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 problems.
Chipboard rot – 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 slope 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.
Ponding – 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 won’t 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.
Speed it up – An 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 any catalytic reaction occurs to stop trims warping with the combined exothermic reaction and the external heat.
It’s probably just easier in winter to use a fast catalyst or cold cure resin see How to mix resin and top coat
I have run 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 is a chance 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 and crazy 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, practice on a very small roof first, apply the recommended amount of resin and use a consolidating roller. You will need to kneel or get close when applying the resin to keep a close eye on what you are doing. Please read the section relating to this first that deals with how to lay fibreglass laminate.
See a video of it being done, an easy way to fibreglass laminate. Always use a consolidation roller. Please don’t be stupid and do this on large roofs, the correct technique can be seen in my article ‘how to fibreglass a roof’ linked at the bottom of the page.
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
Cracking – 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 topcoat 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 you are working on hot a 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 or more 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 and no. If the topcoat is under 12-24 hours old you “should” get a chemical bond to the previous topcoat – no problems. If the topcoat is new, but over 24 hours old, this can create a problem. Topcoat contains a wax that can make bonding to it problematic until this waxy finish has aged and is not longer waxy. This can take one to two years (my experience) so running out of topcoat and not getting it done within a day can cause problems.
If you do need to topcoat, your best bet is to sand the roof with 60 grade aluminium oxide paper thoroughly. Use and orbital sander and a mask working upwind whenever possible. Remove all the dust and clean with Acetone before applying the topcoat.
The same technique is used for top-coating over old or faded topcoat, or applying a different colour topcoat. Although you may want to clean the roof first with lightly soapy water to get ingrained dirt off.
Fibreglass guarantees
I know some branded GRP system’s guarantee their products for 20+ years etc. But here’s the problem, that’s exactly what they are guaranteeing, the product. They know there is very little chance of product fault, and a much higher probability of poor installation. Do you think they will be putting their hand in their pocket without a thorough and possibly biased investigation? Even then if you manage a claim check the small print, it’s probably just for GRP materials and not boards, labour etc. Keep this in mind before installing yourself, or be educated as a paying customer. To this day I have never had a problem with any roofing resin, guaranteed or not.
Just to be clear, I’m not bad mouthing or trying to stop anyone from buying branded fibreglass roofing systems, but just because a company slaps a nice rosette on the tin it doesn’t mean its time for triple brandy’s all round.
Can I fibreglass over an old fibreglass 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.
How do I repair my fibreglass roof if it has a split or hole
The same repair method as described above, if the wood underneath is dry there should be no problem. If the wood underneath is wet or damp make sure that any pressure applied on the fibreglass roof doesn’t squish water out and contaminate your fibreglass repair. Likewise if the sun is out, or heat underneath the roof causes the water to vaporise the steam vapour release can spoil the repair.
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.
Visible matting – Wait a couple of weeks after installing a new roof, fibreglass roofs do not look their 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 top layer of 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
Super smooth – Laminate surface tissue on top of your 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.
Should I screw fibreglass roof trims
I have read in one large fibreglass roof system brochure that they recommend ‘glue and screw’ , screws also have no place in fibreglass trims as far as I’m concerned, firstly they are prone to splitting the trims, and secondly screw heads do not sit flush to the surface of a trim like a clout nail, thus becoming visible through the finished laminate. Choose nails or sealant, or both, never silicone though!
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 fibreglass sitting on top of the felt. I say don’t bother, but if you must, 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, it depends how brave you are and how big the asphalt roof is. Unfortunately asphalt is bitumen based, and bitumen is reactive to almost all things except bitumen, and bitumen hates fibreglass.
Asphalt roof – 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, and 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 or Fixall. 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 previous asphalt roofer used a soft fiber layer between the decking and asphalt. Your roof your choice.
Can I fibreglass onto lead
If we’re 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.
Lead – 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 surfaces are clean of contaminates such as oils or bitumen.
If you are considering fibreglassing over larger 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, it 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
Sometimes 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.
Awkward pipes – My advice is to install a rubber pipe boot similar to the one pictured. It can be cut to suit the bore of the pipe and slipped over the top. A suitable pipe collar can be fitted afterwards.
In normal situations 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.
Pipe through a fibreglass roof – If its a soil pipe, soil pipe collars can be slipped on after the roof is finished by sliding it down from above, or by cutting it, and opening it up. Then use either silicone or PU adhesive to glue 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 advice is work with the lowest square metre figure but use common sense. If its a very large roof you may consider Torch-on felt or EPDM as an option.
What weight of CSM fibreglass matting do I require for a flat roof
A very good question, and 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 advice? Well for a number of reasons and I shall try to explain. Basically they are trying to give you a one size fits all answer to a 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.
CSM – 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 decide how thick to laminate your roof deck, you may find a lot of the fibreglass thickness stated by some suppliers are just pure good willed guesswork as they have not seen the job in question. Some will simply guess the higher the better, if one says 450g and the other 600g it may not be based on any real scientific reasoning. (or indeed any actual experience in the case of a lot of suppliers)
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 heard 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 then left with OSB sterling board and fibreglass?
OSB – 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 characteristics 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 seventeen 8 x 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 to. You can also see that 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, th