Is GRP a good roofing material?

GRP flat roofing Prose and cons


  • Now easily available in DIY shops
  • Good and strong
  • Tested over many ears to perform
  • Comes in any colour
  • Can be formed into any shape
  • Will hold standing water


  • Very rigid and brittle if laid to thin
  • Has to be laid by experienced people
  • Has to be built up to form a good surface and this takes time and materials
  • Weather and temperature dependable

Other options to consider

  • 3M SD650
  • GacoRoof
  • Kemper

All three are based on the principles of GRP resins and glass fiber but they have been modified to be flexible.

My opinion.

If the GRP roof is laid correctly and to the correct depth they are good roofs however I only ever see well laid GRP roofs when Im down the coats these are normally laid by boat builders and are formed from may layers of GRP,  forming a good strong shell.

If you have lots of money and want a really nice looking roof then GRP is the way to go.

Flat roof survey or report

It’s not often we get asked to look at a flat roof as large as this and unfortunately our free estimate wont cover the cost of surveying this roof and reporting back the findings.

This flat roof is a round 1500m2 and is covered with an old type flat roofing membrane with welded joints, this type of roof is still being fitted but we consider it to be one of the older generation flat roofing materials.

I’ve taken a walk around the inside of he building and seen where the water is penetrating, externally I have examined the roof and can see lots of  design faults, I will be submitting a preliminary report  suggesting that thermal imaging of the roof is undertaken to show where the insulation is saturated this will help identify the areas of the roof where the roof has taken on water, then we can  scrutinise each joint and visually inspect the membrane for holes punchers  or rips.

At this stage I think we will be cutting out all the wet insulation then repairing the roof using a liquid roofing systems such as GacoRoof.

Related articles

Liquid roof covering by GacoRoof over a parapet wall

In this video I show you just how flexible the GacoRoof covering is

We have just covered an old asphalt roof in wembley with the new liquid waterproofing system, we dressed the parapet walls with the system so we didn’t have any flashing details to deal with.

No flashing no messing about complete encapsulation, the flexible roofing system will allow for expansion and contraction  over the seasons





Parapet wall and flat roof before and after


Using a liquid roofing system we can encapsulate the parapet walls with ease


Take this old asphalt roof in Wembley, the asphalt has sagged and cracked away from the parapet wall


Cracking is evident and the ceiling below shows water-damage.


The brick parapet wall is  solid construction but the flat roof is built on timber and will settle over time, the asphalt will melt in the summer and go brittle in the winter, with the thermal movement the  cracking occurs.


So after cleaning of the roof taped patches are applied to the cracks before the new roofing system is installed to the roof.



On this roof we are using the GacoRoof a silicon-based liquid roofing materials from the USA . GacoRoof is very flexible and can withstand   the movement described above.


The other interesting factor to all of this is the color of the finished roofing materials GacoRoof  has a white finish and reflects the sun’s heat stopping the  old roof from melting


liquid flat roof gacoroof london 14



Preparation before a new liquid flat roof

We are on day to of this roof, the old roof is an old asphalt roof and was leaking from several places.

In this video I walk you around the roof showing you the condition of the old roof, some of the cracks and detailing.

The new liquid roofing system we are about to use on this roof is imported from the USA and is relatively new to the UK

It’s a silicon-based flat roofing materials called GacoRoof and one of its biggest advantages over other flat roofing materials is its amazing flexibility

Before we cover the old flat roof with the new roofing system we have given it a good clean and repaired all the damage that has happened over the years.

This old roof had Ponding or pooling water all down the center, although a problem to the old roof its not a problem to the new liquid silicon roof.

Why choose a liquid flat roofing system for your new flat roof.

Some roofs just look a mess when covered with a sheet roofing materials, if the flat roof is large and the detailing is limited then a sheet flat roofing materials such as EPDM rubber could be used.

If the roof is covered with obstacles such as skylights, pipes, air conditioning units and so on then a liquid roofing system should be chosen.

I alway explain to my customers we don’t choose the flat roofing system for the roof the roof and your budget chooses the system.

The new Liquid roofing system we are using on this roof is ideal as an overlay system for old flat roofs, the advantages of the GacoRoof liquid flatroofing system is that its snow white when completed reflecting all the suns rays int he summer.

How to price a new flat roof

Recently we have been  overlaying an old asphalt roof in Wembley with a new silicon-based liquid roofing material called Gaco Roof. I took the opportunity and made a video, hopefully explaining how we can provide an estimate of the cost of a new flat roof over the phone, using information given by the customer.

The new roofing system, Gaco Roof, that we are using here is a new silicon-based liquid roofing system and is relatively new to the UK. GacoRoof is made in the USA and has had quite an eventful life over there; we have been using it now for the last 5 months and it’s

liquid rubber SD650 by 3m flat roofing 27

becoming one of our favourite liquid systems.


