I am going to be installing 3/4" x 3-1/4" prefinished oak flooring on a 3-season porch ceiling. The rafters are 2x10, 16" oc. I have two question on the best way to prepare these rafters for installing the hardwood. I am naling the boards horizontally dirrectly to the rafters. The ceiling willl be fully insulated with cathedral insulaiton. Because of the variance in lumber, the instalation area is not going to be a flat plane, there is going to be difference from rafter to rafter. To figure out how much difference there is, I plan on running lines acroos the top, middle and bottom of the rafters to see where the lowest point is and how each rafter varies from it's neighbors. Using the lowest point as the baseline, where necessary, I will use shims.

First question is, I don't know is how much variance is ok. I was thinking that anything less than 1/8" I'd let go and shim anything 1/8" or greater (numbers picked completely out of the air). But, with 3/4" oak and 16" oc, it seems like that is too much. I wouldn't be able to nail the board tight to the rafter.


Second question is, I am not sure of the best way to level the rafters . Since the strips are being installed perpendicular to the rafters I can’t just run furring strips across the rafters. I need to shim along the length of each rafter. What is the best way to do that? For this installation would it make more sense to shim under each board as needed during installation?

 

Your 3/4 boards won't bend much. I would probably shim as needed during installation to keep the tongues running straight.

If you find that you will need a lot of 1/8 shims, you could get a sheet of 1/8" tempered hardboard and tack that on the low rafters as a continuous shim. If you find you have a few rafters that are crowned the wrong way (crowned down instead of crowned up) you could power plane the bottom of the rafter so that you don't end up shimming everything down to match a bad rafter.

I would be tempted to omit the flat top framing until you have the sides completed all the way up. That way you could be sure you pick the best place for the two to intersect... and ensure that the top will be parallel and a good width that works with your 3 1/4" layout. You would want your first and last piece (the ones with an angle ripped on them) to be as large as possible...

I hope you won't be sorry that you don't have an air barrier between your insulation and your oak t&g. Warm air rises to the peak on cathedral ceilings and when it comes into contact with a cold roof it can create a lot of condensation in the rafters/bottom of the roof sheathing.

 

I would pull a string or shoot a laser down the length of the rafters at several points. Then you can see how much correction you will need. If it's only a few places here or there it's easier to shim as you install the flooring. If you have entire joists or large sections that need shimming I would nail a 2x4 to the side of the joists in the correct position so you effectively are shimming the entire joist (or wherever you install the 2x4). You can also shim on the face of the rafters if you want. Non compressible materials like tar paper, sheet plastic and strips of wood can be used.

 

There are a couple of issues you should consider.
1. Finishing that space into an insulated (all be it 3 season) space may require you to insulate it up to current standards. The energy map shows NH to have 2 zones and the 09 energy code shows either r-38 or r-49 depending. For cathedral ceilings there may be an area allowance where you can get by with less insulation. The applicable code is a local decision along with the need for a permit.

2. Once you resolve that your t&g approach does present a moisture problem as X mentioned. T&g is never air tight and even with a plastic vapor barrier it will leak sufficiently to move moist air into where it will create a problem. Here is a link with some related reading:
How to Build an Insulated Cathedral Ceiling | GreenBuildingAdvisor.com

Bud
Before I exit, as for leveling those rafters it wouldn't hurt to plane of a touch of the high (low) spots. No since having to adjust everything to a few bad rafters and 1/8" in a few spots won't make any difference.

 

I would use drywall shims, here's a link:
https://www.homedepot.com/p/Grip-Rit...SHIM/202090729

If you need to use a lot of shims I would purchase a pneumatic or electric staple gun.

 

If using random length flooring, how are you going to adjust to make seams of the ends of the flooring (tongue and grove) line up on the ceiling joists so you can install the flooring. I would rather re comment bead board at 16' lengths that you could control the joints better.

 

At 16” oc I am not worried about the joints that don’t fall over the rafters. I will glue those joints along with a piece of wood on the back for stiffness.

 

Just my spidy senses being raised, but gluing butt ends of flooring is not something that I would be comfortable doing for a client. I have done bead board cathedral ceilings before and have a feel for this. Also done hardwood flooring and have a feel for that as well. Good luck with your project, I fear it will be more involved than you think.