GRP roof problems: cracks, delamination and how they are repaired

Updated 23 October 2026 · SEO Dons Editorial

This guide is for owners and managers of commercial roofs finished in GRP, also called fibreglass or glass-reinforced polyester: plant decks, walkways, canopies and detail-heavy roofs. It explains the usual faults, which can be repaired locally and how, and when something else is the better answer. For specifics it leans on one manufacturer's published manual, Restec's, so read those points as examples of how a named system is handled, not rules for every product.

What is a GRP roof, and why does it crack?

LRWA describes these systems as a glass fibre reinforced polyester resin, cold applied on site, using resins formulated for roofing to give a combination of wear-resistance, flexibility, elasticity and fire performance. The design therefore leans on provision for movement. Restec's manual requires a minimum expansion gap of 25 mm at all abutments, special expansion trims on large roofs, and taping of every trim joint and trim-to-deck junction to prevent stress cracking. LRWA's design guide names upstands, movement cracks and expansion joints as the areas of concentrated stress most in need of reinforcement.

Where do cracks appear, and can they be repaired?

At board joints, trims and upstands. Restec's manual is written for GRP over an 18 mm OSB3 timber deck, so a roof on any other deck type needs the maker's specification for that deck. It says square-edge board joints should have a 3 mm gap and be locally reinforced. A crack that follows a board joint or trim line points to movement or to a detail that was not reinforced. LRWA's refurbishment guidance reads main-area cracks in a bonded coating as a sign of a substrate that has moved, such as timber decks and plywood, and cracks at details as a sign that no allowance was made for differential movement.

In the topcoat. Restec warns that the topcoat may crack if applied too thickly, and may not cure if applied too thinly, and says using too much topcoat is detrimental to the system. It also says not to fill low spots with a deep application of base or topcoat because it will crack. A roof where dips were filled with extra resin may therefore show this cracking.

Local cracks and damage can be repaired (see below). Cracking along many board joints across the roof is a different matter: it points to movement in the deck beneath, and repairing the surface alone is unlikely to stop it.

What causes delamination and flaking?

Most of what Restec's manual says about adhesion is about timing and moisture. If more than 24 hours have passed since the base resin was applied, the surface must be lightly abraded before the topcoat. It says never to apply further topcoat directly onto cured topcoat, as it will not adhere and will flake off, which matters when a tired roof is offered a fresh topcoat. Base boards must be 100% dry, and boards should be overcoated the same day or protected from the weather.

The deck matters as much as the laminate. LRWA lists multi-jointed or moisture-susceptible timber, such as non-exterior grade plywood, as unsuitable for direct treatment, and says chipboard should always be removed and replaced. Restec says a substrate found unsuitable for overdecking, such as one that is defective, decayed or structurally unsound, will need to be stripped and removed. If the laminate has lifted from the deck over a wide area, the question is the deck, not the laminate.

What do tacky patches, pinholes and a worn topcoat mean?

Cure depends on conditions and mixing. Restec's datasheet says not to begin work in wet conditions or if rain is likely, to use the system only above 5°C air and deck temperature and below 30°C air temperature, and never to add less than 1% or more than 4% catalyst to the base coat, because full cured properties will not be achieved. Its manual says temperature has a significant impact on working time and cure time. For a base coat that does not harden, it gives the cause as resin not catalysed, or insufficient catalyst for the conditions, and the remedy as removing and re-laying the affected area. For pinholes in the topcoat it points to a base layer that was not properly rolled out or was applied too thinly.

Wear is different. LRWA notes that where the topcoat is a different colour from the coat beneath, the underlying coat shows through as the topcoat wears, and that ongoing inspections should check for wear and excessive weathering. Wear is a maintenance signal; whether it has become a leak is a separate question.

How is a GRP repair done properly?

Restec's published repair method for its own system is to grind the immediate area well into the original base resin layer, wipe it with acetone, mask the surrounding area and re-lay the full system over it: base resin layer with matting, then topcoat. In other words the repair is a rebuilt piece of the same laminate, not a smear of resin or a patch of topcoat. LRWA adds that any liquid system should be applied in accordance with the manufacturer's specification and instructions, so a repair follows the manual or certificate for the system actually on the roof. The BBA certificates guide shows how to check one.

Restec describes its system as not for consumer use, to be installed only by professionals, and says its 20-year materials guarantee is available only to installers with a valid training ID number. If a guarantee is in force, ask who is entitled to repair under it before anyone starts. The liquid roof comparison sets GRP beside PMMA and polyurethane, and the liquid-applied and GRP overview covers where it is used.

When is an overlay or a different system the better answer?

Ponding. Restec's troubleshooting gives the cause as a deck not laid with sufficient falls or a structure that has deflected, and says not to attempt to level the roof by building up resin.
A failed deck or insulation. LRWA says the roof will need to be stripped.
A different system altogether. That is a design decision; see GRP, felt and single-ply compared.
A new liquid layer over old GRP. LRWA says manufacturers' guidance must be sought on compatibility with the substrate and on preparation, priming and application. The coatings versus membranes guide explains why a coating is not a membrane.
Walkways and plant decks. LRWA says different grades can be specified for different foot traffic, and that walkways should not cross areas subject to temporary ponding.

What should an owner ask before agreeing a repair?

Is the fault in the topcoat, through the laminate, or in the deck beneath?
Which system and certificate is on the roof, and will the repair follow its manual?
What were the weather and deck moisture conditions when the roof was laid?
Is the roof ponding, and has anyone checked the falls?

The commercial flat roof repair guide explains what a repair should involve, and you can send roof details through the site's form to ask about a repair.

Sources

GRPROOF application manual v3.2 (facts about Restec products only), Restec Limited, version 3.2, checked 22 October 2026.
GRP Roof 1010 technical datasheet (Restec products only), Restec Limited, checked 22 October 2026.
GRPROOF 1010 system page (Restec products only), Restec Limited, checked 22 October 2026.
Generic types of liquid applied membranes, Liquid Roofing and Waterproofing Association, checked 22 October 2026.
Design guide for specifiers, Liquid Roofing and Waterproofing Association, 2020, checked 22 October 2026.
Guidance Note 2: substrates, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.
Guidance Note 4: refurbishment, Liquid Roofing and Waterproofing Association, issued 2010, checked 22 October 2026.

Common questions

Can a GRP roof be repaired in winter?

It depends on the system. Restec's datasheet says to use its system only above 5°C air and deck temperature and not in wet conditions or when rain is likely, and other makers set their own limits. Ask the contractor which limits apply and what temporary protection is proposed meanwhile.

Can a worn GRP roof simply be given another topcoat?

Not simply. Restec says further topcoat must never be applied directly to cured topcoat, as it will not adhere and will flake off. Ask the system maker for its refurbishment route for the product on the roof.

Is GRP suitable for walkways on a commercial roof?

LRWA says different grades of liquid waterproofing can be specified for different foot traffic, and that walkways for access and maintenance should not cross areas that might be subject to temporary ponding. Ask which grade is on the roof.

Who should repair a GRP roof that still has a manufacturer guarantee?

Restec says its 20-year materials guarantee is available only to installers with a valid training ID number, which suggests using the original or another trained installer. Read the guarantee for repair conditions before work starts.