Flat Roof Fibreglass Problems
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Why use OSB3 with fibreglass roof construction
Sterling board or OSB board has lots fine rough textures to the surface which provides an excellent key, the roughest face is usually the side with the writing on it.
Both OSB and OSB 3 were designed for the building trade for load bearing and minimal expansion and contraction problems, although both are resistant to moisture OSB3 is specifically designed to moisture resistant. Why use anything else?
I’m using 8 x 4 OSB what expansion gap do I need to allow
A 3mm or 1/8“ gap between all edges of the sterling board are recommended to allow for thermal expansion and moisture absorbance expansion, don’t forget to allow 25mm or 1″ where the OSB roof deck comes into contact with an abutting structure like a wall.
Do I have to use OSB 3 T&G
No you can use normal 8 by 4′s but you do not have to bandage the joints with T & G unless you want to, don’t forget to allow 25mm or 1″ where the OSB roof deck comes into contact with an abutting structure like a wall.
T&G is smaller, lighter and easier to handle, obviously at 2′ by 8′ ( 600mm x 2400mm ) it will have more joints so a little more expansion and contraction will be allowed for as the expansion gaps are built into the board. The tongue and groove design means that no edges lift, although having said that I have never seen edge curl or lift on a properly fixed 8 by 4 OSB3 either.
Can I use Chipboard or overboard chipboard
You can overboard with OSB but I would recommend fixing into the rafters underneath the chipboard itself, personally I would always feel uneasy in case the chipboard rots at a later date due to condensation or other unforeseen means.
Never use chipboard as a roof deck for fibreglass laminate roofing, firstly it can rot with moisture exposure, but more commonly these days from the threat of condensation. Secondly the surface is too smooth and it delaminates.
Do I need a fall on my roof
No you don’t, a fall or slope on a flat roof is always very desirable for the longevity of any flat roof coating however it isn’t necessary. Often flat roofs are designed with the bare minimum of slope and over the years the roof settles under its own weight and the nice little slope disappears, any flat roof correctly installed can still be waterproof.
If you are going to lay any new flat roof without a fall you are going to have to make sure you do a good job though. If the roof holds a lot of water ( Ponding ) you may want to consider the additional weight the water will add to the structure of your roof. All that said why not do it right.
My resin is drying too fast
Always balance the temperature and catalyst ration with one eye on the weather, when the sun comes out suddenly the roof deck temp can accelerate drying times. Never mix too much resin and always have spare rollers and brushes available. Make sure you are using a slow catalyst in summer or LTP in hot conditions. See How to mix resin and top coat for full instructions.
My resin wont dry fast enough
Make sure you use at least 1% minimum catalyst and never more that 4%, in winter curing times will be greatly reduced, do not allow resins to become very cold before use.
Consider the use of a electric space heater below the roof if possible on cold days or a large outdoor pub style IR ( infra red ) heat lamp from above also works. Only use the lamp at a distance to give a gentle heat and remove it totally the moment the catalytic reaction occurs to stop trims warping with the combined exothermic reaction and the external heat.
Also in winter use a fast catalyst or cold cure resin see How to mix resin and top coat
I have ran out of resin can I use topcoat
This is highly undesirable, apart from anything else topcoat is far more viscous and this makes any wetting out hard work, pin holes will be a major concern. If its a very small section say < 1m2 then the answer is probably yes through gritted teeth, but no if the roof is prone to ponding or is near a drip. If you do go ahead, don’t forget to topcoat the top coat. Is it not easier to buy some more locally and chalk it down to experience?
I have white spots in my laminate
Unless it’s been raining the chances are you have rain or water contamination in your CSM matting, have you trodden damp feet onto you matting or has a drip from guttering above caught it? Always keep fibreglass matting bone dry and away from sources of damp or moisture even in storage. If its just a few small patches you can sand, resin and topcoat, or sand, apply another layer of CSM and resin and then topcoat. Never work in the rain or when there are chances of rain.
Can I apply resin on top of matting without wetting the OSB decking
The honest answer is yes you can but… technique is everything and the larger the roof, the more impractical it becomes. Your fibreglass supplier will not want you to do this as human error is greatly increased and this can result in leaking roofs, quite rightly they want you to install a perfect roof as this will lead to smiley faces all round and the bony finger of blame staying in its pocket where it belongs. If you are going to consider this method practise on a very small roof first, apply the recommended amount of resin and use a consolidating roller. You will need to kneel when applying the resin to keep a close eye of what your doing.
