How to Fibreglass a Bay roof
This article deals with how to fibreglass small bay roofs, like the one that can be seen in the pictures and video further down, for large bay roofs please also see the article on how to fibreglass flat roofs in the links at the bottom of the page.
Fibreglass GRP Bay Roof
A very hard roof to better, fibreglass roofs have been around 20 to 30 years now so they have a history to benchmark in terms of how long they will last without leaking or failing. I have seen correctly laid fibreglass roofs in very good condition in excess of 30 years and I cannot see a reason why they should not last a lot longer. A new fibreglass bay roof can sometimes look a little shiny for an day or two after laminating until it reaches a dull satin look, however they are probably not at their best until 6 months to 1 year has passed. Unlike most roofs they tend to look better as they get older.
Build one just like this, a great starter project learn how here.
Please use this written guide in conjunction with the video, rather than just the video alone…
This video shows an easy method of pushing resin through the matting and can be used for very small flat roofs only. For larger projects see how to fibreglass a flat roof further down.
Fibreglass Roof Decking for bay roofs
Sterling board or OSB (Oriented Strand Board) is really the roof deck of choice for laying a new fibreglass roof onto. The rough texture allows the resin to grip and key into the surface of the board, this gives very little change of delamination i.e. the matting peeling away from the surface of the wood. The absolute best is tongue and groove OSB3 but this can be tricky to get hold of unfortunately. Plywood is a good third choice option but you can risk delamination unless thoroughly sanded on the laminate side.
Sterling Board or OSB – New decking – When you have stripped the old bay roof the boards are normally contaminated, or planks of tongue and groove, which are again no good for fibreglass, either way you will need to overboard or fully replace the timber decking. For full replacement use 18mm standard square edge OSB3 or T&G OSB3, but if you use standard square edge you will need to allow expansion gaps of 3mm between any abutting joins in the OSB, then apply tape over the joins afterwards.
Over boarding – If you are over boarding the existing bay roof timber, and it’s in good condition, 11 – 18mm OSB3 will suffice as long as the timber underneath is not chipboard and in a very firm rot free condition. You may also wish to consider cross laminating or laying the boards in the opposite direction so that any joins from the timbers underneath do not stack on top of each other. A 3mm gap should still be allowed between any abutting OSB boards, followed by taping.
Taping the joints – Tape any gap in the joins between abutting boards, this stops excess roofing resin seeping into the gap between the timber decking and hindering the contraction process. Tape over any gaps with good quality masking tape, duct tape or gaffer tape, half onto each side of the board.
Expansion and contraction – Bay windows tend not to extend far out from an abutting wall so do not suffer from large movement problems, but It’s always a good idea to allow a small gap of a minimum of 10mm behind any up stands against brickwork or fascias though, even on a very small bay roof.
Common Roof Trims for Bay Windows ( drip trims )
There are two commonly used drip trims when laying a fibreglass bay roof, the AT195 ex (external) trim and the A170 drip trim. I find when mimicking the other original lead bay roofs in the surrounding area that more often than not the AT195 ex gives the more realistic look of a lead drip.
To see the full range of fibreglass roof trims click here – GRP Roof trims
Common Upstand trims fibreglass roofs
Normally a fibreglass roof or fibreglass bay roof will butt up to a wall or fascia, at this point some thought should be given to how much expansion and contraction room you need to allow. Obviously on a very small roof this is not so important and you will probably get away with 10mm or so on a roof that only protrudes forwards a metre or two, in this case the AT195 int (internal) trim will suffice. Where larger expansion gaps are required the D260 wall fillet is made for the job.
Expansion – Fibreglass has excellent expansion and contraction properties in comparison to lead for instance, however provision has to be made. Whilst the AT195 is not strictly designed for this it’s very useful for allowing small expansion joints also.
Fibreglass matting (CSM)
CSM Chopped Strand Mat – For most small roofs and bay window roofs 450g (1.5 Oz) is more than adequate choice for CSM thickness and weight, on large bay roofs or where repeated heavy roof traffic is expected you may need 600g (2 Oz).
Fibreglass resin application
This video shows a method of pushing the resin through the C.S.M (chopped strand matting) whilst I don’t recommend this for large roofs it can sometimes be preferable when working on a small surface area like a bay window with overhanging slates or soffits. Use a small fin roller to remove air bubbles and consolidate.
How to apply roofing resin the easy way on small roofs like this, also shows cold cure resin for cold weather situations.
