If you've ever dealt with a parapet wall and had to figure out how to drain water off the roof without it pooling against brickwork or soaking through an upstand, you already know this: standard outlets simply don't cut it. Once you see what our GRP Through-Wall Parapet Outlet does, it's easy to understand why it's one of our best sellers. What Is a Through-Wall Parapet Outlet and Why Does It Exist? A parapet wall boxes in your roof on all sides, so where does the water go? You can't run it over the edge like a standard flat roof. It needs to go through the wall. Our Through-Wall GRP Parapet Outlet does exactly that. It passes clean through the parapet so rainwater discharges on the outside of the building, away from the roof deck. And because it's made from GRP, the same material as your membrane, it laminates straight into the system with no awkward joins or weak points. Just a clean, fully bonded outlet that becomes part of the roof. Why GRP Is the Right Material for This When you're choosing a drainage outlet for a GRP flat roof, material compatibility matters more than most people realise. A GRP outlet allows you to laminate the matting and resin directly over the flange, creating a seamless, monolithic connection. Compare that with pushing a PVC or TPE fitting through your membrane and hoping the bond holds, it's not the same thing. Our GRP outlet also handles the kind of thermal movement you'd expect from a UK flat roof over the course of the year. GRP expands and contracts at a very similar rate to the laminate around it, which means the joint stays intact even through harsh winters and hot summers. Under Approved Document H of the UK Building Regulations, flat roofs with perimeter upstands must have drainage provision and ideally a secondary overflow in case of blockage. Installation: What to Expect Fitting our Through-Wall Parapet Outlet is a straightforward process when you're working with GRP. The unit is positioned through a core hole in the parapet wall, the flange sits flat against the roof deck, and you laminate over it using chopped strand matting and your roofing resin of choice, just as you would any other detail. The outlet then becomes fully integrated into the waterproof layer. If you're looking to see the process in action, our fibreglass roofing video tutorial library covers GRP application in detail and is a great reference point whether you're a first-time installer or brushing up on technique. For a full system approach, check out our GRP Roofing Kits, everything you need for the deck laminate is in one box. One thing worth noting: the outlet should be installed before you lay the topcoat, so it sits within the base laminate and gets fully sealed when you finish the deck. This is standard practice and ensures the flange bond is protected by the topcoat layer. If you're doing a refurb over an existing roof, the same principle applies, bed the outlet in, laminate over the flange, then apply your GRP topcoat to seal everything off. The Rest of Our Roofing Outlets Range The Through-Wall Parapet Outlet is the headline product in this category, but we stock a full range of drainage solutions to cover every scenario you're likely to encounter on a flat roof project. Here's a quick overview of what else is available in our Roofing Outlets & Vapour Barriers collection: TPE Through-Wall Outlet A flexible TPE alternative for through-wall drainage that works across both GRP and EPDM systems. Suitable for square and rectangular pipe configurations, this is a solid option when you need compatibility across mixed-system roofs. Shop the TPE Through-Wall Outlet for GRP / EPDM GRP Floor Drain 70mm For roofs without a parapet where water drains internally through the deck rather than through a wall. The 70mm outlet integrates directly into the GRP laminate in the same way as the through-wall version, and it's ideal for larger flat roofs where you need to shift water quickly. Shop the GRP Floor Drain 70mm GRP Soil Pipe Cuff 110mm Designed for penetrations where a soil pipe passes through the roof deck. The cuff bonds into the laminate and forms a watertight collar around the pipe, preventing ingress at one of the most commonly failed details on any flat roof. Shop the GRP Soil Pipe Cuff 110mm GRP Internal Drain Outlet A recessed internal outlet for roofs with central drainage falls. Works well with tapered insulation schemes and is particularly suited to commercial applications where the drainage has to go internally through the building structure. Shop the GRP Internal Drain Outlet Don't Forget: Vapour Barriers Matter Too Drainage is one side of the coin. The other is making sure moisture can't work its way up through the roof build-up from below. That's where vapour barriers come in, and we stock two of the best options available: the StrataShield SA ALU Self-Adhesive Vapour Barrier and the Alutrix 600 Vapour Barrier. Both are designed for use on warm deck flat roofs and help prevent interstitial condensation, a common cause of GRP delamination and premature failure on roofs where the spec hasn't been thought through properly. If you're ever unsure which products to pair together for a specific project, give us a call on 0800 612 7903 - we're happy to talk through the details and make sure you're ordering the right combination for the job. Browse our full Roofing Outlets & Vapour Barriers range or head straight to the GRP Through-Wall Parapet Outlet product page to order.
