Allowing for Wood Expansion in Solid Board Paneling

A woodworker puzzles over the measures needed to let V-groove wood board paneling expand and contract with seasonal moisture changes in a seaside home. January 2, 2012

I've taken a job making and installing solid wood v-groove paneling in cherry for a large room (approx 20' x 40'). The client prefers wider boards (avg. 7 in. width in random widths from 5" and up). I've purchased 5/4 stock, and re-sawn the boards to make 7/16" thick paneling. Tongue length is 3/16". I'm trying to get a handle on what I need to do to accommodate wood movement and to figure on what would be the widest board possible. The house is on the ocean front in RI, so high humidity is a factor.

Current moisture content of the boards is 7.5%. I ran an oven dry test on some samples to determine this. (Curiously, the 10" sample pieces shrank approx 3/32", which is about half what the Forest Products tables said they should). What I determine is that in an average July, the moisture content should get to be about 15% (with a possible high of 17%), which means that a 9" board will expand to about 9 5/32". There appears to be minimal potential shrinkage as the boards drop to a probable low MC of about 7%.

It would make sense that I should space the panels as I install them, perhaps as much as 1/8" (which may look bad in winter). I also should have expansion joints every so often to break up the overall runs (total expansion for the 40' wall should be about 9" or more). Any thoughts or suggestions will be appreciated. I don't have direct experience on a project this large.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor Y:
If you pre-stain and seal your boards before you install them that should minimize the movement somewhat and also make the spaces that open up in the winter less noticeable. Is there a way you can float each board in the field so that they can all move on their own and settle in naturally? Or are they getting nailed to the wall individually?

From contributor D:
Make sure you pre-finish and at least put a clear coat or two on the backs, you'll see a huge difference. We wouldn't do it without pre-finish.

From contributor W:

Trying to get a better picture is the wall 40' long and 20' in height, and the 1xs will go on vertically?

From contributor G:
I think you are planning ahead correctly. There is going to be a small, if any, shrinkage in use. There is potential (as you stated) is expansion. One thing you could do is check the EMC of the wood in the house with a moisture meter. This would give you a base to work from. Also, does the house have AC or are the windows open in July and August?

From contributor N:
Will the design permit some sort of moldings at the corners? If so, pre-finish it and install it in the same manner as you would flooring. Keep everything tight on install, but leave space at the corners for expansion.

From contributor R:
A little late to the party, but how much is 40' of solid cherry going to expand in the summer - a couple feet? I think this is why they invented frame and panel wainscotting.

From the original questioner:
Thanks for all the responses and suggestions. This is where I'm at right now.
The clients don't want frame and panel construction. They want a look that straddles rustic and finish, and v-joint paneling does that for them.

I made up a spreadsheet to track the movement. The overall increase from its current 8% MC to that of an average July (using RH and temp records for Newport, RI ) would take the MC to 14 3/4%, with a worst case of 17 3/8%. There's no AC in the house - windows are open in the summer. Given this, a 10" board will expand at least 3/16" and as much as 1/4". The 36' wall has a total movement of 8 to 11". Shrinkage is minimal -- 3/4" all told going down to 6% in the winter. The house is not used a lot in winter, and the temp is kept relatively low when they aren't there.

Some of this expansion will be taken up by putting a little space between the boards as I install them. All the boards will be pre-finished, front and back. I will also have several larger expansion joints every so often. I had suggested at one time of doing a variation of a "board-and-batten" look, with narrow battens between wider boards that would have a deeper groove in them so there would be no gaps at all.

The clients didn't really like the mockup I made of it, so I will have slightly thicker corner board with a 1/2" deep groove that will allow that much movement at the corners. A similar board will be at the midpoint of the wall, flanking an existing 6x6 post. Faux "keystones" will be installed above the doors and windows also with 1/2" grooves, so a 40' wall will have 5" of expansion with them. A spacing of 3/32" between boards should accommodate all the movement of an average summer, with the expansion joints taking up the extremes.

Also, whenever I use a board wider than 7", I will nail it with brads through the center face instead of the tongue and have a narrower board next to it, so there isn't more than 6" between fasteners. This should ensure that the movement will be more than the tongue and groove (3/16" ). Again, thanks for your help and interest. I'm hopeful that I've anticipated the worst problems.

From contributor Y:
You have to remember that if each board is nailed up separately they are going to move separately so as you said if you leave about 3/32 space installing this time of year come summer the joints will close but not blow the boards off the wall. Pre-finishing them will make the gaps less noticeable.

Another thing, have you considered looking at some of the older houses in your area? Maybe some old Victorians that would have lots of woodwork such as you are going to be installing and looking at the way it acclimated to its home? I will say that I have put plenty of tongue and groove up where I live on Cape Cod and never had a problem with it expanding to the point where it pushed itself off the wall or buckled from the humidity in the summer.

From Gary Katz, forum technical advisor:
I'd be really surprised if the MC of the wood ever came even close to 17% on the interior of the home. Even in Louisiana you wouldn't find that type of interior MC. I think you're taking all the right precautions, and maybe even going a little too far. I think your idea of expansion joints is really good, but no matter how well you pre-finish the material, it's still going to acclimate to the humidity in the house eventually.