Metal Sheet Roofing Query

[iamlee2002]

Hi all, Being half way through building my garage walls im now starting to price up roofing etc.

I want to go for the metal sheet roofing but i heard you have alot of condensation with these, but you can buy the insualted type which are very expensive so thats out of the window.

My question if if i insulate between the joists would these prevent any form of condensation? If so how must i go about this eg line roof with membrane or some sort first?

I plan to also build a false ceiling in there to plasterboard so really need to stop any form of condesation forming before doing this.

Thanks in advance

Lee

 

[freddiemercurystwin]

What kind of metal roof?

 

[iamlee2002]

the box profile type roofing

 

[freddiemercurystwin]

You should use a rigid type of insulation and this should be placed on top of the timber joists not between. This is what is known as a warm roof.

Sandwiched between the insulation and the top of the joists you should have a 'vapour control layer' (a membrane that does not allow any moisture to pass through it) this will minimise any moisture entering the insulation. Above the insulation you should have a 'breather membrane' (a membrane that allows moist air to pass through it but does not let water droplets through). Just in case any moisture does pass through the VCL and the insulation it will be able to escape through the breather membrane where it will condense on the underside of the metal roofing sheets. It can then drip back onto the breather membrane where it will flow down the fall of the roof on top of the breather membrane and (now the important bit) this breather membrane must then allow any of this water to drain into the gutter at the edge of the roof.

So the roof makeup from the top down is as follows

Metal Roofing Sheets
Small air cavity say 20mm
Breather membrane
Insulation
Vapour Control Layer
Timber joists
Plasterboard

Gettit?

 

[iamlee2002]

thanks very much just the sort of answer i needed cheers buddy

 

[iamlee2002]

what is the best way to fix the sheeting to the joists? as the insulation will be underneath it when fixing it wouldnt it just crush the insulation boards?

thanks in advance

 

[freddiemercurystwin]

No the insulation won't get squashed, you need to be using something like this to support the roofing sheets: http://www.rigidal.co.uk/downloads/Corogrid_Brochure.pdf Not sure where you'll get a small quantity (assuming your garage is a normal sized garage) I have no idea what the smallest amount you can buy is, maybe some local roofing contractors will oblige. That's what happens when you deviate from the norm ie felt or epdm, things get more complicated.

 

[iamlee2002]

ah i see things getting more complicated now, i wonder if i could come up with an alternative to using them support brackets? thanks m8

 

[freddewar]

sounds like you may as well go the insulated panel route, it'll also be much easier for you to install. incidentally insulated metal roofs only need a breather membrane for high humidity buildings (swimming pools etc). Kingspan are the main supplier, TATA also manufacture 'composite panels' as they're known. You'll have to go through a distributor i suspect. SIG group do Kingspan (no, I don't work for any of them).

 

[freddiemercurystwin]

sounds like you may as well go the insulated panel route, it'll also be much easier for you to install. incidentally insulated metal roofs only need a breather membrane for high humidity buildings (swimming pools etc).

That's debatable and in anycase for the very small cost involved its foolhardy to leave out.

 

[iamlee2002]

hi all going to go for the composite sheets as ive found them a bit cheaper elsewhere, by the time i get the insulation boards etc may aswell get the pre insulated ones first as last. Works out roughly £16 a square meter thats pretty good for insulated boards aint it?

thanks for all ya advice

 

[freddewar]

£16 a metre for composite panels is very cheap, for a small quantity i'd expect at least £23 -25 + VAT. Is £16 for a rigid PIR insulation board with foil facing to both sides?

 

[iamlee2002]

hi freddewar yep thats correct. not sure if they are all standard thickness but overall thickness on these ones are 72mm