Calculating the size

Size guide

Working out the size of your roof surface

Breaking it down

First of all, in order to make sure your area is square, measure three metres from the corner out along one of the roof edges. From this point measure four metres across the front edge. If the diagonal line that connects these two points is five metres long then your area is square.

When measuring large roof surfaces, make it easier by breaking it down into smaller sections for example if you have an L shaped roof break the space down into two rectangles, measuring them separately and adding them together at the end.

Measure the first of these rectangles, starting with the width, followed by the length, and repeat the process on the second rectangle. Mark these measurements in your notebook.

Calculating the area

To calculate the area multiply the length by the width, so, for example, we have a roof that's 5 metres in length x 3 metres wide then the totals square metres works out at 15m2.

Once you have finished calculating all the measurements, it’s always a good idea to round up your totals just to make sure you have more than enough materials for the GRP roof you are about to take on.

Topcoat Application

Finishing topcoat guide

How to apply the GRP Roofing finishing topcoat system


The Fibrelast finishing topcoat comes in a wide range of colours and along with making the roof aesthetically pleasing, increases the roofs strength and durability whilst also working as a barrier to prevent UV degradation.

Apply the finishing topcoat when the laminate is semi-cured, this means you can walk on the roof without any stickiness and the glass layer is impossible to move with your finger. If you find that you have run out of time for the day and you are loosing light then make sure that you cover and apply the finishing topcoat within 24 hours.

Using sanding paper or a sanding machine, gently sand the surface and corners of the laminate making sure to remove any unwieldy fibres. Next take a sharp utility knife and cut away any excess cured fibreglass matting that has extended beyond the trims, in the industry this is known as green trimming.

Open the can of Fibrelast finishing topcoat and give it a good stir before pouring any into a mixing bucket. The finishing topcoat will require the same amount of catalyst as the base resin, however if you are applying the finish at the end of the day with temperatures dropping you may need to add extra catalyst to the mix. Use an infrared thermometer to gauge the temperature of both roof surface and topcoat.

Mix the finishing topcoat in small batches of 3-5 litres and apply it to the roof using the chemical resistant topcoat rollers. Aim to achieve a thickness of around 1mm, if the finish is applied too thick then it may crack when it cures, too thin and it will not efficiently cure. Use a smaller roller or brush to coat the trims using a discard piece of trim to protect the fascia from any drips.

Resin Application

Resin application

How to apply the GRP Roofing base resin system

Resin guide

As per our fibreglassing guide we recommend that you should use either 1 layer of 600gm or 2 layers of 450gm fibreglass matting, for this example we will be using 1 layer of 600gm fibreglass matting. If you allow for 2.5 litres of Fibrelast base per square metre, this should give you an excellent pin hole free coverage.

1 layer 600gm

Use 2.5 litres per m²

2 Layers 450gm

On each layer use 2 litres of resin per m²


Laminating is a term that describes the application of catalysed resin to the fibreglass matting. Start off by mixing a small amount of resin to laminate any details like pipework and trim overlaps. Laminate the fibreglass bandage around the trim to deck joints along with the decking joints if you haven't used a T&G board.

First, using a resin application roller, generously apply a catalysed base resin mix to the OSB decking on an area no greater than 3 m2

Then place the pre-cut fibreglass reinforcement over the saturated area, applying more catalysed resin over the top of the fibreglass.

Allow the fibreglass and resin mix to activate and start breaking down the binder in the fibreglass matting before working the base resin mix through the fibreglass with a consolidating roller.

GRP Roofing Catalyst


Catalyst guide

How much catalyst to use and how to mix into the resins and topcoats.

Measuring catalyst

Catalyst is an integral part of the whole GRP roofing process and adding the right amount to the Fibrelast system is crucial. Too much catalyst and the product will start curing before you've had chance to apply it to the roof, not enough and it won't activate the resin system, which means you'll have to scrape it all back and start again.

4% Catalyst

Cold day

From 6°C to 12°C

3% Catalyst


From 13°C to 20°C

2% Catalyst

Warm day

From 21°C to 28°C

1% Catalyst

Hot day

From 29°C to 35°C

Adding the catalyst

Both base resin and finishing topcoat require catalyst to cure and once added will start to cure in the bucket. Make sure everything's ready to go and always mix in small batches of around 5 litres or 1 to 2 litres for detailed areas or bandaging.

Open a can of the resin or topcoat and give it a good stir before you start to use, some of the additives in the base could have settled in transit or storage so you will need to make sure everything is thoroughly mixed in. After stirring pour the product into a 5 litre bucket.

Measure out the desired amount of catalyst with the special measuring bottle and pour into the bucket with the resin and mix it in really well with a square-ended flat stirring stick or paddle. 5kgs of base or finish will need to be mixed for at least 1 minute to ensure there are no un-catalysed parts in the bucket.

Curing Times

The weather and temperature play a large part in how long it takes for the resin system to cure. When calculating cure times make sure you take into account the temperature of the deck and resin system along with the ambient air temperature.

