I just bought a 1830 Post and Beam house. I love post and beams and I am currently exposing all the beams in one of the rooms. I currently have a plan for the walls and have begun that work. Come spring I am going to tackle the ceiling. Currently there is 3/8 sheet rock nailed to the joists. The joists are 4x6's and they are 24 oc give or take. My plan was to pull the old sheet rock down and put new 1/2 sheet rock on top of the beams. Then I would either put 3/4 plywood on top of the sheet rock to make it stiff enough that you can walk around in the attic with out having to guess where the joists are. Once the plywood is down then I will put the insulation (8 Inch Poly Iso boards) back on top of the plywood.

My worry is that when I go to the attic to check on the slate roof, my walking around may crush the sheet rock that is between the joists and the plywood.

 

You never want to put a load on drywall! Why not install a plywood walkway in the attic?

 

I know you never put load on Dry wall. Thats why I am putting plywood over the top of the drywall. This plywood would be the walk way. The only way to get the drywall in would be to either cut each piece to fit between the joists, screw it up from the bottom to the plywood walk way, tape all the seams, figure out how to fishing the edge of drywall to beam, and them paint.

Or lay sheet rock on top of the joists, lay plywood on top of drywall, tape seams, paint. finished.

 

I don't think I understand what you are wanting to do

Won't you have insulation between the ceiling joists?

 

If you use 3/4" T&G plywood in a good grade, it should be fine. Want to be really sure? Install a second layer, with glue and short ring-shank nails to join the two together. Plus the flooring nails into the joists, of course.

 

so right now the Drywall is on the bottom of the joists. I would like to move the drywall to the top of the joists. Exposing the beams. IfI just move the drywall to the top of the joist, I wont be able to walk or move around in the attic because the joists will be under the Drywall and I wont be able to see it. I will put the plywood over the drywall so I can walk in the attic. A break down of the layers from the attic to the living space would go like this. Air - 8 inch polyiso insulation - 3/4 inch plywood - 1/2 drywall - 4x6 joist - Air

 

I wouldn't use 1/2" drywall. I would use 5/8" rock. That will really help ensure that it can maintain a level surface, given that it will be spanning 20", +, between joists. Plus added strength to resits pressure.

I'd forgotten about the insulation. Given that, the plywood between the drywall and the insulation could probably be 5/8" - maybe even 1/2" - provided you can get T&G sheets in those thicknesses. Then the 8" of polyiso. On top, 3/4" T&G ply, with construction adhesive under it and several spikes through each sheet to stabilize it on the insulation.

With 8" of rigid foam plus a layer of plywood, the drywall probably won't even register that you're walking around up there. The top layer of plywood is to give you a sturdy, level surface to easily walk on on your roof inspection trips. You really don't want to be planting your weight and motion directly on the rigid foam.

 

Getting full sheets of T&G plywood into the attic might be a problem.... or do you have enough access?
I've never used them but they sell 5/8" T&G that is either 2'x4' or 2'x8' designed for attics.

btw - I finally got my head wrapped around what you are trying to do
Any special reason you want drywall to show between the beams? wouldn't wood [maybe bead board] look better?

 

I think bead board would look too busy. Just plain old sheet rock painted flat white would look really nice. Know that you have an idea of what I want to do what are your thoughts on it?

 

When the old Drywall comes down the ceiling will be completely open to the unfinished attic. There will be just joist there so getting 4x8 sheets up won't be any issue at all.

 

My main objection to drywall is taping/finishing and painting between the beams. IMO wood would be less work.

I don't know why I didn't think about the joists being open, my brain was in gear - maybe the clutch was slipping

 

If the beams are 24" OC, a 48" sheet would span two bays without tape. If the is less than 12', one sheet of 12' drywall would cover without a seam in that direction as well. This might not be that bad to do.

I also agree with Nashkat, I would use 5/8" material due to the 24" OC distance.

 

To do this in 5/8 sheet rock proves to be substantially cheaper than to do it in bead board. The plan would now be 5/8 rock layed on top of the joists, 23/32nd OSB with tongue and groove on top of the sheet rock, 2 layers of 4 inch Polyiso foam on top of the osb.

 

And then a surface that you can walk on and set things on on top of the polyiso?

 

If you truly are concerned about cracking the drywall by adding weight on it from above, just do like this (from top to bottom):

OSB/plywood as a walking surface
Rigid foam insulation
vapor barrier (if you have a heated room in colder climate, and yes I know the foam will do some of this, but a VB is a cheap added insurance)
OSB (to support the foam)
1/2" furring strips on top of beam on top of drywall to create airspace
drywall
beam

All that said, I thoroughly hate drywall. It is a cheap building material that can't take much abuse, hard to work with (compared to wooden sheets) but I can see the contrast between the exposed beams and white drywall. Could look nice.

When we took our stick framed roof off and exposed the ceiling joist (2x6, 16" on center) in our house (early 1960s construction) in order to put on new roof trusses for a new attic and a new story, we took all the old ceiling plywood and spread it out on top of the 2x6 joists. We did get a flat work area (over 1500 sqf!), but we also did get cracks in the drywall under the 2x6s in two areas. Bugs the hell out of me today, hard/impossible to repair, So I will probably put a wooden 1x6 T&G celing up under this drywall to cover it all up. Will also allow me to properly put in a vapor barrier. And a white washed or painted T&G ceiling looks so much better than (****ty) drywall.