Lead Flat Roof Ventilation required?

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I'm drawing up a lead flat roof. (Its a square extension built off 2 existing walls with a parapet wall to the remaining 2 walls - see roof plan below)


Reading up on lead detailing seems to require that a lead flat roof (min. 1:80) requires ventilation of 50mm below the deck. The makeup for the roof I'm proposing is as follows:

Code 5 lead on
Geotextile membrane on
12mm WBP Plywood above
50m ventilation gap (formed with 50x50 battens) above
insulation sat on
VCL sat on
19mm WBP Plywood sat on
Joists with
Plasterboard beneath

I was planning on venting it from back to front (along the pitch) now assuming the above sounds ok I've found a detail (see below) for venting at the top of the roof where it abuts the existing house external wall, at the lower end I have a small parapet which obviously also needs to be vented too. Q1 Do I use the same detail or is there alternative?

Vent detail

Realistically I appreciate that any decent lead worker will be able to detail this ok but I'd like to understand how the detail works.

Q2Alternatively could I just vent the roof from side to side and use this vent detail at each side instead? and Q3 Do I have to vent round the rooflight as well?

Q4 Why does it need ventilation anyway, why is it different to a conventional warm roof?
 
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Answered my own queries with a 2 minute call to the Lead Sheet Association :rolleyes:
Q1 Yes
Q2 Yes
Q3 Yes
Q4 Because its lead and impermeable.
 
Hi. I may have miss read but is the decking 19 mm wbp ? not 12 mm.
 
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Hi. Yes, 19 mm under lead sheet (i guess 400 centre joists) among other things is needs to be stable when dressing the bays and good fixings for wood core rolls, that act as expansion joints between bays. On this link you will see a small porch roof, with rolls fitted indirection of fall. http://justlead.co.uk/news/ Good Luck
 
Hi thanks for that, some sweet looking details there, another question though, realistically is it overkill/the norm to be doing this venting malarky or is it standard practice?
 
Hi. In the extreme i have seen a large lead roof code 5, corroded, near to renewal in under ten years. This was a change of use situation(into a kitchen) The down lighters were the large type, which allowed the condensation generated in the kitchen to enter the void above the suspended ceiling and condensate on the underside of the lead sheet. We installed vents at opposing ends of the roof to delay the big spend. (and they have not yet)
The hot/cold roof argument is often ignored, but its a difficult call.
The detail you have shown is easy to achieve. Just leave the ply decking and 1" short of the wall / abutment, using say 80 mm screw attach a batten (2" x1") along the line of ply and then another on top giving you the 4" upstand. Once the roof has been laid with sheet lead, screw a 3" triangular length of timber to wall , incorporate insect screen and add sheet lead flashing. It may add a day to the job and cost an extra £80-00 in timber and wider lead flashing. Good Luck
 
Ok after a little more research of my own its all making sense, if the underside of the lead is not open to the atmosphere it can't patinate and is vulnerable to corrosion from condensation. Therefore why are lead valleys or lead lined internal gutters not ventilated should there be a possibility of condensation occurring on their underside which there may well be? I'm not playing devils advocate here just trying to understand the thoughts behind the system and (in reality) preparing for some inevitable questions from my client (who happens to be a surveyor (as in levels & buildings)) and also seems to be a bit of a know-it-all and thought I was mad when I mentioned it to him in passing about ventilating the underside.
 
Hi You are not alone with your thoughts. Historically buildings have been poorly heated, well ventilated and not insulated. Churches are a prime example, as you will never get a sweat on, perhaps to conserve the lead roof? As for the valid point Re:- gutters, valleys, flashing etc, in general a modicum of ventilation is present through tiles and slates surrounding the items. However when lifting sheet lead from tapered gutters, valley etc. it is common to find degrees of corrosion, perhaps reducing the thickness of the lead by 10 percent or more. But maintaining the weathering properties. The white powder and flake that are a result of corrosion are highly dangerous to health and should be treated with respect when removing. As for the client, i would point him in the direction of the codes of practice published by the Lead Sheet Association in an effort to reinforce your knowledge and design. Good Luck
 

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