How the materials have been stored and how long they've been kept will also affect curing times. The standard shelf life for these resin products are between 2-6 months so don't buy stock you know you're not going to need and use the resins and catalysts as soon as possible to ensure optimum performance.

Fibreglass Matting Guide

Fibreglass guide

What is fibreglass mat and how to prepare before getting started

Choosing the correct weight

Fibreglass Matting is available in various different weights, however for standard fibreglass roofs 450gm and 600gm per m² are the best choices.

There's all kinds of conflicting suggestions on just how many layers and what weight of fibreglass to use on your roof. Some people say it’s 1 layer of 450gm or 600gm while others say it’s 2 layers of 300gm, 450gm, 600gm or a combination of the two, as you can understand the waters get muddied and it all becomes very confusing.

And while people will always have different opinions, heres our own answer on the subject, the fibreglass layer is the first line of protection and having more of it on your roof is certainly not a disadvantage, we recommend you never use 300gm fibreglass matting on a roof and use only 450gm on roofs of up to 10 square metres with no foot traffic, for everything else you can use 1 layer of 600gm or two layers of 450gm depending on the use.

Preparing the mat

You will need to need to pre-cut the fibreglass into lengths across the roof parallel to the drip trim, start by rolling out the fibreglass overlapping by at least 50mm. Choose the best place to finish laminating the roof, always best to work toward the ladder, you don’t want to be stuck in the corner of your roof with no escape.

When this is done roll up the lengths and keep them in their position just in case their might be a different size or shape from one length of fibreglass to the next. Next job is to sweep the roof, you need to make sure the roof is as clean as possible before you start.

GRP Roofing Trims

Installing Trims

GRP roofing trims

Product information and application guide

Product overview

GRP roofing trims are often seen as a commodity item by some contractors and over the years cheaper products have flooded the UK market and reduced both strength and quality.

When choosing a trim you need to be looking for strength, some cheaper products are almost transparent in nature and can be flexed really easily. While the price might be good they can be cracked really easily which could mean an expensive repair job.

The Fibrelast GRP roofing trims that we supply are manufactured in the UK to ISO9001 standards and are made with 600gm fibreglass making them super strong and able to withstand ladders and other objects. Here's a quick run down of all the trims and corners available.

Corners and closures

Although the corners aren't really used by the professional GRP roofing contractors (they make their own on site using the fibreglass and base resin) they are a quick way to get a professional uniform finish without the hassle of making them from scratch.

Flat flashing

This product comes in 20 metre rolls or can be cut and sold by the metre. Flat flashing is mainly used at the intersection of a pitched roof and flat roof often found on dormers.

Fascia trims

These trims are sold in both 3 metre and 1.5 metre lengths. The trim is fitted to the lowest edge of the roof usually where the rainwater flows into the gutter.

Simulated lead flashings and closures

These range of trims are designed to simulate the look and feel of a lead roof.

Raised edge trims

Sold in 1.5 and 3 metre lengths, these up-stand trims are installed to contain and direct the flow of water into a gutter.

Wall fillet trim

Sold in 1.5 and 3 metre lengths, these angle fillet trims are fitted where the roof abuts a wall or vertical face, against parapet walls and in box gutters etc..

Expansion joints

Expansion joints allow for GRP roofs over 50m2 to cope with thermal expansion and contraction.

Angle trims

Angle trims are used when the GRP roofing laminate needs to cover a surface which continues perpendicular to the current level.

Application guide

Take extra care and use gloves when handling as the rough edges of the trim can be sharp. One side has been created with a high adhesion matt finish, which you must always bond to.

When cutting trims down to size always make sure that you are wearing a safety mask and never breath in any of the dust. Recommended tools for cutting the trims are hack saw, small angle grinder or multi tool. Another useful tool to cut trims are tin snips, these take much longer to cut than the others but they reduce the amount of dust being generated.

Bond the trims into position using polyurethane adhesive, you can either apply small beads at 300mm intervals or a small lines of adhesive under the fibreglass trim and then apply to the roof.

The trims can be fixed to the decking board using 15mm galvanised clout nails or staples with a compressed air gun. Simply hold the trim in position making sure the face is vertical, start by fixing the ends, then the middle and then 200mm thereafter.

When joining the trims together, simply overlap by at least 50mm applying a bead of PU adhesive to one of the surfaces. If a ladder is going to be leant against the trim for regular roof access or widow then you may want to either double up by slotting in an extra section of trim where the ladder would go or reinforcing with an extra layer of GRP laminate and then surface tissue to maintain the smooth finish.


GRP roof decking

Choosing the right decking and how to apply

Recommended Decking

When re-boarding for a fibreglass roof, we recommend using only 18mm 8X2 OSB3 (Oriented Strand Board) these boards not only provide the perfect surface for the resin to bond with but also provide a number of other labour and monetary savings.

Most of these boards also carry FSC Forestry certification, which means the wood used to manufacture the boards has been sourced from well-managed and maintained forests.

Perfect bond

The textured surface allows the resin to secure a firm bond and key into the board, virtually eliminating de-lamination.

