New roof on a cedar home... Some questions?

Darren · Mar 30, 2005

Try to find someone in your area that does the standing seam metal roofing. That eliminates your potential problems with corrugated metal and burning embers on the roof. If I recall correctly those roofs are good for a 100 years. Most of those roofs get put on commercial buildings but I've seen them on homes. When you get the estimate, you can pick the builder's brain about additional insulation and sealing out critters.

They may be able to do a sandwich installation with insulation between the T&G and the metal.

bugstruck · Mar 30, 2005

Eddie, I understand what you are saying but I want to point out a situtation someone could get into with this no-venting technique. If you have a cathedral type ceiling and install insulation and leave a small airspace above the insulation, with no venting provisions; that it a prescription for real problems in most colder climates. I've torn a few of these apart. The initial visible sign is water stains on the ceiling. Once you get underneath you usually have a mold/ mildew mess and sometimes rot to contend with. The worst I ever saw actually had visible water collecting on the underside of the drywall along the ridge. It was not a roofing material problem (leak). It was a condensation problem. We removed and replaced the damaged materials and vented that 1" airspace. Problem solved.

I just want anyone trying a no-vent solution to realize that no vent means no airspace too. Zero airspace. That's why either the jobsite or plant-installed (preengineered panels) liquid foam works so well when it can be applied to this type of application. It fills all the voids completely if installed properly. I assume you'd be fine with sheet foam if you fill the cavities tight. This type of no-vent construction is never done with fiberglass in our area as that material has some free flow air in it that can create some issues even with a fully filled cavity. At least that's how we approach it in Maryland and the surrounding states. Here no-vent = no fiberglass = no airspace. This keeps us out of trouble in these latitudes.

I have to say though that no-vent also = very hot substrate immediately below the roofing during summer. Theoretically, that could be a glue integrity issue for sheathing materials with long term exposure in southern latitudes. Another consideration is that by having the airspace/venting you do create a thermal break between the actual roof material/substrate and the insulation. Most people don't consider this and I don't think the energy people do much either, but insulation is a pretty good slow release heat sink. That's it's purpose. Ever wonder why your AC has a much easier time pulling indoor temps down after you get home on a cloudy hot day vs. a sunny hot day (humidity being similiar). The AC system has to over come the added stored heat in the ceiling insulation while trying to cool the house down to temperature. Point being the heat sink (insulation) is going to be super heated in any no-vent condition. It will without doubt drive energy costs up in the summer months. Winter would be negligable although you could apply the same theory to get a little winter heating savings, assuming no snow with the limited sun angle and duration.

Whew. That's a mouthful! /forums/images/graemlins/blush.gif /forums/images/graemlins/smile.gif

tomrscott · Mar 30, 2005

Darren,

Can you describe this "standing seam metal roof?" I don't think I know what that is.

The only metal roofing I am familiar with are the corrugated panels that you overlap and screw to a sub layer. They are pretty easy to handle, and quite durable if installed correctly.

Also I have a wonderful Hypertherm plasma cutter that would make real fast work of any metal trimming I might need. The roof would need to have some ~45 degree tapered edges for the push-out walls facing the deck on the south side (see first photo above), as well as trimming around vent stacks and a couple of wood-stove chimneys.

/forums/images/graemlins/smirk.gif

tomrscott · Mar 30, 2005

One difference here is that there is no drywall on the inside to trap that moisture. The 2x6 T&G is the the ceiling and it breathes. Actually it breathes too well right now, sucking wasps and things through the gaps.

If I put a 30# felt above the T&G ceiling, they will become part of the interior air space of the house. Any moisture above that felt is outside. The question then becomes whether I can seal the space inside the insulation "frame" sufficiently to prevent moisture accumulation inside the framing. The frame will sit on top of the 30# felt, then inuslation, OSB or plywood sheathing on top of the frame, more 30# felt, and then the metal.

I also have a question about the sheathing. What are the tradeoffs between OSB and plywood? Is OSB cheaper, or does it have more uniform density (fewer ply voids), making it a better choice, or is plywood better but more expensive?

