A flat roof is a roof with a very shallow pitch - typically no more than 10 degrees (1:6 fall) and usually much shallower (1:40 to 1:80 minimum). Unlike pitched roofs that shed water by gravity between overlapping tiles, flat roofs rely on a continuous waterproofing membrane bonded to the structural deck. Common waterproofing systems include EPDM rubber (single-ply membrane), GRP fibreglass (polyester-reinforced resin), and built-up felt (two or three layers of bituminous felt torched together). Flat roofs are common on extensions, garages, and commercial buildings.

What is the difference between a warm deck and cold deck flat roof?

In a warm deck (or warm roof) flat roof, insulation is placed above the structural deck and below the waterproofing membrane. This keeps the structural deck warm and eliminates the risk of interstitial condensation within the roof structure. It is the preferred modern construction. In a cold deck flat roof, insulation is placed between the joists below the structural deck, leaving the deck cold. Cold decks require ventilated air space above the insulation to prevent condensation - a requirement that is difficult to achieve correctly and leads to frequent failures. Cold deck construction should not be used for new flat roofs. An inverted (or upside-down) roof places the insulation above the waterproofing membrane, protecting it from UV and temperature extremes.

What is the best flat roof material?

EPDM rubber (ethylene propylene diene monomer) is the most widely recommended flat roof material for domestic extensions and dormer roofs. It is available in large single sheets (minimising joints), is highly durable (20-25 year typical life), resistant to UV and temperature extremes, and relatively easy to repair. GRP fibreglass is seamless (applied as a liquid that cures in place) and very hard-wearing but less forgiving on large areas prone to movement. Traditional built-up felt has largely been superseded for new work - it has a shorter lifespan and is more prone to failure at laps and terminations. All systems require proper drip edges, outlets, and flashings to perform.

What falls are required on a flat roof?

BS 6229 (Code of practice for flat roofs) recommends a minimum design fall of 1:40 for most flat roof waterproofing systems - in practice a fall of 1:60 to 1:80 is the minimum acceptable to avoid ponding when deflection and construction tolerances are accounted for. Ponding water on a flat roof accelerates membrane degradation and increases dead load. Falls are achieved by cutting structural joists or decking to a taper, or by using tapered insulation boards (which simultaneously provide the fall and the thermal insulation in a warm deck build-up).

Roofing noun

Flat Roof

/ flat ruːf /

Also known as: warm deck roof, cold deck roof, inverted roof, low-pitch roof

Definition

A flat roof is a roof with a very shallow pitch (typically 1:40 minimum fall in practice) that relies on a continuous waterproofing membrane rather than overlapping tiles to shed water. Modern flat roofs use warm deck construction - insulation above the structural deck, below the waterproofing. Common membranes include EPDM rubber, GRP fibreglass, and built-up felt. Flat roofs are standard on domestic extensions, garage roofs, dormers, and many commercial buildings.

In practice

A modern warm deck flat roof build-up (from inside out): structural timber joists; vapour control layer (VCL) on the warm side of insulation; rigid insulation board (PIR typically) to achieve target U-value; OSB or plywood structural deck; EPDM or GRP waterproofing membrane. Tapered insulation boards can combine falls and insulation in one layer, eliminating the need to cut joists to a taper. The membrane terminates at upstands (minimum 150mm high at abutments to walls) and at a drip-edge or metal-capped fascia at the eaves.

Falls must be maintained across the whole roof - any areas of zero fall or reverse fall will pond water, which accelerates membrane breakdown and structural timber decay. Outlets and gutters must be positioned at the low points of the falls. On larger roofs, multiple drainage outlets with 100% overflow capacity are required to prevent catastrophic overloading if the primary outlet blocks. Roof lights (VELUX-style) and penetrations through the membrane for pipes and services must be properly detailed with upstands and flashings to prevent water ingress.

Building Regulations

Thermal (Part L)U-value 0.15 W/m2K for new flat roofs

Moisture (Part C)Warm deck required - cold deck construction prone to failure

DrainageBS 6229 - minimum 1:40 fall; overflow provision required

Fire (Part B)Roof covering must achieve minimum fire rating for position

Approved Document L requires new flat roofs to achieve a U-value of 0.15 W/m2K - typically requiring 140-160mm of PIR insulation (lambda 0.022 W/mK) in a warm deck build-up. Approved Document C requires roofs to resist water penetration and interstitial condensation - warm deck construction inherently satisfies this but must include a vapour control layer on the warm side. Approved Document B requires flat roof coverings to achieve at least BROOF(t4) fire classification if within 6m of a boundary, to prevent spread of fire to or from neighbouring properties.

Full Building Regulations guidance

Flat Roof

Just so you know...