Prevents distortion

The tongue & groove profile will stop the boards from distorting as the laminate cures.


The smaller 8×2 tongue and grooved boards have been designed to reduce expansion and contraction and a much easier to carry up ladders.

No jointing required

These boards do not require any additional fibreglass bandage on the joints saving both time and money.

Fixing the deck

For optimum performance a GRP roof should always be applied to fresh OSB3 decking and you should be wary of contractors who suggest boarding over the existing surface. Boarding over is a serious cut corner and will save money but any underlying issues will still be there and may come back to bite you.

If your roof has been holding water (ponding) then you need to fix it at this stage, your roof should have a minimum fall of 1:80 which can be corrected with firring strips on the rafters, before laying the boards. Do not prepare the deck in bad weather, the boards must be completely dry before the resin is applied so if you are leaving overnight make sure you protect from rain or moisture.

The easiest way to fix the boards to the joists is to use a nail gun with a 63mm or longer galvanised ring shank nail at 200mm centres with 4 nails across a 600mm board, if you don’t have a nail gun then you can use screws or nails with a minimum penetration of 40mm.

Start and place your boards from the furthermost edge of the drip section, staggering the joints to create a strong structure, if your boards are going to be laid along a wall or any abutting up stands, always leave an expansion gap of 25mm.

Lay the boards lengthways at an angle of 90° to the roof joists, it’s important to lay the boards “writing side up” this sides rougher texture not only allows the Fibrelast GRP roofing base coat to flow better getting a firm hold and key into the board it also allows the base coat material to run into the boards joints giving a better bond.



Information to help you plan for the UK weather

Planning ahead

The weather and temperature play a large role in the life of a fibreglass roofer, so you always need to have one eye on the sky before getting started.

Both the OSB boards and the GRP roofing materials don't cope well with water of any kind falling on them from the sky. Before you start doing anything get the weather from either a local radio station, online or just by going outside and looking up.

Heavy rain on a partially cured roof will inhibit the cure and turn the resin white which means stripping back and starting again, so always have a polythene cover on site and quickly accessible so you can throw over the entire roof. The polythene cover will protect the resin without sticking and will just peel off when the resin has cured.

Never leave the boards bare, always try and get the base layer down and cured in good time. The new deck will soak moisture in like a sponge making laminating impossible. If you have to leave the boards bare then securely cover overnight adding timbers between the deck and cover to allow air movement.


You can optimise the GRP roofing materials to work better in different temperatures by adjusting resin system and catalyst. The Fibrelast base resin and finishing topcoat system can be ordered in a ColdCure version which will allow you to work in temperatures down to 0°c.

Health & Safety

Health and safety

Information you need before you start using the products

Safety information

Over the years we've heard about silly accidents that could have been avoided with better training and understanding for these GRP roofing products. Here are some of the main common sense safety steps to keep you safe when using these materials.

Read the product safety information

Read the MSDS (Material Safety Data Sheets) for fibreglass roofing products before starting the job and familiarise yourself with the storage and safety procedures.

Wear protective glasses

Make sure you have on a pair of wrap around safety glasses before handling or mixing any resin, topcoat, catalyst or cleaning solutions. Always have two bottles of sterile eyewash on site – one for the active site area and one for backup.

Wear gloves

Never let base resin, finishing topcoat, catalyst or acetone come into contact with the skin. Always wear either heavy duty latex disposable gloves or heavy duty reusables available to buy online.

Wear protective clothing

Always wear protective clothing when using fibreglass materials, the disposable coveralls available in the store are the best way to protect any uncovered parts.

No smoking or naked flames

Never smoke on site and keep all GRP roofing materials away from any naked flames.


The fibreglass roofing base resinsfinishing topcoatsacetone and catalyst are all classed as hazardous materials and are delivered to you with transport companies who are trained and licensed to do so. Road regulations also demand that you must comply with certain aspects, particularly when transporting these materials to site.

For any vehicle below 7.5 tonnes the driver doesn't need an ADR licence however they must have received details and instructions from the company to ensure the driver is aware of the hazardous nature of the materials being transported and how to react to an incident.

The panel above details the product transport information and packing sizes for the 3 hazardous products in the Fibrelast range. This will not necessarily apply to most people purchasing GRP roofing materials, but professional contractors who carry and transport stock should have an understanding of current regulations.

GRP roofing professionals who transport these hazardous goods should have eyewash, first aid kit and fire extinguisher in their vehicles at all times to meet safety regulations. Transport emergency cards or information in writing for contractors regularly buying are available to download and should also be kept in the vehicle on display at all times.


Along with keeping you safe, storing GRP roofing materials in the correct way will also maintain the life of the system allowing for optimum performance.

The base resin and finishing topcoat should be stored in a dry and well ventilated room, below 20°c and away from the catalyst, acetone and any heat source.

Store the acetone in a tightly closed container in a dry, cool and well-ventilated place separately to the other fibreglass roofing materials. Keep away from heat, sparks and any open flames.

Catalyst needs to be stored in cool dark room below 25°c and away from flammable liquids and direct sources of ignition.