fibreglass supplies for flat roofing and all types of fibreglass projects

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Fibreglass Roof Building

Fibreglass Roof Building

A properly laid fiberglass roof will last for at least 30 years with no measurable deterioration.

Roof Preparation

You may be working with a new or existing flat roof. All surfaces should be clean & dust free. Allow to dry and clear all unnecessary items off the roof before mixing your first batch of resin.

You should ensure that the plywood boards (typically 18mm exterior plywood or sterling board) are laid to adequate falls in order to allow good drainage, so that there is no standing water on the roof after completion.

You cannot laminate effectively onto a bitumous or painted surface as this will retard the cure of the resin used. It is possible that some of the boards underneath will be rotten or have rotten areas which will need replacing.

A possible alternative to stripping back to a clean sound surface is to simply board over the existing surface with fresh boards and then apply GRP to them.

Affects of temperature

The ideal working temperature for GRP laminating whether moulding or sheathing is around 18°C, at temperatures below 15°C there is a great risk of the resin or flowcoat 'undercuring' and not reaching it's formulated properties. At high temperatures (30°C or above) materials will cure quicker, even in small quantities. It is advisable not to use Flow Coat in direct sunlight.

Heating a workshop is of course fairly easy and cooling can be in the form of extraction, both resulting in a comfortable, dry environment to work with temperature sensitive resins, careful consideration therefore should be given when working outdoors. Any commercial GRP roofing company should expect inclement weather to play a part in week to week work, therefore a temporary workshop might consist of just plastic sheeting should it rain, or better still, some form of support or ridge pole to Create a basic tent structure.

The roof must be laid onto a dry deck in dry conditions at a temperature not below 10°C. The resin and Flowcoat/Topcoat should be suitable for use in roofing.

Heating MUST be dry heat, not of the type that creates moisture (ie: not calor or oil filled space heaters as these produce large quantities of water vapour and will inhibit resin cure). Electrical or convector heaters are ideal, and once under a supported cover, can provide good working conditions for completion of the job.


You should always use 2 layers of 450gm chopped strand mat. The chopped strand mat should already be cut to size and resin and catalyst and containers placed within easy reach. ONLY mix sufficient material for you to use within 9 to 10 minutes otherwise it will start to cure in the bucket!

The catalyst ratio for most resins is 10 to 15ml per kilo and again needs to be well mixed into the resin. As the catalyst is mixed you will notice a slight change of colour which will tell you when the catalyst is thoroughly mixed in.

Deck/Resin 29-35°C 21-28°C 13-20°C 6-12°C
Percentage Catalyst 1% Catalyst 2% Catalyst 3% Catalyst 4% Catalyst
Table of Percentages in Millimetres, Per Weight of Resin Used
Amount of Resin Catalyst Usage
1 Kilo 10ml 20ml 30ml 40ml
2 Kilo 20ml 40ml 60ml 80ml
3 Kilo 30ml 60ml 90ml 120ml
4 Kilo 40ml 80ml 120ml 160ml
5 Kilo 50ml 100ml 150ml 200ml
6 Kilo 60ml 120ml 180ml 240ml
7 Kilo 70ml 140ml 210ml 280ml
8 Kilo 80ml 160ml 240ml 320ml
9 Kilo 90ml 180ml 270ml 360ml
10 Kilo 100ml 200ml 300ml 400ml
11 Kilo 110ml 220ml 330ml 440ml
12 Kilo 120ml 240ml 360ml 480ml
13 Kilo 130ml 260ml 390ml 520ml
14 Kilo 140ml 280ml 420ml 580ml
15 Kilo 150ml 300ml 450ml 600ml
16 Kilo 160ml 320ml 480ml 640ml
17 Kilo 170ml 340ml 510ml 680ml
18 Kilo 180ml 360ml 540ml 720ml
19 Kilo 190ml 380ml 570ml 760ml
20 Kilo 200ml 400ml 600ml 800ml

Catalyst Addition

There are a number of important rules of to follow when deciding how much catalyst to add:

  • Never use less than 1% or more than 4% catalyst.
  • Never underestimate the effect of temperature. Resins will not cure at or below freezing and will always cure much quicker in direct sunlight.
  • When using top coat late in the day, add more catalyst to allow for the lack of sunlight
  • In winter use fast catalyst, in summer use standard catalyst, in very hot conditions use LPT (Long Process Time) catalyst.
  • Remember: Any catalysed resin left in the bucket will exotherm. Heat is generated as the resin cures, so it should be kept well away from other stored materials. Water can be poured over the resin to suppress the heat gain.
  • Always mix the catalyst into the resin thoroughly before using the resin (a couple of minutes for a 10 litre bucket.) Failure to do this can result in 'streaking' on the laminate, where streaks of uncured resin will remain visible and ultimately lead to failure in the laminate.