27th of march 2009

Hi Al,

Good progress and looking a lot warmer up there now.

I had a question about steel roofing which I’ve been discussing on another site too.
Here in the southern hemisphere, steel roofing is used almost universally but never is it applied over a solid roof decking. Instead purlins are laid across the rafters, then roofing felt, then the steel is screwed to the purlins. I can’t seen any drawback to this really. It does provide more space for insulation ( not that Kiwis and probably Aussies make much use of this useful feature)

Is it a bad idea to put steel over solid decking? Is there an issue with condensation on the underside of the steel? Is it necessary? Why wouldn’t it be done here? cost?
Is there some structural reason for the roof decking or is it all a plan to use more wood by the mills?

Thanks for your posts and all your help.

there will not be any condensation since we have a “cold roof”. In other words, the attic is not conditioned, and our ceilling is airtight. the actualy roof’s solid decking is OSB covered with ice and water shield. Our building code tells us to have atleast the first 3’ of roof coveres with ice and water shield (to protect against icedaming, something that happens when you have snow over your roof thawing and freezing), we did the extra labour and did the whole roof. It will never leak.

The solid decking also provides a channel for air to travel. Natural ventilation, between the steel and the deck. this allows any possible condensation to be dried up. You may even see purlins on a solid decking to provide a bigger air gap(depending on the roof style).

I have heard about installing steel roofing directly on purlins and the rafters, but never seen it. Apparantly you could do it eather way. Cost is defenetly more with the decking. one possibility of why you dont see it too much on your end is hte fact that condesation builds up on the warm side of things (your exterior, our interior). we put roughly R-30 + on our ceilings. For structural, we also needed it for our trusses, so we didn’t really have a choice.

Not to mention it helps against the rain noise. It must get preatty noisy in a storm if you only have purlins…

I hope this answers a few questions.

Al

PS if it sounds complicated its that im headed to school and dont want to be late :D If you want me to clarify more, let me know. Thanks for your question.

-- BigAl, Ontario Canada

Al,

thanks for the detailed answers. Hope you made it to your class on time!

I know a bit about the building code in Canada, I’m from there, eh. Lived in Ottawa, Bowmanville, Waterloo and grew up mostly in Drummondville, Quebec. It’s probably why I have so many questions. I mean why are things done one way here and so different from the way I remember it being done in Canada. Which one is better?

Contrary to what you might believe, it does get cold(ish) here – not by Canadian standards, but it does get below freezing some days for 3 or 4 months and we do get some snow. The winter here is more like in Dallas, TX, where I also lived for 10 years.

But, it gets cold enough, that you want to build a warm house. The building standards in New Zealand seemed to be in denial about how cold and uncomfortable the homes are here in the winter.

So, I’m really looking for the right blend of efficiency in cost (for here) but quality of construction to make it warm, airtight and easy to heat and comfortable to live in. A lot of the materials used in houses in NA don’t even exist here. There is no OSB here and even if there was, it would be probably be $50/sheet. Obviously we don’t have to worry about ice-damming, at least not at low elevations. We get plenty of rain in the area where I will be building. 1.8m/year. Believe it or not, rain on the roof is not an issue. I have plenty of insulation in my current home ( added after I bought it ). We put 200mm of blown wool insulation in. Wool is naturally fireproof, absorbs humidity (it actually gives off heat as it absorbs it) and is abundant and cheap here.

Our house is definitely not airtight and that’s something I’ll be looking to do more of in our next home.

Thanks again for all your help. You are definitely learning your craft well!

yeah, your right, I didn’t think it did get to cold over there. I think I should travel more. I’m from Ottawa myself and might move back in a few years. I guess the reason for building the way we build is probably local availability of materials and/or the skills/traditions of local builders…

For example: in the town i live now (perth) we timber frame everywhere. One hour east, in Ottawa, you barely see it anymore. and there are barely any carpenters who can work with big timbers. You need to hire a skilled carpenter from Perth to come to Ottawa.

I suppose your plywood would be very expensive if you OSB would be that much (our 7/16” OSB is around 7-8$ for a 4’x8’ sheet). That could probably make people not want a solid deck for the roofs.

I suppose i might not really know why the two areas are so different, but I do know why WE build the way we do.

so I guess the right answer is: I don’t know, but this might be why.

Al

PS. Don’t forget Canada is a heating climate and Dallas Tx is most likely a cooling climate. This would probably affect the way we build our homes in relation to the local climate.

-- BigAl, Ontario Canada