I am planning to have a new raised seam metal roof put on a summer lake house in Maine next spring. The main roof has a 11/12 pitch, and a gabled roof on the side is 8/12 pitch. It is not insulated. The interior of the roof is an open cathedral with the rafters and roof sheeting visible from the inside. We do not want to change the inside.
The building is not insulated, and we use this house for a week or two at a time in late spring, summer, and early fall. Even though we do not use this in the real heating season, we do use a wood stove in the spring and fall.
I plan to have the old asphalt roofing stripped off, so the base will be the 1 by planking on the roof.
I think it would be beneficial to add insulation under the new roof. The question is how it should be applied, and if it needs to be ventilated, what is the best way.
One way that we are considering is to cover the roof with 1/2 sheeting, then put 2×4 sleepers horizontally with sheets of 1 1/2 rigid foam insulation between the sleepers. Cover this with roofing felt, and then the raised seam metal roofing. The only ventilation would be in the gable ends of the building below the roof.
Adding ventilation or condensation drainage seems to have several variations. One is to put the sleepers and insulation on the old roof, run 1 by stringers up the roof, and cover this with 1/2″ sheeting, roofing felt, and then the steel roof. The stringers creat an air channel over the insulation. This channel may need a ridge vent. Since ridge vents are very expensive, we would like to avoid that unless it is really important.
I would appreciate any thoughts and recommendations that you have for determining if ventilation is necessary or desirable. And if it is, how it should be done on an installation such as we have.
Thank you
Allen
Replies
Vented roofs do two things. One is to get rid of interior moisture before it can condense on the underside of the roof sheathing and cause rot.
The other is to keep the roof surface cold, so snow doesn't melt and refreeze at the eave to form ice dams.
So the question is, do you keep the lake house heated all winter? Or do you want to make sure it will perform well if it ever is heated in the winter?
Mike,
Good question. We do not heat it in the winter now, so I think there is no problem with moisture.
However, metal roofs are supposed to last for a very long time, and I would like to make sure that it will work ok if things change and we decide to "winterize" the building.
Thanks
Allen
Go to the Timberpeg wqeb site to see how this is done as part of their timber frame "wrap and strap" system.
I'm doing one of these now. Over the ceiling planks/T&G, you first apply a 6 mil plastic vapor barrier, being careful to lap and seal the seams. Then apply a framework of true 2x blocking (not nominal 2x4s). You screw the blocking in with long "log hog" (Headlock) screws. Then put courses of 2" polyiso faced foam inside the blocking. Then horizontal by vertical 1x4 strapping, then plywood, then felt, then metal roofing.
That sounds more complicated, and still allows thermal bridging, more of an issue here in the cold north than down your way
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The 2x blocking is screwed into the timber frame (rafters, plates) at the roof perimeter (to create a nailing/screwing surface for horizontal strapping), not inside the roof field. If thermal bridging still is a concern, one probably could add a course of 1" foam over the 2" foam layers and blocking.
In my case, the blocking is 4" thick, and that is over 6"-8" timbers. The wood does have some R-Value of its own (R1.5/in for White Pine?).
But I'd have to agree (based on painful experience), the "wrap and strap" system is pretty labor-intensive. I don't think the OP would want to copy it exactly, but I offered it to show one approach to the general idea. Ceiling planking ...vapor barrier ... foam insul... sheathing+felt+metal, with or without some system of blocking and strapping (to create a ventilated roof) between planking+foam and sheathing.
With a timber frame, one of the advantages of the strapping is to create a channel for electric cable. Assuming you are not using SIPs with pre-formed channels (which I'd recommend for anyone who can afford them and who does not want , as I did, to do much of the work as a DIY project).
You do NOT want to be separating the foam between sleepers. You want a tight integrated insulation where no vapour passes through it. That cuts any need for ventilation by about 95% while increasing your insulation value 50%.
Strip roof
Apply tarpaper.
