Fibreglass GRP Roofing - Frequently Asked Questions

Below we have provided a listing of the most frequently asked questions arising in regard to GRP - Fibreglass - Composite flat roofing.

These have been refined from real user queries and we trust that their queries will help you with any such concerns you may have. Many have attached articles for more detailed responses

If you cannot find what you are seeking please do not hesitate to contact us.

Fibreglass, GRP flat roofing  bandaging and trims installed

The top five questions we are asked

"Whilst not a top five query we would hammer home one vital message - for fibreglass roofing installation your roof must be dry, no moisture as this prevents curing. If the forecast says it's going to rain probably best not to start - if it might rain unexpectedly mid job be prepared, have some Visqueen sheeting available to cover up (unlike tarps etc this will not stick to resin or topcoat even if still wet."

1 - You have multiple fibreglass roofing kit ranges - what's the difference?

Standard kit - engineered especially for roofing - very cost effective, yet really good quality.

Standard EXTRA COLD kit - designed exclusively for the winter months - cost effective installation from zero degrees up

Standard colour kit - cost effective standard resins combined with premium clear topcoat  and pigment - mixed pre dispatch - 35 colour options

Premium kit - the very highest quality kit - 25yr product guarantee - 100% pure resin - lloyds approved- add value to your property!
Premium colour kit - the very best of Premium with our new range of 35 colour options

Platinum Fire Rated kit - the very best of premium combined with the top rated fire retardant topcoat

All our roofing kits, standard, premium or platinum FR, are available in 3 options - 450gsm, 600gsm and double layer of 450gsm

450gsm is the standard choice of fibreglass roof.
600gsm is designed for roofs with regular foot traffic
A double layer of 450gsm is designed for heavy foot traffic.

2 - How much Hardener/Catalyst ?

RKD understand that this element probably causes most concern when installing a fibreglass flat roof. This is the area where most potential issues can arise in the roofing installation. So first tip -

1. Always make sure that you thoroughly mix catalyst into resin and topcoat (resin in a pot or bucket - not in the tin)

2. Always use a proper catalyst dispenser to ensure your measure is accurate 

3. Remember too much catalyst can cause failure to cure just as easily as too little.

4. Potlife - resin will cure quicker if left in the bucket (about 20mins) so mix little and often as you go. 

5. The climatic conditions (temperature) will effect curing time - generally the hotter the faster. (Are you in direct sunlight?)

There is not an exact definition of how much catalyst to add to resin as it depends on the temperature and local conditions. This is why you mix little and often, it allows you to adjust the catalyst percentages as you go.

Resin and topcoat catalyst addition is the same – your base layer and topcoat should cure in about 30 – 40 mins if the mix is right

The industry provides two types of catalyst - Summer and Winter - RKD provide winter catalyst by default fro November to Feb/March

Here' are our catalyst addition charts

summer catalyst addition chart
winter catalyst addition chart

3 - Can I apply the topcoat layer of fibreglass roof on another day - will my roof still be waterproof?

We receive this query quite often, especially from customers who are first timers and those with larger roofs and limited daylight in the Autumn/Winter months.

The Simple answer is yes and yes - however you will need to be aware that there are extra elements to consider.

Firstly, with the resin/matting basecoat layer correctly applied your roof will be fully waterproofed.

Secondly, moisture - the roof and basecoat must be dry before topcoat application - moisture will prevent the topcoat from curing.

Topcoat - just a general note, although when mixed with catalyst the topcoat is applied just like a coat of paint it doesn't just add a colour finish. The topcoat is designed to increase structural integrity of the roof and chemically bonds to the basecoat/resin layer. It also adds UV damage prevention attributes.


Catalyst ratios are the same for topcoat as the resin layer, but take note, conditions may be different on another day so check your charts and the climatic conditions.

If you apply topcoat same day or within circa 12 hours there's no need to sand the laminate for bonding purposes. Outside this time frame you will need to light sand and wipe down with acetone. Sanding gives a key, acetone helps the topcoat to chemically bond to the resin matt/base layer.

However many installers like to give a light sand to check for any bumps and smooth trim edges etc. (obviously clean off with acetone properly)

Again for topcoating always wear mask, goggles and gloves from A-Z. As with laminating get your tools ready and to hand. Mask off with masking tape any edges to insitu fascia boards etc.