See the video of it being done an easy way to fibreglass laminate. Always use a consolidation roller.
My topcoat has pin holes in it
Did you put enough resin on when you laminated? If you didn’t resin starvation can make topcoating harder and more difficult to cover. Never skimp with topcoat either, it makes life hard work, make sure you apply it at the rate suggested in How to mix resin and top coat . Make sure you keep the new laminate clean before top coating, don’t tread dirt, grease, oil from driveways or any other contaminant onto the laminate before topcoating.
My topcoat has cracks or my topcoat has flaked
Do not apply too much topcoat, see the above link on suggested topcoat application rate. Keep topcoat and laminate decking away from sources of moisture like fog, dew or other sources of damp. Topcoat within 24 hours.
My topcoat is sticky or my topcoats is tacky
On very hot days the sun can evaporate the solvent too quickly, this can happen when the sun comes out unexpectedly in mid summer or when your working on too hotter day. The roof deck can heat up fast, you may even find in shady areas topcoat goes off normally but remains tacky in the sun. It will cure but may take around 48 Hours to do so. Just hope too much wildlife and leaves don’t stick to it. Try using LPT catalyst as described in How to mix resin and top coat
Can I topcoat another day or re topcoat an old roof
Yes sand the roof thoroughly with 60 grade aluminium oxide sandpaper first, use and orbital sander and a mask working upwind whenever possible.
Can I fiberglass over an old fiberglass roof
As long as its structurally sound yes, again sand with 60 grade aluminium oxide grit paper first, you may also want to clean with acetone and a cloth also. I have only had to do this once, it was for an engineer who had installed his own fibreglass roof 30 years previous. I asked him why he wanted it done and if it was leaking, he said ” no I just like it ” . It was very reassuring to see a 30 year old roof in such good condition.
I can see the fibre pattern of the fibreglass matting – can I get rid of it
Have you used enough resin and topcoat? Skimping on either of these will show the matting fibres up more than it should.
Wait a couple of weeks after installing a new roof, fibreglass roofs do not look there best when brand new, in fact it’s one of the few roofs that actually improve with age. A fibreglass roof will not look its best until 6 months to a year old in my opinion.
I have visible ridges in my fibreglass roof
Always keep any bandages across the joins on OSB boards away from a joint in the fibreglass matting. You may want to consider ripping the edges of the bandages to feather them out to make them less visible, this is also true for any edges on the main fibreglass matting laminate itself. Use a consolidation roller to flatten down any joints or double layers and sand thoroughly when dry. This has always given me a good finish.
I want a really smooth fibreglass finish to my roof
Laminate surface tissue on top of you main roof laminate with a thin coat of resin for good measure. When dry sand off any high spots or snags. If the resin and topcoat is older that 24Hours, sand with 60 grade aluminium oxide sandpaper first.
Can I fibreglass onto roofing felt or on top of bitumen felt
I have seen it done straight on top of roofing felt with mixed results, worst case scenario is that the bitumen bleeds through into the resin and inhibits the cure leaving you with a nasty mess. If this doesn’t happen it may go off fine and delaminate later leaving you with a shallow bubble of fibreglass like a blister, or best case it delaminates later and becomes a waterproof skin of fiberglass sitting on top of the felt which is the more common result with mineral felt. I say don’t bother, but if you do, try a coating of G4 primer first.
Can I overboard a felt roof for fibreglassing
I have seen this done with no problems, however this could be dependant of the quality of the decking underneath the felt which will not be visible. One slight concern may be any effect it may have on the breathability of the roof when finished, vapour barriers etc. This is more of a problem as modern house insulation programs increase condensation levels within the home without thought as to indoor moisture levels and the consequences.
Can I fibreglass onto Concrete
Yes make sure the concrete is in good condition, bone dry and free of grease or moss and seal with G4 as per instructions. If the concrete is in poor condition with loose top surface or a worn jagged top surface consider over boarding with OSB3 by screwing and plugging with an SDS drill.