Apply two plies of 1-1/2" EPS foam ( the blue or pink stuff) sealing any open cracks and joints with spray foam from cans. stagger joints to prevent convection loss as well.
Over that, you can use plywood, then tarpaper the roofing, or alternatively, you can use sleepers for a cold roof, then the metal.
Some standing seam roof manufacturers require the firm plywood under, so if you want both the cold roof and the plywood and the solid surfacce for the roof then you could go foam first, sleepers, then plywood, then metal.
To hold the wood down through the foam, use GRK Reisser screws. They are available from Viking lumber in the midcoast region and come in all lengths you might need.
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Piffin and Tom,
One concern that I have is that I do not want a million nails sticking through the existing 1 x roof decking into the living space. If I was not insulating at all, I would put a 1/2" sheeting layer on top of the existing roof to give me the thickness to keep the roofing nails from coming through to the inside of the building.
The Timber??? website showed sheeting over the T&G. Perhaps this has the same purpose. I think this might still be needed even if I put sheeting over the insulation. What do you think?
Also....
I spoke with someone at a metal roof company who suggested that an alternative (to strapping for ventilation) is to use a strip of foam approx 1/2 x 1/2 in in cross section running down the roof directly under each panel of the raised seam roofing. This creates a convex surface for the panel, and enough space for condensate to run down and exit the roof. Have you heard of this? What do you think?
Thanks again for your input.
Allen
The Timberpeg literature shows two layers of plywood in the roof system, a layer of 1/2" just above the T&G and a layer of 5/8" just under the metal (with vapor barrier, blocking, foam and strapping in between). The 1/2" first course is not there as a nailing surface. It is there to strengthen the system (to meet code requirements in some areas.) It is not even specified in my plans which are for a non-residential space off a garage.
In their Wrap and Strap system, roofing fasteners go into the perimeter blocking or the timber frame. If the installer does not goof, they don't go into the T&G ceiling decking at all. But the T&G is 2" thick anyway.
I am not quite picturing what you describe there with the half by half insulation strip.You are using the term raised seam rather than standing seam.
Is this the type where the seam is a box rather than a flat stand up seam?The methods I described would not leave you with ANY nails poking through your sheathing because you would use screws locating them at the existing rafters to attach things.
On some jobs that can get tricky keeping track of where those are as you build things up and out, but you can have someone checking inside occasionally and it is easy to back a screw out and move it over.
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Piffin,
The man that described the 1/2 by 1/2 strip of foam did not give me a specific type or name. I envisioned it as a long strip of foam insulation, possibly from a roll which could be laid down on top of the roofing felt, from the ridge to the eaves, in the middle of where the next panel will be installed. Then when the panel is placed on top of it, it squashes the foam, but not completely, leaving the middle of the panel spaced away from the roofing felt. The panels are cleated to the deck on the edges before they are joined as is normally done. So the panels are slightly bowed up in the middle; leaving room for condensation to run down the underside of the metal roof.
I believe I should have been saying standing seam roofing. It is put up in about 2' wide panels and the seams, bent up at 90 degrees, are overlapped and crimped together. Once crimped the seams are about an inch high and 3 or 4 thicknesses of metal wide.
I will check with my builder on the nails. I think it was his suggestion to use the sheeting to keep the nails from coming through to the inside.
Thanks again,
Allen
I understand the metal venting now. That sort of foam is kind of like a sponge - very open celled and comes in various thicknesses, It come on a roll and is sticky one side or two sides according to how you order it. I would be leery of doing that without consulting themanufacturer of the standing seam product, because They often specify the type of underlayment needed.For instance, leaving that pucker-up can drastically change the way the roofing reacts to wind uplift foirces and whether your material stays on the roof iona high wind.Some manufacturers spec solid plywood for that reason. With others a sleep venting sytem under is fine.Oilcanning can be another concern, but my instinct s that this sort of detail could prevent that occourance.
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