When people discuss flat roofing, most of the talk usually centers around the membrane, the resin, the kit, the main part. The trims and edges don't get much attention, and yet, those are the details that decide whether a GRP roof actually holds up properly over time, or starts letting water in where it shouldn't. This post covers three trims that come up regularly on fibreglass roofing jobs: the D260/D300 Heavy Duty Wall Flashing Trim, the B300 Large Raised Edge, and external corners for GRP roofing. We'll keep it practical, what each one does, where it goes, and why it matters. Why Roof Edge Trims Aren't Just a Finishing Touch It's easy to think of trims as the last step, something you slap on once the resin has gone off and the topcoat is done. But they're structural in purpose. Roof edge trims have three main jobs: they mark the edge of the roofing material, help direct water off the roof, and protect the GRP laminate edge from damage or separation. Building Regulations Approved Document C say that flat roofs need to be designed to stop water from getting in, and having the right edge finish is an important part of this requirement. In case you miss or rush the trims, you may leave a gap where leaks may occur. The trims mentioned here are all of hard GRP (Glass Reinforced Plastic). This means they fit well with GRP roofing systems and stick together properly when layered. They won't corrode, they're UV stable, and they'll outlast most alternatives. You can see the full range of fibreglass roofing trims here. D260 / D300 Heavy Duty Wall Flashing Trim What It's For The D260/D300 is the trim you need where your flat roof meets a vertical wall, like a parapet, house wall, dormer cheek, or an upstand around a rooflight. It is a significant point on a flat roof since it is where water may accumulate and pour through in case it is not covered well. The "D" in the name refers to the profile shape, it's designed with an upstand section that goes up the wall and a horizontal flange that laminates into the roof surface. The D260 has a 260mm overall width and the D300 extends that further, giving you a larger flange for roofs where there's more movement or where the GRP needs to run further up the wall. Why It's Heavy Duty Not all wall flashing trims are equal. Thinner, cheaper options can flex and crack at the junction point, which is the exact place you cannot have a crack. The D260/D300 Heavy Duty Wall Flashing Trim is made from 600gsm GRP, the same weight used in heavy-duty roofing kits. That rigidity matters. It holds its shape when laminated and gives the GRP membrane a solid, stable edge to bond to. Each length is 3 metres, which means fewer joins on longer runs, fewer joins means fewer potential weak points. Installation Notes The upstand section should go against the wall first, fixed mechanically if required, before the GRP laminate is applied over the horizontal flange. The flange gets laminated in during the base coat stage, not stuck on afterwards. Top of the upstand should be sealed with a GRP topcoat or a compatible flashing detail. If you're working around a corner, you'll need an external or internal corner piece to avoid cutting and bending the trim, more on that below. For installation guidance on fibreglass roofing, the Apex fibreglass roof video tutorials walk through the full process step by step. B300 Heavy Duty Raised Edge Trim What It's For The B300 is a raised edge trim, it goes around the perimeter of the flat roof at the eaves or at any edge that doesn't meet a wall. Its job is to create a contained boundary for the GRP membrane, stopping water from running off the edge of the board uncontrolled and directing it towards a drip edge or gutter. The B300 is the largest of the B-series with a depth of 140mm.Where B230 and B260 are used on residential flat roofs of typical size, the B300 is used on roofs with a deeper fascia, parapet or where you require a heavier edge detail. Sizing It Right One question that comes up often: do you need the B300 or will a B260 do? The answer depends on the fascia depth of the roof. The trim needs to sit against the fascia board and overhang slightly so water drips clear of the board below. If your fascia is deep, a B230 or B260 won't cover it properly, that's when you reach for the B300 Heavy Duty Raised Trim. Again, 3-metre lengths, heavy-duty GRP construction. Where It Fits in the System The B300 is typically installed before the GRP laminate goes down. It's fixed to the fascia board, and the GRP membrane is then laminated up and over the horizontal flange of the trim, locking it in. The outer face of the trim gets topcoated along with the rest of the roof for a seamless finish. If you're also fitting a drip edge trim on the same roof, the A200 Medium Drip Trim or A250 Large Drip Trim can work alongside the raised edge at different sections of the roof perimeter depending on where water needs to go. External Corners for GRP Roofing Why You Need Them Corners are where most flat roof trim installations run into trouble. You can't bend a rigid GRP trim around a 90-degree external corner, not without creating stress fractures in the material. And if you try to mitre-cut two lengths of trim and butt them together, you end up with a joint right at the most exposed point on the roof: the corner. That joint will move, open up, and eventually let water in. The proper solution is a purpose-made external corner piece. These are pre-formed at 90 degrees and designed to sit at the junction of two trim lengths, giving you a watertight, rigid corner without any cutting or bending. The C3 External Corner For GRP roofing, external corners need to be compatible with the trim profile they're joining. The GRP Corners collection includes external and internal corner options that work with the standard trim profiles. If you're using the B-series raised edge trims, you'll want a matching external corner. If you're using the D-series wall flashing trim, the same principle applies, the corner piece profiles to the flashing trim and keeps the junction sealed. Don't Forget Internal Corners Too The attention is paid to external corners, however, internal corners (the 90-degree recessed junction, most often this is the corner of a parapet and a wall in two directions) should also be detailed properly.Internal corners can be handled using a fibreglass tape and more laminate, however, pre-shaped internal corners are time-saving and neater. The AT300EXT External Angle Trim is another option worth looking at for angle trims and external junctions. Bonding It All Together However you're installing these trims, the bond between the trim and the substrate matters. A flexible PU adhesive is ideal for initial fixing of trims to timber or metal before laminating in, it stays flexible, so it accommodates the minor movement that happens with thermal expansion without pulling the trim away from the board. Once trimmed out and fixed, the GRP resin and matting go over the flanges as part of the lamination process. If you're at that stage and need to order resin, the GRP Roofing Resin is available with free catalyst, and you can pick up Chopstrand Matting in 450g or 600g weights depending on which specification you're working to. For a complete job, check out the GRP Roofing Kits 600GRM, everything you need in one order. A Note on Building Regulations In England and Wales, flat roofs on extensions, garages and new builds generally fall under Approved Document L (energy efficiency) and Approved Document C (moisture resistance). This document talks about how edge detailing is connected to managing moisture. In case you are not certain whether your plans are suitable to the project requirements, it is always a good idea to enquire with your local building control or use a certified GRP roofing system that has evidence of compliance. According to research by the Building Research Establishment (BRE), poor edge and junction detailing has always been listed as one of the main causes of flat roof failures.Achieving the trims at the very beginning is, in other words, much more cost-effective than repairing water damage in the future. Wrapping Up Trims don't make the headlines, but they do a lot of the real work on a flat roof. The D260/D300 wall flashing, the B300 raised edge, and purpose-made external corners are all part of a properly detailed GRP roof, and they're all available in the fibreglass roofing trims section at Apex. If you're planning a new GRP roof or replacing an existing flat roof, take a look at the complete GRP roofing kits for a full material list, and don't skip the trims when you're ordering. Get them right the first time and the roof will look after itself for decades. Any questions, give us a call on 0800 612 7903.
