# Garage roof insulation?



## gasmansteve (10 Feb 2009)

Hi all
I`m in the process of insulating my workshop (ie. concrete garage on concrete base) and wondered how others have insulated a similar roof. Its a corrugated asbestolux type roof with angle iron beams spaced about 1.5m apart and each beam has a 25mm slot along its length. I had the idea of slotting some ply or such like through the slots and then filling in the gap between the ply and the roof with some foam sheeting?? does there need to be an air gap or can the insulation touch the roof?
Cheers
Steve


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## MikeG. (10 Feb 2009)

Steve,

I am now too tired to think straight.............but if this one gets bumped up to the top of the list tomorrow, or if someone reminds me, I'll have a look at it at lunchtime.

Mike


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## neilyweely (10 Feb 2009)

gasmansteve
Mike is the man to talk to, he helped me with mine.
I used kingspan stuff (celotex?), which was put in the gap between the roof and the chipboard I screwed to the rafters.
Works a treat. Another thing Mike told me to do, and I am SO pleased he did, is to raise the level of the insulation and panelling on the wall so it is about an inch above the concrete.
There was a leak the other day, which,as the insulation was not touching the floor, didn't soak upwards.

HTH

neil


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## Russell (11 Feb 2009)

I built a frame on the outside over the top of the corrugated asbestos filled it with loft insulation and covered the lot with metal roofing sheets cost me about £180 and solved the problem of a leaky roof at the same time.


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## Paul Chapman (11 Feb 2009)

Steve,

In my last house I had one of those concrete garages with an asbestos roof. The roof was fixed to the beams with bolts which had rubber or nylon washers. I found that the washers would perish or split over time, causing leaks. Maybe worth checking these and replacing if necessary before you fit the insulation to the roof.

Cheers :wink: 

Paul


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## MikeG. (11 Feb 2009)

Steve,

I remembered! The pills must be working :wink: 

Firstly, yes, you must leave a ventilated gap to the underside of your roof-sheet. The difficulty with this is that you also have to keep birds, insects and mice out of the gap, as well as allowing good airflow. All builder's merchants stock something called an "eaves comb", which should keep the bigger things out for a bit.....but wont stop wasps etc finding a way in.

If you put you insulation hard up against the underside of the corrugations, any condensation will run down the valley sides of the corrugations and accumulate at the very point where the insulation is touching. You should, therefore, space your insulation below this. To do this, you might consider battens strategically placed.

I would suggest 100mm of Kingspan.......but certainly don't go under 50. It is hardly worthwhile going to the trouble if you only use 50. If you are going to use a ply or OSB liner, it should be on the underside of insulation, not above it. I would have thought it would be well nigh impossible to do a decent job of this, considering the air-gap requirement and the need for continuity of insulation, if you tried to use mineral wool ("Rockwool" or similar).

I would invest in a foam gun, or at least in a can or two of the usual expanding stuff. Blow foam into all the inevitable little gaps, and you'll be amazed how it locks everything in place, as well as cutting out all the draughts.

Your wall insulation should, of course, be contiguous with the roof insulation.

Russell makes a very good point.........this is something I suggest to OPJ a few months back for his leaky asbestos roof..........but again, beware that you must leave an airgap under the top sheet. Insulationg on the outside would make the eaves junction between roof and wall insulation just a bit more tricky.

Sorry for the delay in responding.......I've got a streaming cold and so just ran out of energy last night. Hope this helps.

Mike


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## gasmansteve (11 Feb 2009)

Hi all
Thanks for the replies.
Not too sure I can achieve 100mm plus air space with the angle iron beams I have for my roof, might have to be a `best endeavours`. I even thought ofloft type fibreglass between the roof and any ply I fit but then again the air space would disappear using that. Thanks again
Cheers
Steve


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## ByronBlack (12 Feb 2009)

I hope no one minds me hijacking the thread, I'm in the very same situation at the moment and looking for solutions. I have 6x2 beams at 600 centres running the length of my shop (from side to side), the roof is OSB + felting + bitumen based corrugated sheeting. I've learnt that I must leave an air-gap between the insulation and the OSB, but I'm confused as to what I need to do for ventilation. Any idea's on what I need to look into?


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## MikeG. (12 Feb 2009)

BB,

when you say beams.........do you mean rafters? Are there any joists? As I understand it, you are looking at the underside of the OSB when you look up at your roof from inside?

Was this an old felt roof that was then overclad with something like Onduline?

Mike


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## ByronBlack (12 Feb 2009)

Mike

Yes, you see the OSB as you look up at the roof, the 'beams' are suppose what you would call joists, I'm unsure of the terminology, it's a flat-roof so these 6x2's go from one side to the other (with a slight angle for water run-off). The OSB is on top of these, followed by the felt.

The bitumen roof panels are these ones from wickes:
http://www.wickes.co.uk/Bitumen+Accesso ... nvt/240059

I was unhappy with the felt so decided to put these on over the top for extra protection as the felting didn't go too well and looked ugly as hell.


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## MikeG. (12 Feb 2009)

Right, got it now. 

You have done exactly the right thing overcladding with that product (Onduline by another name)........on battens I hope?

The OSB and felt provide a very effective vapour barrier, but unfortunately on the wrong side of the insulation. If you were to insulate between your joists with say 100mm of Kingspan, but at the lower part of the joist depth, that would leave a 50mm gap above the Kingspan and the OSB. What we need to do is to get a decent air supply to that gap.

What is your wall construction? Brick, block, timber, feather-edge etc......?

Do your joists overhang the wall, or do you have a fairly flush fascia board over their ends? If the latter, would it be possible to replace the fascia with something an inch narrower, and have on over-fascia vent running along the top to provide both the ventilation and the insect-proofing? This will only work properly if the OSB of the roof overhangs the fascia a bit.

Mike


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## ByronBlack (12 Feb 2009)

Mike - thanks for your help on this..

The walls are 2x4 construction with 50mm wool-type insulation between studs. The interior will be clad with 18mm MR-chipbboard, and outside it's clad with 19mm shiplap over a vapour-barrier.

The joists overhang by about 6 inches, the facias are attached to the ends and there is a soffit board underneath - I did think about maybe taking off the soffit and fitting a few lengths of grill vents with some anti-insect mesh - would that be appropriate or would it be better to vent directly into the facia? (which I would like to avoid, as it was a real pain fitting them).

The OSB + Onduline over hang the facia by about 2 inches for water-run off into the yet to be installed guttering.


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## MikeG. (12 Feb 2009)

ByronBlack":25iq6hax said:


> The joists overhang by about 6 inches, the facias are attached to the ends and there is a soffit board underneath - I did think about maybe taking off the soffit and fitting a few lengths of grill vents with some anti-insect mesh



Perfect!!! 
Just make sure you do it at both ends of the roof, for good cross-flow whichever way the wind is blowing. 

Mike


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## DoerupperRP (2 Oct 2018)

Hi just wondered how you got on with your roofs? I’m just trying to get mine done but don’t really know where to start. If I can post a pic I will to show you it. I don’t get the ventilation thing?!


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