New Flat roof - Single Ply membrane or Zinc/lead


29 Jul 2008
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United Kingdom
We are planning a smallish flat roof extension, which has been drawn up with a warm deck flat roof (apparently we are no longer allowed cold deck flat roofs in Scotland?).

The original design had a single ply membrane covering (sarnafil or similar) and I had been thinking of used Zinc or lead instead.

However, someone (not a roofer!) has suggested that I would need to ventilate between the warm deck and zinc/lead. Is this correct?

The flat roof will abut a wall of the house on one side but also join to an existing pitched roof on another side and I was told that might be where it gets difficult, in terms of venting where the lead/zinc is dressed to the pitched slate roof?

Appreciate its difficult to comment with a drawing/photo, but is the priniciple of venting the lad/zinc correct?
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I think technically lead does. Zinc no, as mentioned PM hardmetalking, he'd prob do a quote for ya too.
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Venting, cold or warm roofs is now regarded a must generally with most roof coverings.
Condensation occurs when warm moist air hits a cold surface. Whatever the roof covering when covered in snow it will be considerably colder than the inside air temperature. There are 2 ways to combat this--
1. Vapour barrier, my preferred method would be to bituthene the ply deck before the covering is installed.
2. Vent the roof! This could never be a bad idea could it?? Even with single ply membranes what happens to the adhesive with all that moist warm air build up?? How can it be that the roof has to be dry for the adhesive to be guaranteed when laid but not once it's covered.

It's not always practical but I'd always do 1 of the 2 if I can't do both.

This applies to lead, zinc, copper and stainless steel.
I'd be surprised if the pitched roof isn't vented. Like I said a good vapour barrier would solve any issues.
If you give me some rough measurements I can give you a ball park figure.

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