How do I protect and insulate a ceiling built against the roof?

Question

My upper floor used to be a large attic. Sometime in the past the entire space was converted to living space: 2 bedrooms, 2 finished closets, and two unfinished storage "closets".

Much of the ceiling is built against the roof, and angles down towards the exterior walls. The walls and ceiling are all lathe and plaster. On the ceiling they simply nailed the lathes across the inside of the roofing members and then plastered over it. There is no insulation, and no barrier between the roof shingles and the interior lathe and plaster.

The roof is composition shingle, and doesn't have solid sheathing underneath.

I'm going to remodel the walls and ceiling this summer. I'll knock out the lathe and plaster and install drywall.

The roof will need to replaced within the next five years. It will require a full tear-off and installation of new plywood sheathing.

OK, enough background. Thanks for reading this far!

My questions:

1. What is standard practice when building a ceiling against the roof?

2. How will I protect my new drywall ceiling from being damaged when the roof is torn off? Should I install plywood or OSB under (above) the drywall? Or maybe just put off the remodel until the new roof is in place?

3. I want to put insulation between the roof and the ceiling. Can this be done when replacing the ceiling, or would it be best to wait and have it put in when the new roof sheathing is installed?

Answer 1

It doesn't make sense to do the inside before the roof has been replaced. The drywall will most likely end up getting damaged when the roof is being demoed and new sheathing is installed. The contractors may inadvertently step on the backside of the drywall and cave it in. There is also a big chance of some water damage. Keeping the original lathe for the roof replacement would also help protect your belongings on the inside. You definitely wouldn't want to be able to see the sky by looking straight up from the bedroom.

Once the roof is complete, then you can demo the inside. Since there isn't much space in between the roof and the ceiling, your best bet would probably be to have spray foam insulation installed. A closed cell foam will be a good choice here because it acts as a vapor barrier and will help prevent moisture from damaging your ceiling. Depending on your climate, the spray foam may not provide enough of an R factor. If that is the case, you can then install XPS foam panels on your entire ceiling over your rafters, and then drywall over that. That should be enough to keep your house warm.

Answer 2

It really depends on a lot of factors...namely your climate and how much space you have in the ceiling rafters. Ideally, you'd insulate with spray foam. That will give you the best r-value and act as a vapor barrier. You should be able to do it separate from the new roof but it may be a lot easier to just do it all at once. (It's a lot easier to toss lathe and plaster out the roof than it is to haul down bucket by bucket.

Answer 3

When it comes to what comes first both ways are acceptable but would be better if you do it all at once. Now when we speak about new roof cover material and procedures depend on type of covering material and there are so many options. But to put it in one word just apply standard procedures (if you use tiles go with the lathe etc.).Now once you cover the roof, taking that it is waterproof you can place rock or mineral wool between roof beams. You can fix those by using wire fixed in the "X" position and you just nail it to the beams, I’m sorry if this is poor explanation if you aren't clear just ask for details. Next step is to put barrier. Over the barrier you can fix either manufacturer substructure for plaster boards or just plain lathe. I think that covers everything...if you have any additional questions feel free to ask.