What is GRP?

GRP is short for Glass Reinforced Plastic. GRP is also commonly known as Fibreglass.

A GRP product is a hard plastic reinforced with strong fibres of glass strands. GRP products are light, strong and durable. They are often used in boat construction, vehicle components, aircraft components and propeller blades. GRP has excellent properties in relation to weather and water and withstands all types of conditions, stresses and strains. GRP copes well with long term exposure to water at elevated temperatures.  A puddle on the surface of a yacht in a hot Mediterranean harbour, for example, poses no problems. GRP has good qualities in relation to acidity and can certainly cope with “acid rain”.

GRP was developed in the late 30’s and by the end of the 40’s was widely used.  Many products made in the 40’s and 50’s are still in use today.

The combination of durability, strength, lightness and water resistance makes it ideal for roofing applications.

Is Stronghold GRP Fire Retardant?

Yes, it is. All GRP roofs should be. Only Stronghold acheives FAA rating straight out of the tin.

Most good quality systems have some rating, maybe FAB or FAC.  Poor quality systems rarely have any fire rating and we advise not to use them. Using them may be illegal… be careful.

 If your not sure then check with your local authority.

All Stronghold GRP systems contain fire retardants and meet the highest rating for flat roofing products

BS 476-3: 2004 EXT.F.AA

If a system has a fire rating, then it will be clearly marked. If it is not, then it is probably not fire rated.


What colour is GRP?

GRP can be any colour. Commonly light, medium or dark greys are applied as these colours “work well” for roofs. Our standard colours are RAL7046 which is a light to medium grey and RAL7011 which is a dark grey. However, Stronghold GRP can be any colour as it depends on the pigments added to the finishing “topcoat”.

How long will it last?

GRP lasts for decades if properly constructed or laid. Products made in the 40’s are still in use.

May I use normal polyester resins?

It is not advisable. Stronghold roofing resins are based on good quality polyester resins but are modified to improve elasticity, to make them fire retardant and to cope with variable installation temperatures. Standard resins may be flammable and too rigid. They will have a shorter lifespan if used for a single layer application such as roofing.

Can GRP be refurbished or repaired?

GRP is not classed as biodegradable. However, over many years the pigments may fade or the roof simply needs altering or repairing. Maybe the owner simply wants to change the colour.  GRP is an excellent material to work with. It is easy to cut with the correct tools, easy to bond to and generally easy to alter or repair. A new layer can be applied to an old roof in about a third of the time it would take to change the whole roof. Installing a 20 metre roof that would last 30 or 40 years may take 2 workers a day. Refurbishing it to add another 30 years to its lifespan would take 2 or 3 hours!

Is GRP difficult to install?

Roofing is hard work in general and may be dangerous. Only competent people should consider it. The GRP layer is relatively easy to work with and much safer than systems requiring heat.

Some knowledge is critical and we recommend our training courses. Reading our installation manual and watching our on-line videos is also strongly recommended. For our guarantees to be valid we would expect the installer to follow our procedures. Courses are not obligatory, following correct procedures is.

For any installer, the first one or two roofs can be a learning curve and it’s probably wise not to undertake a large roof as a first job.

However, GRP is not difficult with some knowledge and there is no reason why a first “job” should not be a fine job for roofers and laymen alike.

Nobody should work on a roof unless competent and confident. If unsure, then we strongly recommend a professional roofer or builder undertakes the work.

How long does it take to install?

A professional installer should be able to install about 10 square metres a day.

We strongly recommend and follow the practise of a minimum team of 2.  Never work on a roof alone. Accidents do happen and somebody may need to call or go for help.

How strong is GRP?

Boats are made of GRP.  They withstand extremes of temperature and obviously, lots of contact with water. They rarely leak unless damaged. GRP roofs are fully bonded and wind uplift is not a problem if the decking boards are securely fixed. There are cases where a complete GRP roof has been torn off a building and yet landed intact. We have seen a caravan torn in half by a tractor yet the roof remained intact. GRP is very strong and is often used for applications where few other materials will survive.

What Does GRP look like?

GRP has a smooth surface and can be any colour. A non-slip, textured surface may be applied. Standard colours are more cost effective as we manufacture them in large batches. However, GRP roofs can be any colour. We can supply colours from the British Standard range or the German RAL range. In theory pigments, can be mixed and any colour can be achieved.

Would a roof contractor require “Work with heat” insurance?

No. GRP is supplied as a liquid. It cures with the addition of catalyst and as such no heating equipment is required.

How long does it take a GRP roof to cure?

Normally within an hour.  Catalyst levels are adjusted to achieve this.

When not to install?

Wet days or extremes of cold or hot.

Avoid water when installing. Resin will not bond to wet wood. Water mixed with resin results in a sticky mess.

Avoid very low temperatures. Above 0°C is possible with experience but above 5°C is more practical. Whatever it says on the tin, polyester “goes to sleep” at zero and cure will stop. An uncured roof left overnight will absorb dew and be spoilt. Avoid very high temperatures, say above 27°C, as the materials will cure very quickly and it requires lots of experience to deal with this.

Is training a requirement for the guarantee to apply?

No. However, if you’re not sure then it is advisable. Please contact us.

If you already know what to do, then training is not required but we do insist that you follow our procedures. Our installation manual is based on many years of experience and the procedures within it must be adhered to for our guarantee to be applicable.

Where can I purchase Stronghold materials?

Our products are available through all the UK via a vast network of distributors. Please contact us to find your nearest.

What are the common mistakes?

Not using enough resin and / or topcoat.

In the interests of gaining a sale many systems now quote by the metre rather than by the volume of materials. The contractor is then trying to spread the materials too thinly and failure occurs. Many systems for example only recommend 1.5 litres per metre for the area, and some even less ! From experience we would never ever attempt a 30 metre roof with 45 litres of resin. Good luck to anyone trying !! Stronghold materials are based on 20 years’ experience as installers and allow for wastage, overlaps, etc.  Follow our guidance when purchasing.

No reinforcing of butt jointed decking.

We strongly advise using T+G decking. If butt jointed products are used, then joints should be bridged as in our installation procedures.

How do I stock the Stronghold Roofing Range?

Contact us. We are loyal to our existing stockists. However, if we have no stockists in your area we will be happy to discuss supplying you.

Frequently Asked Questions

02920 888 391


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