4 - I see you recommend using 18mm OSB3 Tongue and Groove decking which are 8 x 2 in size but I've already laid 8 x 4 OSB3 plain boards?

This is very common, 8 x 4 sheets are often more readily available and installation over an 8x4 sheeted deck is easily overcome. We recommend T&G because they include natural expansion properties within edging joints. With 8 x 4 sheeting expansion is generally included by allowing a 3mm gap between sheets.


This gap will therefore require fibreglass bandaging to ensure full fibreglass roof integrity. Bandaging is applied by laying the bandage over the join and applying catalysed resin to it, letting it settle then paddle rolling just as you would for the base coat layer. Thus any bandaging installation becomes an ideal test bed for catalyst addition .


All RKD fibreglass roofing kits allow increased quantities of resin and bandage to cope with nominal wastage and edge trim sealing. If your roof is above 15m2 and you have  8 x 4 deck each kit also includes an opportunity to purchase a little extra resin or bandage to resolve this requirement

5 - I will need to purchase edge trims, but which ones and what size?

GRP Fibreglass Trims generally come in 3m lengths. We stock a full range –

Advisory Guide

  1. Small roofs/Bay roofs. - 65mm depth - Use the A170 Drip and the B230 Raised edge trims together

  2. Standard roofs (most common) - garage/extension etc - 90 mm depth - Use the A200 Drip and the B260 Raised edge trims together

  3. Warm roofs - 140mm depth - Use the A250 Drip and B300 Raised edge trims together

  4. C1,C2,C4 preformed corners work with all the above

  5. D260 Wall fillet combined with a C100 flashing trim are the most common solutions for adjoining walls and upstands

  6. Use D300 Wall fillet combined with a C150 flashing trim wall fillet where an existing chase from a felt roof is evident

  7. Use F300/600/900 where flat roof adjoins a tiled roof

RKD GRP fibreglass roofing drip trim

The Drip trim -
A170, A200, A250 - used at the roofs lowest point directing the water into the gutter.

The Raised Edge trim – B230, B260, B300 - used to prevent water dripping off the roof edges and guides waterflow to the gutter edge

The Wall Fillet trim -
used to create a waterproof upstand against a wall or upstand (Fits over the 25mm gap between decking and wall)

Simulated lead flashings -
fits over the wall fillet trim and slotted into a groove in the wall and fixed with PU adhesive. Finished off with silicone bead to groove to waterproof

AT 195 rt angle trims, external and internal are used on steps but most commonly around the base of skylight upstands

Flat Flashing trim – F300, F600, F900 – used where the flat roof is adjacent to a tiled roof (sold by the metre)

Further FAQ's

How do I stop my roller going hard while I'm using it?

Have a litre of acetone in a tub for cleaning during the Process of laminating.

My roller Barrel head is jammed - everything has seized up?

Unscrew the barrel head between sessions allowing the frame to soak clean - if rock solid, unscrew the barrel head and wire brush clean & Soak overnight in acetone - NB in a well vented area.

My boards got wet in the morning dew, what now?

Don't worry, it often happens when you remove the old roof covering one day ready to fix your new roof the next. Simply you need to make sure they are properly dry before starting. A delay yes but they have to be dry

My resin has failed to harden, why?

Uncured resin is nearly always attributable to poor or wrongly measured mixing of the catalyst. For example - not enough catalyst, too much catalyst or the wrong type of catalyst (e.g. LPT or summer catalyst used in winter.) Remedy..... Read more -

My resin is going off while i'm still applying it?

This is typically either the use of winter catalyst in the summer - (the wrong one), or it's just too hot (direct sunlight in summer) We would recommend reducing the size of batches ensuring you are laying the shortest possible runs across the roof Check your catalyst measures just to be sure.

What if it rains mid-job?

If you think it may rain mid laminating don't even start. If it starts or even looks like raining while laminating STOP straight away and get the roof covered with visqueen sheets. Resin contaminated with water is a NO NO and you will need to reskin.

My topcoat has not cured?

This is due to poor mixing of catalyst or not enough catalyst. Potentially it could also be that topcoat is contaminated by water. You can remedy by adding another, very thin layer of topcoat with more catalyst added - vigourously rolling in to the unciured layer. Read more -

When should I use an expansion joint?

In general the professionals insert an expansion joint when the roof is over 50 m square. At this size there is a danger of cracking laminate due to expansion and roof failure if not incuded. If your roof is below 50m2 you should be OK without one.