Liquid roofing systems all have their respective advantages and disadvantages. If you have been following my blog, you will know that over the years I have been trained to use several different liquid roofing membranes, and after extensive testing, if a system is found not to be that good, we will stop using it. Ive found in general it take around about a year to understand a new liquid roofing system you need to go thorough all 4 seasons before  a complete understanding can be  achieved.

liquid rubber SD650 By 3m flat roofing 8

Problem with Green flat roof

Green flat roofs may be nice on the environment and nice to look at but when you get a problem they can be very expensive.

This green flat roof is probably going to have to be completely striped as the flat roofing membrane under it is leaking somewhere and we cannot find the problem.

Realistically under a green flat roof you don’t want to have joints that can go wrong at sometime, most new flat roofing materials will last for many years but when they do go wrong they will alway go wrong where there is details.

Remove the details remove the problem.

liquid rubber SD650 By 3m flat roofing 8

Garden Office Shed roof.

Hi,I saw your videos on You tube and was impressed.

I have a garden office, just installed bu it only has a temporary tyvec cover on at the moment (the company only did felt roofs and we asked for a shallower pitch to the roof for extra height height).

It’s under a tree, the floor space is 15m2 (3.75m x 4m) so the roof is slightly larger due to overhangs. Due to height restrictions, I cant’ do a warm roof, so it will be insulated inside with a boatload of cellotex I have.

Do you work in Se London? Is the best form of roofing for this fibreglass (GRP?). Do you have any idea how much it would cost? The access is through the house (it’s a victorian terrace) but it shouldn’t be a problem and the are no parking restrictions and we work form home.

Any advice on the insulation and the amount of air gap I need to leave would also be appreciated.

Many thanks,

Reply from:

London Flat Roofing.

GRP flat roofs or Glass Reinforced Plastic flat roofs, have in the past been a good alternative to the older tar based flat roofing materials such as Asphalt and mineral felt.

Time has moved on, second generation Liquid roofing systems evolved from older GRP systems, a glass fiber matt impregnated with a resin, the resins had been modified to stay flexible unlike Traditional GRP that’s rigid.

Third generation liquid flat roofing systems have no glass fibber matt to reinforce them, new resins bond together giving a tough durable materials that’s stays flexible but also very strong.

If you have not already seen the 3M video that promotes there new flat roofing system go and have a look they show there old system at the start with the glass fibers this system was good and flexible  the new system douse away with the glass fiber saving the material cost and the time but most of all removing a procedure that if not carried out correctly could over time be the downfall of the system.

The new system being easier to install means that there are less procedure to undertake on site, less chance of a mistake being made, less chance of a failure in years to come.

The 3m System on 13m2 will cost around £1500+vat

No air venting is needed if this is a warm room. Warm flat roof video

Yes we will work out your way

Send me some photos of what you have  I’m in office all-day today if you want to call 078023 00099

EPDM rubber will be cheaper still very good  around £1000.00+vat   black and not as good looking.

Estimate for new EPDM roof or 3M Scotchkote Urethane Roof Coating SD650 liquid roof.

Information from customer regarding his leaking flat felt roof.


We have a leak on a felt flat roof extension in London NW6 4SS, which is about 10 square meter. We are looking to replace with EPDM. Could you please come around for a quote?

Reply By email from London Flat Roofing.

EPDM over felt is not recommended as its black and attracts the heat in the summer melting the felt and making the rooms under the roof uncomfortable. To stop this happening the roof will need a OSB Board overlay  before the EPDM then flashing’s so I would suggest a price of £1400.00 +flashing’s and scaffolding if needed.

An alternative roof is the 3M Scotchkote Urethane Roof Coating SD650  the finish is light gray  so reflects the heat, although much more expensive that EPDM rubber, easier to lay and now over boarding needed  so would suggest a cost of around £1000.00+vat + any flashing’s and scaffolding needed
Both prices will be subject to how clean the old roof is and its accessibility.
All +vat
Guarantees up to 25 Years are available
Have you got access to the roof, if so can you send me some photos? if So can you send me a photo from outside the property
If you also send me the address I can look this up on Google earth.
These may be of intrest to you.

Time-laps video of a new flat roof

Cleaning a old asphalt roof

liquid rubber SD650 ny 3m flat roofing 8

liquid rubber SD650 ny 3m flat roofing 8 (Photo credit: EPDM Rubber Flat Roofing)

First coat detail round skylight

EPDM rubber roof northwood 6

EPDM rubber roof northwood 6 (Photo credit: EPDM Rubber Flat Roofing)