Can I fibreglass onto asphalt
There is no straight answer for this I’m afraid, I depends how brave you are and how big the asphalt roof is. Unfortunately asphalt it bitumen based, and bitumen is reactive to almost all things except bitumen, and bitumen hates fibreglass.
The good news is some grey surfaced asphalt doesn’t seem to react, or reacts slowly allowing you to laminate to it without bleed through inhibiting the cure. I have seen it be successful, I have seen it bubble after time like a shallow blister, and I have heard that it may not cure leaving you with a sticky mess.
You may have better success if you try to render the asphalt inert first with G4 primer or by bonding a layer of CSM fibreglass matting to the asphalt first with a thin screed of PU adhesive. I have only done it myself once and it was successful straight to the asphalt with G4 but the roof was only 7m2. Since then I have always stripped the asphalt and overboarded, It should come up easily as long as the asphalt roofer used a soft fiber layer between the decking and asphalt. Your roof you choice.
Can I fiberglass onto lead
If were talking about fibreglassing onto small new lead drips or outlets you may be fine to wipe the lead clean with acetone to remove any surface oil form the face of the new lead and sand it with 60 grade grit paper. If it’s old lead that is clean of bitumen you can wire brush any oxidisation off first and prime with G4.
For bonding purposes sand, coat lightly with G4 as per instructions or apply a very thin layer of PU adhesive and tap a little bit of fibreglass matting gently into the surface of the PU. Always make sure surface are clean of contaminates such as oils.
If you are considering fibreglassing over larger the pieces of lead, expansion and contraction will become your enemy, the larger the piece of lead the more movement it will have. Lead expands and contracts a lot more than fibreglass, my advice is no in this instance.
Can I fibreglass onto asbestos or fibre cement sheeting
Yes is sticks like hell as long as the sheeting is clear of moisture,algae, moss, lichen or dirt. Personally I never work with asbestos or anything that looks like asbestos.
Fibreglassing around soil pipes, Vent pipes or downpipes
Some time soil pipes or vent pipes can be in very inconvenient places like in the middle of a large roof, this may mean you will have to allow for expansion and contraction of the roof deck and fibreglass against the pipe. One good tactic is to loose tape some layers of corrugated cardboard around the pipe before laminating to provide a gap around the pipe, simply take them out when the roof has cured. Again a pipe collar can be retro fitted or a water proofing silicone bead can be applied.
In normal situations simply wet out two layers of matting around the pipe to a height of 75mm or more when laminating then apply a little bit of flashband or flashing tape when the roof is fully finished to the join between the plastic and laminate. If its a soil pipe, soil pipe collars can be slipped on after the roof is finished by cutting it, opening it up and either silicone’ing or PU adhesiveing it into place.
What size of roof and how big does a roof have to be before I need to fit specialist expansion joints or expansion gutters to the fibreglass decking
There are two schools of thought I have come across, some say 50m2 and some say 100m2 . The truth is though there is no easy answer, expansion and contraction values will vary depending on, construction materials and quality, geographical location, exposure to moisture, sun, peak summer temperatures and lowest winter temperatures etc. My advise is work with the lowest square meter figure but use common sense, If its a very large roof you may consider EPDM as an option.
What weight of CSM fibreglass matting do I require for a flat roof
A very good question, one that I suspect you may not find a satisfactory answer to anywhere. If you look at different fibreglass suppliers answers go from 450g to 600g and sometimes 900g matting ( 2 layers of 450g ). Why are the suppliers giving contradictory advise? Well for a number of reasons and I shall try to explain why. Basically they are trying to give you a one size fits all answer to potentially complicated question where there isn’t a simple answer, and everyone loves a simple answer don’t they. The problem is expansion and contraction, movement of timber joists and decking.
I have found 450g to be fine for most small to medium sized roofs of good solid construction, you may want to consider 600g for a larger project or where frequent footfall is expected. On a large roof 2 layers of 450g may be advisable.
Of course it’s you who decides how thick to laminate your roof deck, you may find a lot of the fibreglass thickness stated by some suppliers are pure good willed guesswork as they have not seen the job.
Expansion and contraction of OSB and GRP fibreglass
Everything expands and contracts, plastic guttering and copper pipes are a good examples of this as they expand and contract a lot. Have you ever heard a creaking noise from under the floor boards as your central heating warms up or cools down? or hear the clicking of plastic guttering expanding in gutter brackets as the sun comes out on a hot day. These are examples of expansion and contraction that happen so fast we can hear it, so where are we left with OSB sterling board and fiberglass?