Very interesting discussion. /forums/images/graemlins/smirk.gif

Darren · Mar 30, 2005

Following is a link that explains standing seam steel roofing. It's basically a system in which no fastners or penetrations in the roofing are exposed to the weather. The seams both conceal the fastners, lock the panels together and prevent water from entering.

http://www.bobvila.com/BVTV/HomeAgain/BTS-1205-MetalRoofBasics.html

bugstruck · Mar 30, 2005

Tom, The 2 x 6 lock deck planking you have or drywall makes little or no difference in the condensation issue I discused with no-vented roofs combined with air spaces. Just avoid the air space between the roof sheathing and the interior finishes and all should be well. A little venting in the open folds of the metal roofing wouldn't hurt anything. I do believe, you have this construction methods nailed down to near the last tack.

Regarding OSB.... not on my roof or any of my Customers!! It's fine for walls. Any properly installed roof is capable of developing a small leak over time. It usually takes some time or a major weather event for the leak to show up inside. So any roof that is developing a slow leak will wet and re-wet the substrate for many, many cycles before the leak becomes apparant. Plywood has a chance at surviving this for some sensible period of time. OSB doesn't. Even with the newer high performance resins it's much more prone to structural degredation than quality plywood under these conditions. Keep in mind that 3-ply 1/2" pine plywood is not the best either for roofs, but better than OSB. I'd try and locate some 4-5 ply fir plywood. It's worth the price. OSB is no bargain against 3-ply pine anyhow. If you don't believe me fill a 5 gallon bucket with water and throw a small piece of each in, cover it for a couple to three weeks and then look. Yowsa, that 1/2" OSB is over twice the normal thickness and you could eat it. /forums/images/graemlins/shocked.gif /forums/images/graemlins/grin.gif Did I say I don't like OSB for roof sheathing? /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Darren · Mar 30, 2005

Bugstruck is correct. OSB should never be used were it might come into contact with moisture. I've seen OSB underlayment around toilets and hot water heaters destroyed by leaks. Then the owner has the problem of ripping up the floor in addition to fixing the leak.

tomrscott · Mar 30, 2005

Thanks for all the explanation about OSB vs plywood. I guess I knew that about OSB, but it sure gets used a lot. Fortunately I'm in one of the timber capitals of the world, so you would think plywood costs shouldn't be that bad.

The standing seam metal looks very interesting. I guess I have seen it but didn't realize it was different from the stuff I've used.

That ridge vent shown on Bob Villa's site would never work here though, the winds we get on top of the mountain would drive rain right into the vent. I've seen exactly that happen with a vent like that on my workshop. It would either have to be about two feet wider, or completely sealed up.

If I can get that material rough pre-fabbed and delivered, I could finish cut it, but if it has to be formed on the job, it would be harder to do myself. In that case, I guess I could do all the pre-work, if I arranged it with the contractor, but that's still probably a lot more cost than doing it all myself.

I'm not really that cheap, just that poor! /forums/images/graemlins/grin.gif

EddieWalker · Mar 30, 2005

I'm not sure why you would put sheething on a metal roof. It isn't neccessary, doesn't accomplish anything and your just throwing money away.

Lets back up a little. What is your ceileing made of? Is it tounge and groove wood?

What is holding it up? Do you have trusses or beams?

I'm thinking that since your current roof is leaking and you want to switch over to metal, which I think is an excellent idea, then you will need perlins for the roof, not sheeting.

It also sounds like your getting advise from people familiar with stick frame construction, but not solid wood homes. There is a huge difference.

Is the roof currently vented? I didn't notice it from you photos, but they aren't very clear. What are you using for insulation right now?

Why couldn't you just strip off the shingles, install perlins, fill the void with solid foam insulation, cover it with felt and install the metal? The perlins will add at leas an inch and a half more space for insulation. The roof is already built, so all your doing is installing a metal roof.

EddieWalker · Mar 30, 2005

The question about OSB versus plywood for sheeting a roof isn't a good one. Most new home costrucion uses OSB. I hope your not confusing OSB with MDF. Every home build from the 70's and before used one inch pine for the sheething, and it was in every size imaganable. The gaps can be several inches apart in most homes. The sheething is just to provide something to nail to. 7/16's is thin for roofs but it's used. If your really concerned, then use 5/8's OSB.

As far as which one last longer if it gets wet, neither. The roof is leaking and you got other issues to deal with. Neither material is used for keeping water out. That's what the felt is for.

Paying more for plywood because it might last a year or two longer before rotting out if your roof is leaking really doesn't make any sense to me.

I'm not trying to be aurgumentative here, just offering my opinion and enjoying the discussion.

Eddie