Video and downloadable pdf’s re fibreglass installation plus a Text Guide for quick review.

Fibreglass Installation Guide


Instruction videos are by courtesy of Fix My Roof

How to install a fibreglass roof


​Roofing Kits Direct specialise in fiberglass- GRP roofing kits and supplies – kits combined with a large selection of roofing edge trims form a roofing system which, when properly installed, will have a life expectancy of 25 years or more.

​An RKD kit will include –

  • Chopped Strand Matting  – according to size purchased +11% for contingency and overlapping this may come in 450, 600 or double layer 450 grams per square metre weight options
  • Polyester resin which may be standard or 100% pure premium
  • Roofing topcoat  – which may be standard, fire retardant or colour pigmented as purchased
  • Catalyst – sometimes referred to as hardener, enables the resin and topcoat to cure
  • Fibreglass bandage to seal edges, corners, trims etc

​Complimenting our kits are a full range of roofing edge trims and supplies each and every kit offers the opportunity to purchase  fiberglass installation Tool kits designed to suit your roof size.

Below is an outline guide for Fibreglass roofing installation. As every project is different this guide cannot be a definitive but provides good basic direction to install your project.

Roof deck installation

Your timber roof structure

​Your new fiberglass roof may be part of a new build or a replacement of an old roof.

Regarding replacement we thought it advisable to consider some key elements.

Having removed the old roof covering this is your vital opportunity to examine the roof joists, firing strips and decking. As everyone knows the key to any successful project lies in good foundations and preparation. No matter how good the fibreglass roof installation is, if the timber work supporting it is suspect it will always cause issues down the line.

Check –

  1. Is all the timber-work sound and rot free
  2. Does it fall correctly and is it free of any dips (ponding potential) – check the firing strips
  3. Is the deck 18mm thick
  4. We don’t advise use of ply which consists of laminated layers – your fiberglass roof will only bond to the top layer of ply deck timber
  5. Is the deck bitumen free – fiberglass reacts badly with bitumen – this will cause the roof to fail – you can strip and replace or overboard with new decking

If the above elements are fine then you are good to go, if not we strongly advise any problem areas are resolved before you install your new roof

18mm OSB3 T&G


​RKD roofing kits are designed to be used in conjunction with 18mm OSB3 T&G decking boards. (Oriented Strand Board) 

​These come in 8’ x 2’ sheets and include natural expansion properties within the T&G edging joints. Thus you do not require any bandaging over the joints before the main fiberglass roof deck is laid.

RKD prefer that these sheets are screwed to the joists at 200mm centres with at least 40 mm joist penetration. Remember your screws need to end up flush with the board surface.

​Tip –  fix boards with writing on the topside – this gives superior bonding capability of the resin to the decking

Fibreglass Trims

​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
A170 drip fibreglass trim

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

B230 raised edge fibreglass trim

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

D260 wall fillet fibreglass trim

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

C100 GRP Roofing Flashing Trim

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 GRP Roofing Internal trim

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

F300 GRP Roofing Flat Flashing Trim

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

Fixing edge trims –

Use ring shank nails/clout/felt nails (screws can split the trim) You can also use PU adhesive where you think expansion needs to be considered. Below are diagrams of how to fix various trims within the installation of a fiberglass roof. As above these are nailed (flush is essential) to the deck.

Cutting trims

​RKD recommend that you invest in a good pair of snips.  Old tech but they work well – You can also use a hacksaw or grinder but remember that here dust will arise – a mask is mandatory for obvious safety/health reasons.

​Joining trims – simply overlap by 50 – 75mm applying a bead of PU adhesive or sealant between the trim surfaces. Using a small amount of manual persuasion they will clip together nicely.


C1 and C4 Universal External  and Internal corner trims – used on corners where raised edge and/or drip trims meet – eg detached roofs, front edge extensions etc
C7 internal and external flashing corner trims – used where flashing trims placed over wall fillets meet adjoining wall corners
C2 fillet to trim LH or RH – used where raised edge trims meet the wall fillet trim
C3 internal and external wall fillet corners used with wall fillet trims where walls or upstands adjoining the roof meet

Begin your fibreglass installation –

A170 drip fibreglass trim

Time to fix your trims

Before you start – clean your trims down with acetone and light sand smooth those surface areas where bandage or matt will overlap to create good bond capabilities.

Corners first, then edging – with the flashing trim grind the groove but leave fixing until after the matt is laid

​Nail them down making sure nail heads are flush and sealant applied to any joins or mitred areas. You can bandage these as well.

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 matt and topcoat should cure in about 30 – 40 mins if the mix is right

There are two catalyst versions – Summer and Winter

Summer Catalyst Addition chart

Bandaging your fixed trims and deck if needed –

Remember use your pots/buckets and mix resin with catalyst in small amounts

  1. Apply the mixed resin to the trim and adjacent deck with a small resin roller.
  2. Roll the bandage on top of the resin and then apply more resin.
  3. Give it a minute or so to settle then work with a smaller bubble buster roller (paddle roller) – this will break down the binding agents and as its name suggest remove any bubbles or trapped air.

When complete your bandage should be translucent (white matting disappears) and no bubbles within it evident.

Bandaging 8′ x 4′ sheets

(You don’t need to bandage decking if you have used OSB3 T&G boards)

Lay your bandage over the 3mm gap, apply mixed resin and bubble bust as above.

You may like to feather the edges of the bandage pre resin this help provide a smooth final finish.

