GRP Roof Trims Explained: A200, B230, D260, C100, F300

How trims work and how they're sold

GRP trims are pre-formed fibreglass profiles fixed around the roof perimeter before laminating. The main laminate is then bandaged and lapped onto them, so trim and roof cure into one continuous waterproof shell — nothing is relying on sealant.

All the standard profiles come in 3 m lengths at around £13–16 each. They're bedded on trim adhesive (roughly one cartridge per 6 m) and pinned or stapled while it grabs, then the joints and edges are bandaged with 75 mm matting strip before the main lay-up.

The letter in the code tells you the family: A = drip trims, B = raised edge/kerb trims, C = cover flashings and corners, D = wall fillets, F = flat flashing sheet. The number is the girth (the developed width) in millimetres.

A200 drip trim — the gutter edge

The A200 goes on the lowest edge of the roof, where water discharges into the gutter. It's a simple L-profile: a 95 mm flange on the deck and a 90 mm face that drops into the gutter line, throwing water clear of the fascia.

Fit drip trims first, before the felt-side edges, so every other trim laps over the drainage path. Check your gutter line height before fixing — the face needs to finish inside the gutter, not behind it.

Every flat roof needs fall towards this edge — 1:80 minimum, ideally 1:40. A perfectly flat GRP roof will pond, and although GRP tolerates ponding far better than felt, standing water always finds the one detail you rushed.

B230 raised edge — verges and kerbs

The B230 goes on the edges where you want to keep water ON the roof — typically the two side verges of a garage or extension. It forms a low rounded kerb about 40 mm high at the edge, then a 105 mm fascia face down the outside.

The kerb steers water along the verge and down to the gutter edge instead of sheeting over the sides and staining the walls. The fascia face also covers and finishes the raw board edge.

Where a B230 meets an A200 at a corner, use the matching pre-formed corner unit rather than attempting a mitre — the corner units are moulded to marry the two different profiles and are the only reliably watertight way to make that junction.

D260 wall fillet and C100 cover flashing — where roof meets wall

Where the roof runs into brickwork, the D260 wall fillet forms the upstand: a 70 mm flange on the deck, a 45° splay, and a 120 mm upstand against the wall. The laminate carries up and over it, giving you a seamless GRP upstand instead of a mastic-and-hope joint.

The top of that upstand is then weathered with a C100 simulated-lead cover flashing, chased about 25 mm into a mortar joint above and dressed down over the GRP. It does the job of traditional lead flashing without the cost or the theft risk.

On a parapet wall the same pair applies. Building regs convention is a 150 mm minimum upstand height above the finished roof surface — the D260's geometry gets you there against a standard wall, but check your levels where a door threshold is nearby.

F300 flat flashing — slopes, mansards and felt joins

The F300 is a flat 300 mm GRP sheet for the awkward edges: dressed up under the tiles as a layboard where a pitched roof drains onto your flat roof, folded down the slope where the flat roof runs off onto a lower pitched roof, or bridging onto an adjacent felt area you're not replacing.

Unlike the rigid profiles, flat flashing is cut and folded to suit on site and fixed along one edge only, so the roofs can move independently. It's bandaged into the main laminate the same as any other trim.

If an edge doesn't obviously match A, B or D — it's meeting another roof rather than air or a wall — it's almost certainly an F300 edge.

Counting what you need

Walk the perimeter and label every edge by what it does with water: into a gutter (A200), holding water on (B230), against a wall (D260 + C100), onto another roof surface (F300). Divide each edge length by 3 m and round up per edge — offcuts from one edge rarely suit another.

Then count the corners: each external corner where two trims meet needs a corner unit, and internal corners (an L-shaped roof has one) need the internal version. Add trim adhesive at one cartridge per 6 m of perimeter and enough bandage to run every trim edge and board joint.

Or draw the roof in the kit builder, click what each edge is, and it counts the lengths, corners, adhesive and bandage for you — with the working shown per line.

Frequently asked questions