Fibreglass Roofing

GRP stands for Glass Reinforced Polyester. This is a composite material formed by strengthening plastic with glass fibers. GRP flat roof is one of the most popular flat roofing solutions in the United Kingdom and is more commonly known as fibreglass roof. Being in use for over 60 years with soaring popularity in recent years, GRP is used in several commercial and domestic buildings and other applications from water tanks to boats. Having a lifespan of at least 30 years with easy installation and maintenance, fibreglass roofs are already making a statement.


How To Install GRP Fibreglass Roofing

 We will briefly describe how we install a GRP Fibreglass.  The steps are:

  • Installing the GRP trim
  • Preparing for lamination
  • Laminating
  • Laying down the laminate
  • Applying the topcoat

This is not a DIY guide, we are describing how we work.


Installing GRP Edge Trims

Edge trims stop the run off of water over the edges of a roof and channel it towards a drip outlet. Trims must be fixed to the decking board with nails or staples. Using Polyurethane Adhesive, bond the trims into place. General purpose mastics and Silicone sealants are not suitable adhesives for installing trims. If trims need to be joined, they should be overlapped and bonded with polyurethane adhesive then bandaged together.


Preparing For Lamination

Air temperature is an important factor and must be taken into account since planning for the laminating depends on it. With this, long runs should be done in cooler temperatures with short runs in warmer temperatures.

Resins for laminating, require the addition of a catalyst (hardener and accelerator) especially if the resin has not already been accelerated, to initiate the curing process and must be done before use. This is why air temperature is important since resin cures faster in warm temperatures with an allowance having to be made when deciding how much glass is to be laid in one mix. Note that the Glass:Resin ratio should be 1:2.5.

The glass must be pre-cut to to the desired length before the laminate is applied. All unnecessary items must be cleared off the roof. Apply the resin to the corners and bandage the trims first, ensure all the trims are fixed firmly into place. Overlap the feathered edge of the mat on top of the cut edge then lay it parallel to the drip trim.

Overlap the trim by approximately 50mm but be careful not to go over the edge. The edge must be left long for the time being. Each roll must follow the same pattern and the long ends can then be cut off to create a straight edge.



With resin on the deck, wet out both sides of a 200m square mat piece using a roller and position the mat onto the face of the adjoining trims making sure the bottom edge is on the radius of the trim. The mat should be folded around the corner, over the top of the trim itself and then back down to the deck.

Cutting the mat from the top of the trim upwards makes it easier to dress and feather.

With a paintbrush and a roller, feather the corners into place. Use the resin and bandage any joint in the trims left over. Any trims or nails that are not due to be covered can be laminated with a small piece of mat. The deck can be laminated even before the bandages and corners have cured.

Flat roof laminating

Laying Down The Laminate

Place the mat along the lowest part of the roof in such a way that when it is unrolled it will not run off-line, before carefully rolling it back. You must apply one-third of the resin to the board and two-thirds of the resin on the mat. Unroll the mat onto the resin coated board. Allow the resin to soak into the mat for a few minutes to allow it break the emulsion binder.

Run the roller across the whole area of the roof to help the binding process. In colder temperatures, it will take a little longer.

Make regular checks to see if it has consolidated, make sure no air is trapped inside the laminate. Take extra care to look out for any small holes that might lead to water penetration later.


Topcoat the laminated roof as soon as possible when the laminate can be walked on without any stickiness. This should be done within 24 hours or less. You can test the laminate to see if you can stand on to complete the topcoat by using slight finger pressure. If it is difficult to move the glass fibre within the resin then the laminate is about half cured and can withstand light foot pressure.

Like the resin, the topcoat needs to be cured with the addition of a catalyst. The topcoat should be even and not more than half a mm thick. If the top coat is over applied, there is a danger that it will crack. The fibreglass roof is complete and will take several days to fully cure. Clean occasionally with soap and warm water. DO NOT USE BLEACH or any strong alkali on the roof. The edge trims may be painted if required.

Frequently Asked Questions


Q: How Does GRP Compare To Other Flat Roof Coverings?

GRP fibreglass is a safer solution compared to the other types of roofing as the chances of an on-site injury occurring are significantly less since fibreglass flat roof systems do not require any complicated or hazardous tools. Having a reputation for their strength and durability is one of the main reasons GRP fibreglass roofs are used with additional practical features such as there being no joints or seams on the main part of the roof. This means less chance of water creeping through gaps and into the decking. A complete waterproof solution, the reason it’s commonly used on boats Topping it all is the topcoat finish which makes the roof resistant to the natural elements. Though slightly more expensive than alternative options GRP fibreglass roofing is generally considered a premium customer choice.


Q: What Are The Advantages Of Fibreglass Roofing?

Strength and durability are big reasons enough but not the only reasons. It is also a great option for lightweight and green roofs as it is considerably lighter in weight than other types of roof systems. Repairs are made incredibly easy due to the material being used with even simpler maintenance. With a wide range of colours, GRP fibreglass provides a great level of choice for property owners. If you have a specific colour scheme in mind with regards to matching with your property, it is far more likely to be possible with GRP.


Q: How Much Does A Fibreglass Roof Cost?

GRP fibreglass roofing costs will depend on the scale of the job at hand. Whether a new roof is being constructed or an existing roof is being repaired will all have to be factored with the latter sometimes being cheaper depending on the level of extent of repairs required.

Given its strength and superior durability you might think that GRP fibreglass roofing would cost a great deal more than other roofing systems. This is not the case. Yes it is more expensive than felt or EPDM rubber roofing systems but when you factor in its longevity, incredibly simple maintenance, impressive resistance to all mother nature can throw at it and beautiful aesthetic, fibreglass roofs are worth the slightly higher costs.

Matt HAuthor: Matt Holder

Matt Holder is our in-house writer. Matt has been working in the construction industry for 10 years. From working on-site to integration himself into our office.


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