In both these applications the resin should begin to cure in around 30mins – If more than 45mins increase catalyst percentage on your next batch – if less then decrease

Bandaging corners and open ends

Corners (using bandage) – rip off an appropriate length of bandage, wet the bandage on one half in the bucket and then apply over the corner joins holding the dry half.

Add resin to the bandage and smooth over the corner join.

Obviously a bubble buster roller may not fit here – the same effect is achieved by adding and working the resin with a stippling motion of the brush

Open ends – (when preformed corners are not used and trims are mitred)

​Cut a piece of matting which will overlap all surfaces by about 50mm and face up to the corner – Snip the bandage where folds are neede

Apply resin at the bucket to the bottom half of the bandage and press to the corner. Add more resin to the bandage to work the folds.

​Again use stippling motions with the brush to remove bubbles and be careful to smooth the bandage on all surfaces away from the open end to give a flat finish over the gap

Laminating your fibreglass roof

This is, providing you get the resin/mix right, actually a straightforward and repetitive job. Much of the work is in preparation –

1. Wear mask, gloves and safety goggles throughout the installation process A-Z. Lightly sand the bandaged areas to enable a good bond here – remove dust diligently make sure the entire roof is clean and dust/debris free

2. Take you’re matting and measure out each length required – physically lay it out for certainty. Make sure it’s square and true overlapping any trims properly. Roll the length of matting up. Stack it at the beginning of where it will be applied. Repeat the process allowing for an overlap of each sheet until you have enough to cover the roof. You should then have a set of rolls stacked in order of usage ready to go.Matting rolls have one straight edge and one feathered edge this allows blending between sheets for aesthetic smoothness of finish. (You can feather an edge yourself by pulling firmly on the edge of a sheet and thus feathered edges overlapping for a totally smooth result.) So make sure your rolls are stacked so that feather edge meets straight edge when you actually start wetting out.

3. Plan your process and leave yourself an escape route – you’d be amazed how often even experienced installers can ‘paint themselves into a corner’

4. Get your tools ready to go and to hand where you will need them. If you are nervous about curing time lay out more rollers, brushes, bubble busters than you need – better to be safe than sorry.

RKD have tool kits available with each kit purchase or stand alone –

Tip – Now you are ready to go – If you are new to this you may still be worried about laying out and can you do this in time to meet curing time? Don’t worry – this is a methodical process so work in 1m2 sections – Step by step and repeat. As with any job it gets easier as you progress and become more comfortable with wetting out and curing times. You can adjust each mix as you go to reach a comfortable time line

If you can have a second member of the team to hand use them. Don’t mix too much resin, 4 or 5 litres is plenty enough to start, you can always mix more. If the mix starts to cure quicker than you thought and brushes/rollers are getting hard – well that’s why you laid out extra and you can lessen the % catalyst added on the next mix.

1. Mix your resin and catalyst (in a pot or bucket)– stir the resin first then add catalyst and mix very thoroughly – take note of the temperature and if you are in direct sunlight – use your chart to calculate the right amount of catalyser for the number of litres of resin in the mix. Tip – User a proper catalyst dispenser – Mix thoroughly!

2. Your going to use about 1.2 -1.5 litres per sq.m on a 450 gsm laminate

3. Get your first roll ready and correctly aligned.

4. Start in manageable sections – using a roller place resin out on the section of deck – roll out the matting onto the resined section and roll in. repeat with the next section then go back to section 1 adding more resin on top of the matting to wet it out. Don’t skimp on the resin it makes consolidating hard work (don’t go mad either)

5. Repeat

6. Consolidating the matting and resin – here’s where the second person comes into play

7. By the time you’ve got to your 3rd section (this could be on your first roll if large area) sections one and two will be ready to consolidate using a bubble buster roller. This entails smooth yet firm rolling up and down to help break down and remove bubbles. Take care over joints, work these well. If you come across dry areas add more resin, if areas remain opaque consolidate until gone.

8. Continue as a team one wetting out one consolidating – move from row to row of matting ensuring smooth overlap and keep going ( you shouldn’t stop for a break unless there’s subs to take over.)

Tips – make sure you overlap trim flat edges and bandaged sections neatly with enough to ensure watertightness. On wall fillet trims overlap as high on the upstand as possible. ( ensure this will be covered by the flashing when finished)

fibreglass roofing Topcoat application

Topcoat –

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 help the topcoat to chemically bond to the resin matt

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

Start with a small amount of topcoat say half a litre 

Check the temperature and apply catalyst from dispenser in the correct ratio, just like you did with the resin – mix thoroughly.

Use this for cutting in corners and trim edging with a paint brush.

You will use about 1 litre of topcoat for about 2 square metres of roof. 

In winter this would reduce a little to about 1.5 square metres.

You will use about 1 litre of topcoat for about 2 square metres of roof. ~In winter this would reduce a little to about 1.5 square metres.

OK you’re good to go with the full topcoat – use a normal size roller for the main section and a small one to cover edge trims as this will provide a more pleasing finish. (An extension handle here makes it easier on the back)

Installers often use a pour and spread technique to apply the topcoat tipping small amounts straight onto the laminate and spreading the liquid evenly around the deck. Again, as with resin, don’t skimp as you will need to have a thickness to accommodate good UV protection. (again don’t go mad either)

Use your small roller to neatly finish off trims and edges. It gives a more pleasing visual effect than a brush.

There you go – done! – your roof will appear glossy initially – this will cure to a more satin finish in a few days.