Flat Roof Replacement glossary of terms & specifications

15 technical terms, material grades and Irish standards explained in plain English so you know exactly what is on a written quote.

Sorted A to Z for quick reference when comparing specifications.

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Key flat roof replacement terms explained

BS 6229:2018 (Flat Roofs with Continuously Supported Flexible Waterproof Coverings)

The primary British and Irish code of practice setting design rules for warm, cold and inverted flat roofs, minimum finished falls (1:80 after deflection), vapour control layers, and upstand heights.

Cold Flat Roof Deck

Traditional flat roof build-up where insulation sits between the timber joists below the structural deck and requires a 50mm continuous cross-ventilated void above the insulation to prevent interstitial condensation.

EPDM Single-Ply Rubber Membrane

Ethylene propylene diene monomer synthetic rubber sheet (typically 1.14mm or 1.52mm thick) cold-bonded in a single piece up to 15m wide with no torch flame, carrying a 40 to 50 year working lifespan.

Firing Piece (Tapered Furring Timber)

Wedge-shaped softwood timber strip nailed along the top of flat roof joists before the OSB3 deck is fixed, creating a positive drainage fall of 1:40 (design) so rainwater runs cleanly to the gutter or outlet.

GRP Fibreglass Wet-Lay Laminate

Cold-applied polyester resin reinforced with 450g/m2 or 600g/m2 chopped strand fibreglass mat and UV topcoat cured onto an OSB3 tongue-and-groove deck to form a rigid, joint-free waterproof shell.

Inverted Warm Flat Roof

Flat roof construction mainly used over concrete decks and roof terraces where extruded polystyrene (XPS) insulation boards sit above the waterproof membrane and are held down by washed pebble ballast or paving slabs on pedestals.

Minimum 150mm Upstand Rule

BS 6229 design standard requiring the waterproof membrane at walls, parapets, roof lanterns and door thresholds to rise at least 150mm above the finished roof surface or top of the insulation to stop splash-back and snow ingress.

OSB3 Sterling Board (18mm Tongue-and-Groove)

Load-bearing moisture-resistant oriented strand board conditioned to IS EN 300 Class 3, used as the structural substrate for GRP fibreglass, EPDM and torch-on felt flat roofs.

Parapet Box Gutter and Scupper Outlet

Internal drainage channel set along the perimeter of a parapet-walled flat roof discharging rainwater through a lead or preformed membrane scupper chute into an external rainwater hopper.

PIR Foil-Faced Rigid Insulation Board

Polyisocyanurate closed-cell thermal insulation board (thermal conductivity lambda 0.022 W/mK) fitted in 120mm to 150mm thickness over the vapour control layer in a warm flat roof to reach a 0.18 W/m2K U-value.

Ponding Water (Deflection Allowance)

Standing water left on a flat roof more than 48 hours after rain because joists have sagged or firing pieces were omitted; BS 6229 specifies designing to a 1:40 fall so the roof still achieves at least 1:80 after timber creep.

S.I. No. 338 of 2026 (Part L Thermal Upgrade Trigger)

Irish Building Regulations requirement under which replacing more than 25% of a flat roof surface triggers upgrading the roof's thermal performance toward the 0.18 W/m2K flat roof backstop U-value where feasible.

Tapered PIR Insulation Scheme

Factory-mitred PIR insulation boards cut to an inbuilt gradient (typically 1:60 or 1:40) that simultaneously insulate a warm flat roof and create positive drainage falls without carpenter-cut timber firings.

Two-Layer SBS Torch-On Bitumen Felt

Polymer-modified styrene-butadiene-styrene bitumen waterproofing comprising an elastomeric underlay and a 4.5kg/m2 mineral-surfaced cap sheet heat-fused to give a tough 20 to 25 year membrane.

Vapour Control Layer (VCL)

Continuous foil-backed bituminous or high-resistance polyethylene sheet sealed onto the structural deck beneath warm-roof PIR insulation to block warm indoor moisture from condensing inside the cold outer roof layers.

Questions people ask

How much does flat roof replacement cost in Ireland?

A kitchen extension of 25 to 40 m² costs €3,000 to €6,500, a single garage €1,800 to €5,200 and a porch or dormer €1,800 to €2,800. The cost guide breaks it down by size, material and county.

Which is better, EPDM, GRP or felt?

EPDM is one sheet with no seams on most house roofs and needs no flame. GRP is hard and can be walked on. Felt costs least and lasts least. See EPDM vs GRP vs felt.

How long does it take?

One to three days on most house flat roofs, weather allowing. See how long it takes.

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