covering and down the fascia.
EPDM: EPDM is a synthetic rubber roofing membrane (ethylene propylene diene monomer), usually laid in large sheets so that smaller roofs can often be covered in one piece. With good specification and maintenance it has an indicative life of 25–50 years.
Falls: Falls are the slopes built into a flat roof so that rainwater drains towards gutters or outlets. Most roofs need a minimum finished fall of 1:80, and a design fall of 1:60 is a common starting point to allow for building tolerances and deflection.
Firring: Firring pieces are tapered timber strips fixed on top of roof joists to create the falls. Re-firring a roof is one way to correct ponding caused by poor falls.
Flashing: A flashing is a strip of material that seals the joint between a roof and a wall, chimney or other projection. A cover flashing, often lead, is tucked into a mortar joint and dressed down over the upstand so water cannot get behind it.
GRP: GRP (glass reinforced plastic, or fibreglass) is a roof covering made by laying resin and glass-fibre matting over a board deck, then finishing it with a coloured topcoat. It cures to a hard, joint-free surface with an indicative life of 20–40 years.
Hot works permit: A hot works permit is a written authorisation used on many commercial sites to control work involving naked flames or heat, such as torch-on roofing. It sets out the fire precautions, extinguishers on hand and the fire watch kept after work stops.
Inverted roof: An inverted roof has its insulation laid on top of the waterproofing, held down by ballast, paving slabs or a green roof. It protects the membrane from damage and temperature swings, and is often used on roof terraces and green roofs.
Kerb trim: A kerb trim is a raised edge trim fitted along roof edges that do not drain into a gutter. It holds water on the roof and guides it towards the drip edge or outlet, and is widely used on GRP roofs.
Laps: Laps are the overlaps where two sheets of membrane join, sealed by torching, hot-air welding, adhesive or tape depending on the system. Split or lifting laps are a common cause of leaks on felt and single-ply roofs.
Liquid-applied membrane: A liquid-applied membrane is waterproofing applied cold as a liquid, usually with a reinforcing fleece, that cures to one continuous layer. It suits awkward details and overlays on sound roofs, with an indicative life of 10–25+ years depending on the product.
Mastic asphalt: Mastic asphalt is a waterproofing material of bitumen and fine mineral aggregate, laid hot by hand, usually in two coats, most often on concrete decks. With maintenance it has an indicative life of 30–60 years.
OSB3: OSB3 is oriented strand board graded for structural use in humid conditions, commonly used as the deck on timber flat roofs. It should be fixed with gaps at board edges to allow for movement and kept dry until it is covered.
Outlet: An outlet is the fitting where rainwater leaves the roof into a downpipe, either through the deck or through a parapet. Blocked or badly positioned outlets are a common cause of ponding.
Overlay: An overlay is a new waterproofing system laid over the existing roof covering instead of stripping it off. It only works where the existing roof is sound, dry and firmly bonded, so the roof should be surveyed first.
Parapet: A parapet is a low wall that rises above the edge of a roof. The waterproofing has to be turned up and over or under its capping, and water usually leaves the roof through an outlet or scupper.
PIR: PIR (polyisocyanurate) is a rigid foam insulation board, usually foil-faced, widely used in warm flat roofs. It gives good thermal performance for its thickness, which helps meet U-value targets without raising the roof too much.
Ponding: Ponding is water that stays on a flat roof after rain because of poor falls, backfalls, a sagging deck or blocked outlets. A patch repair does not fix a fall; re-firring or tapered insulation does.
RBM: RBM stands for reinforced bitumen membrane, the modern form of roofing felt, made of bitumen with a polyester or glass-fibre reinforcement. It is laid in layers by torch, cold adhesive or self-adhesive methods, and BS 8217 is the code of practice for it.
Scupper: A scupper is an opening through a parapet or upstand that lets rainwater run off the roof into a hopper head or gutter. It needs to be kept clear, and its edges sealed carefully into the waterproofing.
Sedum: Sedum is a group of hardy, low-growing succulent plants widely used on green roofs, often supplied as pre-grown mats. A sedum roof needs a root-resistant waterproofing layer, drainage and a structure checked for the extra weight.
Single-ply: Single-ply is a roof covering laid as one layer of PVC, TPO or FPO membrane, with hot-air welded laps, and either mechanically fixed, bonded or ballasted. It is common on commercial roofs and has an indicative life of 25–35+ years.
Tapered insulation: Tapered insulation is insulation board cut to a slope, so it creates the falls on a warm roof while also insulating it. It is a common way to cure ponding without rebuilding the joists.
Thermal element: A thermal element is the building regulations term for a wall, floor or roof that separates a heated space from the outside or from an unheated space. Renovating more than half of a flat roof over heated rooms usually means upgrading its insulation where feasible; an unheated detached garage normally isn't a thermal element.
Topcoat: The topcoat is the final pigmented resin layer on a GRP roof. It protects the glass-fibre laminate beneath from sunlight and weather and gives the roof its finished colour.
Torch-on: Torch-on is the method of laying reinforced bitumen membrane by heating the underside of each sheet with a gas torch so it bonds to the layer below. Because it involves a naked flame, it needs fire precautions, and flame-free systems are used where hot works are not allowed.
U-value: A U-value measures how much heat passes through part of a building, in watts per square metre per degree of temperature difference (W/m²K); the lower the figure, the better the insulation. In England, the renovation target for a domestic roof is 0.16 W/m²K.
Upstand: An upstand is where the waterproofing turns up a wall, parapet, rooflight kerb or other detail at the edge of the roof. Upstands are usually taken at least 150 mm above the finished roof level to stop water getting over them.
Vapour control layer: A vapour control layer is a membrane laid on the warm side of the insulation to stop moist indoor air reaching it and condensing. In a warm roof it sits on the deck, beneath the insulation.
Warm roof: A warm roof has its insulation above the deck and below the waterproofing, with a vapour control layer under the insulation. The deck stays warm, which reduces condensation risk, and it is the preferred design for most flat roofs.
Wind uplift: Wind uplift is the suction that wind creates over a roof, strongest at its edges and corners. The fixing or bonding of a roof system is specified for the building's height, location and exposure so the covering stays in place.

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