GRP Resin – I strongly advise the use of resins specifically developed for roofing, whilst standard polyester resins and re-blends can and are used on small roofs, for the small additional cost use a dedicated roofing resin as this has been blended with greater elasticity for expansion and contraction.
When wetting out 450g fibreglass mat I find around 2 Litres of resin per 1 square metre of matting provides about the right ratio for a nice finish.
In my opinion there is no major advantage in buying a branded roofing system such as the Cure It grp waterproofing system, Topseal or Cromar Pro Grp. Whilst they are perfectly good systems, other standard roofing resins, colours and mattings sold by some big fibreglass suppliers are equal, in fact I prefer to use some of them. I am however not knocking them, they are very good and I use them myself from time to time.
In the video you will notice I laminate the whole bay window with a small 100mm mini roller, you should also have a small 150mm paddle roller on hand too to consolidate the matting to the deck. Gently pass the roller backwards and forwards 2 – 3 times over the laminate when it starts to turn transparent to remove all traces of bubbles.
For small roofs like bay widows within close range of double glazing and fascias I would use a small PTFE fin roller or aluminium V-notch roller, splashing and flicking of resin is reduced to virtually zero over a traditional horizontal finned consolidation roller.
Fibreglass resin ratios and drying times
To Learn all about mixing topcoat and resin, including cold cure resins for cold weather application click Roofing resin and Topcoat
Temperature and weather are the arch enemies of the fiberglasser but let’s deal with weather first.
Rain – Do not fibreglass if it looks like rain, or you think it may rain. Fibreglass hates rain which is good when the roof is complete but your absolute worst nightmare with matting on the roof, exposed roof decking or halfway through applying the resin. If it rains on you at this stage you may have to have a little cry.
Fog or damp – This may coat your decking or roofing trims and cause de lamination.
Temperature – Obviously the weather will play a big role in this and please study the Catalyst and resin drying and curing times provided below. Temperature does not only mean ambient air temperature but the temperature of the decking and the resin itself as these will affect the curing times greatly. A laser thermometer is a great tool for measuring roof deck and resin temperature.
Very Cold weather – Below 5C and down to 0c and below, there are now several cold cure resins on the market for working in these temperature ranges but bear in mind fog or damp associated with cold weather can cause delamination.
Cold weather – 5C to 15C use a normal fast Catalyst or winter Catalyst, you can also possibly use a reaction accelerator like ‘accelerator G’ with normal resin.
Good weather – 15C to 28C standard Catalyst or summer Catalyst
Hot weather – 26C – 36C Ideally use LPT (Long Process Time) Catalyst or slow Catalyst and keep resins and topcoats cool before use. Top surface may stay tacky for 48 hours or more in temperatures above 28C, this can be an issue for working on the surface or debris and insects sticking to it.
Roofing Resin and Hardener Cure times
To Learn all about mixing topcoat and resin please including cold cure resins for cold weather application click Roofing resin and Topcoat
Lead flashings for GRP roofs
For full instructions on creating your own lead flashings from start to finish including videos see the links at the bottom of the page…
Flashings – If you need to install lead over flashing either into the brickwork above a bay, or underneath the roof against a fascia do not use a continuous length. When fitting lead flashings I tend to use about 1 metre or 3 feet lengths to allow for expansion and contraction up to a maximum of 1.5 metres then overlap any joints by 100mm minimum (4″)
Finishing with fibreglass flashings
Personally wherever possible I finish with real Lead flashings not the fibreglass simulated lead flashings C100 or C150, as I think it gives a much nicer period look to the property when the roof has weathered, but it comes down to cost and personal preference.
GRP Simulated lead flashings – Best fixed in place with P.U. adhesive and the chase sealed with mastic or a product like ‘Lead mate’ or ‘Roofers mate’
- Fibreglass roofing supplies – Buy GRP roof supplies Trade or DIY
- How to fibreglass a flat roof – Bigger and a bit more in depth.
- How to install lead flashing – Full instructions
- How to fit a new Lead roof – See the lead to use and method
- Flat roof costs – Fibreglass bay roof costs now added
- Fibreglass bay windows – A superb lead alternative
- Lead bay roof – Lead bay roofs and how to repair one
- Bay window choices – Roof covering choices for bay roofs
- Fibreglass Problems and FAQ’s – Questions and answers
- Roofing tools – Some of the tools I use daily