CSCS Roof Felting cheat sheet

Work at height hierarchy: avoid work at height, prevent falls using collective measures, then mitigate residual risk with personal protective equipment.

The Work at Height Regulations require a risk assessment and a written plan for every roof felting job; rescue and emergency arrangements must be planned before work starts.

Hot-works control: gas torch work needs a permit, a fire watch during work and a minimum 60-minute post-work fire watch, with two suitable extinguishers on the roof.

Liquid petroleum gas cylinders must be stored upright, secured, away from ignition sources and never inside a building or below ground level.

Bitumen fume is a respiratory and skin sensitiser; local exhaust ventilation, cooler torch settings and correct RPE reduce exposure.

Welfare facilities on site: drinking water, washing facilities, sanitary conveniences and a means of drying wet clothing must be provided.

A 2-metre fall can cause fatal head injury; edge protection or a fall-arrest system is required where a person could fall 2 m or more.

Manual handling: two-person lifts for rolls over 20 kg; rolls should be carried on a barrow or hoist, not shoulder-carried up ladders.

Membrane types: BS EN 13707 reinforced bitumen membranes; torch-on APP (plasticised) and SBS (elastomeric) are the two main polymer-modified bitumen classes.

APP membranes resist high temperatures and UV better than SBS; SBS is more flexible at low temperature and better for cold or movement-prone roofs.

Membrane designation uses a letter code: e.g. a 4 mm SBS torch-on cap sheet is often specified as a two-layer system with a base sheet beneath.

Mineral-surfaced cap sheets provide UV resistance and slip resistance; sanded or foil finishes are used for underlays or reflective detail work.

Vapour control layers (VCL) are specified where warm-roof construction requires control of interstitial condensation; a typical VCL is a bitumen or polyethylene membrane.

Insulation for warm roofs: PIR or PUR boards typically laid over the deck, with a separate VCL below and membrane above.

Fasteners and accessories: membrane-lined edge trims, drip trims, upstand termination bars, outlet and penetration collars, and adhesive or torch-bonded laps.

Lap widths: side laps typically 80–100 mm and end laps 100–150 mm for torch-on reinforced membranes, unless the manufacturer states otherwise.

Substrate moisture content must be checked before felting; trapped moisture causes blistering and premature failure of torch-on membranes.

Timber decking: boards should be close-butted, fixed to joists, free from deflection and have a maximum gap per specification (commonly 3–6 mm).

Concrete decks must be dry, sound and free of laitance; a primer or bonding agent is applied before the first layer.

Metal decking requires a suitable deck profile and a separating or vapour-control layer before the membrane is applied.

Existing felt removal: strip back to a sound substrate unless an overlay system is specified; overlay requires a mechanically fixed or bonded carrier board.

Surface preparation includes cleaning, degreasing and removing loose aggregate; primers cure before the membrane is laid.

Fall assessment: check the finished fall (typically 1:40 to 1:80 for flat roofs) and confirm ponding risk before work starts.

Roof edge and upstand dimensions are checked against drawings before setting out the membrane layout.

Torch-on technique: heat the membrane underside and substrate together until the bitumen is molten, then roll and press to expel air and achieve a continuous 10–20 mm bleed at laps.

Lap sequence: side laps should face the direction of water flow and be laid so water runs over, not into, the lap.

Stagger end laps of adjacent rolls; end laps of successive layers must be offset to avoid aligned joints.

Two-layer systems: base sheet is laid first (often partially bonded or mechanically fixed), then the cap sheet fully bonded over it.

Mechanical fixing of the base sheet uses washers and screws at the specified density; the cap sheet is then torch-bonded over the fixings.

Membrane should be laid at right angles to the fall on flat roofs, starting at the lowest point, so water flows over laps.

Avoid over-torching: scorched or burnt-through membrane and exposed reinforcement are defects that must be cut out and replaced.

Ambient temperature and wind affect torch work; membrane should not be applied in wet conditions or below the manufacturer's minimum temperature.

Upstands: membrane should turn up a minimum of 150 mm above the finished roof surface, with a termination bar or chased-in edge fixing.

Upstand height above finished surface must also account for any insulation, screed and paving; check the drawing detail, not just the deck.

Edge details: drip trims or edge trims are fixed before the membrane is dressed over and bonded to the trim.

Rainwater outlets: outlet collars and outlets are installed before the main membrane so the membrane can be dressed into the outlet flange.

Penetrations: pipes and flues require a collar or sleeve with a minimum 150 mm upstand and a weathertight seal at the top of the collar.

Internal corners and changes of direction need a reinforcement layer or a preformed angle piece before the cap sheet is applied.

Parapet and wall abutments: use a chased-in edge or a termination bar with a sealant bead at the top edge to prevent water tracking behind the membrane.

Expansion joints and movement joints require a movement-joint detail with a compatible flexible membrane, not a rigid lap.

Core tools: gas torch with regulator and hose, seam roller, trowel, knife, chalk line, measuring tape, and a nail or screw gun for fixing.

Gas torch safety: check hoses and regulators before use, use a flashback arrestor, and never leave a lit torch unattended.

Edge protection: guardrails at 950 mm and 470 mm heights with toe boards are typical collective fall-prevention measures.

Fall-arrest systems must be anchored to a suitable anchor point and used with a rescue plan; a harness is a last line of defence, not primary protection.

Roof ladders and crawl boards: ladders must be secured, extend 1 m above the landing point, and be used at the correct 1-in-4 angle.

Fragile roof surfaces (roof lights, asbestos cement sheets) require a risk assessment and suitable crawling boards or staging.

Hot-works equipment should be inspected and PAT tested; extension leads and transformers must be kept clear of the torch and water.

Weather monitoring: stop work in high winds, heavy rain or lightning; wind can lift membrane and torch flames are a fire hazard.

Visual inspection: check for blisters, wrinkling, fishmouths, open laps, exposed reinforcement, and insufficient or excessive bleed at laps.

Lap integrity testing: probe or peel tests on sample laps confirm bonding; open laps are re-heated, pressed and re-rolled.

Water test: flood test or hose test on completed areas before handover; ponding indicates inadequate fall or a low point.

Blistering causes: trapped moisture, insufficient priming, or over-heating; repair by cutting out and patching with a lapped patch.

Wrinkling causes: membrane laid too loosely, uneven substrate, or thermal movement; repair by cutting and re-laying if structural.

Ponding water in excess of 5 mm after 24 hours of drying is a defect; the fall or outlet should be investigated.

Repair method: cut back to sound membrane, clean and prime, then apply a patch extending at least 100 mm beyond the defect, fully bonded and lapped.

Record inspection results with photographs, location references and dates; the handover pack should include guarantees and maintenance instructions.