Fibreglass Roof Cleaning and Maintenance

How a fibreglass roof is built

A GRP roof is made from glass-fibre matting set in resin over a timber deck, finished with a pigmented topcoat. Edge trims are bonded around the perimeter and into gutters. The topcoat is the layer that faces the weather, so most maintenance is about protecting it.

What builds up on a GRP roof

Leaves, seed heads, grit and dust settle on flat roofs because there is little slope to carry them away. Where water sits, algae and moss can form a green film. Debris also collects against upstands and in the corner where the roof meets the wall.

Safe cleaning methods

Remove loose debris by hand or with a soft brush. A gentle solution applied at low pressure lifts green growth without abrading the surface. Avoid stiff wire brushes, scouring pads and solvents, all of which can dull or scratch the topcoat.

High-pressure washing is a poor idea on GRP too. It can lift edge trims, force water under flashings and erode worn topcoats.

Ponding water

Small puddles after rain are common on flat roofs. Larger areas that stay wet for days usually mean the deck has dipped or the fall towards the gutter is not right. Ponding encourages algae and puts extra weight on the roof.

Cleaning will not change the fall, but keeping the outlet and gutter clear stops ponding getting worse.

Checking edges and trims

Most leaks on fibreglass roofs start at edges, upstands and around rooflights rather than in the middle. While the roof is clean, look for cracks along trims, gaps where the roof meets a wall and crazing in the topcoat.

What not to do

Do not stand heavy ladders directly on the surface without protection, drag furniture across it or use it as a balcony unless it was designed for that. Do not paint over cracks with whatever is in the shed; a proper repair uses compatible resin.

Rooflights and lanterns on GRP roofs

Many fibreglass roofs carry rooflights or glass lanterns. The upstands around them are common places for debris to gather and for leaks to start. While the roof is being cleaned, the frame edges and seals around them can be cleared and looked at. Avoid harsh cleaners on the seals, which can perish them.

Fibreglass compared with felt and rubber

Felt roofs have a mineral or granular surface that can shed granules into gutters, and blisters can form as they age. Rubber (EPDM) membranes are smooth and flexible, and need care around seams. Fibreglass is rigid and shiny. Each is cleaned gently, but the weak points differ, so it helps to know which you have. Our flat roof cleaning page covers flat roofs more generally.

Outlets and gutters on flat roofs

Flat roofs drain through outlets in the surface or over the edge into a gutter. Both block easily with leaves and grit because water moves slowly across a flat surface. A blocked outlet is the most common reason for deep ponding on a GRP roof, and clearing it is often the single most useful maintenance task.

Moss on fibreglass

Moss rarely roots deeply into a sound topcoat, but it grows readily in debris and along trims and upstands. Removing it by hand and treating the surface gently keeps the finish clean without scratching.

Timing

Clear debris after autumn and check edges after winter.

Get help with your flat roof

We clean flat roofs including fibreglass finishes and will tell you if we see a problem needing a specialist repair. Quotes are free across Hampshire, Surrey and Sussex.