Luckily OSB or even better OSB3 ( or OSB4 ) have very desirable expansion and contraction properties because it was manufactured for the building industry for load bearing, resistance to expansion and contraction, and in the case of OSB3 and 4 moisture resistance too.
Fibreglass polyester resin or specialist roofing resins also boast very favourable characteristic’s so fibreglass and OSB are an Ideal pairing. The trouble is every single roof construction, shape, roof size and roof location is different, this where problems arise from specifying which grade of fibreglass mat or how many layers to use.
Lets give some examples of how size and shape affect expansion and contraction
The example below are true of any roof, not just fibreglass…
Roof 1 – Is one sheet of OSB 2.9m2 otherwise known as an 8 x 4 ( 2400 x 1200 ) as you can imagine very little movement in this board, you could probably laminate 1 layer of 300g matting on it and it would last forever, would 900g CSM make it last longer? Err..no.
Roof 2 – Is perfect square 50m2 , this would be 17 – 8 by 4 boards each with an expansion allowance of 2 – 3 mm between the edge of each board, here you can start to see that the roof deck and laminate is now much thinner in comparison to its footprint. Also the joists underneath will be susceptible to movement and expansion and contraction too, in fact the OSB3 will probably be more stable than the joists they are fixed too. You can also see that a it may be wise to use 600g or 2 x 450g matting to help compensate for this.
Roof 3 – is a ridiculous 500mm wide by 20m long like a covered walkway roof, the interesting thing though is that the expansion characteristic’s have changed. It’s unlikely to expand much in width but the length now has great capacity for expansion and contraction, and the whole roof is still only 10m2 . Now you can see the problem faced by fibreglass suppliers with a one size fits all suggestion.
Of course your roof could be an unusual shape and have all of these factors.
Other factors that will affect expansion and contraction
Large temperature swings, a southerly facing exposed flat roof could be exposed to -20 or +500C , moisture and humidity changes will also cause it to expand and contract. And then there’s movement caused by foot fall and poor constriction of the roof itself, nothing to do with expansion but another factor never the less.
Can I glue the OSB joints together with resin
Yes you can, but I don’t think its a good idea. Firstly you are stopping the contraction process by filling the joints with resin, and secondly I have never seen any glue that can stop expansion when it chooses to happen, especially on the edge of wood. Even brickwork and block work have maximum sizes before expansion joints have to be incorporated in case they split.
Does UV damage fibreglass roofs
Fibreglass roofs are extremely resistant to UV damage, some fading of the topcoat may occur over the years as the sun takes it toll but this is aesthetic, and can be re top coated if necessary.
Fibreglass roof leaks – I’ve read on the internet forums fibreglass roofs leak
A fibreglass roof just like any roof can be ruined by the installer, usually by not reading the instructions properly. I am a second generation roofer and have been in the roofing industry for over 30 Years now. I have seen every kind of roof leak prematurely and in 99.99% of the cases its human error. If installed thoughtfully a fibreglass roof will last in excess of 30 years, as I have seen them do this with my own eyes.
My fibreglass roof is leaking
Are you sure? I have never had one fail on me yet. Have a professional check the roof for cracks, splits and pin holes, silly mistakes or poor workmanship will almost certainly be the cause it if is. Near the edges of the roof check flashings, sometimes fibreglass trims are pointed with mortar and these are prone to cracking or hairline cracks, rain can then drive down the brickwork and in the cracks creating a leak. I recommend using a flexible water proofer as described in my how to fibreglass articles. How to fibreglass a flat roof and How to fibreglass a bay roof
My fibreglass roof is leaking the OSB boards are wet
Is the damp on the boards localised or generally all over?
Leaks are normally localised to a specific area unless something has gone really wrong which is very unusual, see above for localised leaks. Condensation is the prime suspect for large areas of damp. Condensation is moist air reaching dew point on a cold surface like the underneath of any poorly insulated or poorly designed roof. Unfortunately these days with the large scale insulation of old property’s, condensation is becoming more and more of a problem. If poor construction is to blame them a competent builder should be able to advise on changes to incorporate breathability into the roof design. Also see Condensation cures ( opens in new window )