How to account for wood expansion when installing moulding?

Question

I plan on installing some board and batten frame moulding in my foyer. Basically, 1x4 and 1x6 pine boards in the shape of squares nailed/glued to the wall. My question though is how do I account for wood expansion. I know that wood will swell in the summer humidity and shrink in the winter humidity. All of the websites talk about accounting for this shrinkage as it could be up to 1/8" for the 1/6's. But they don't say how to account for this. How do I account for the shrinkage so that come winter time all of my moulding isn't full of gaps.

I've done some frame moulding in the past and it looks like the day I installed it, but I think I did it in the winter. Apparently it only tightened up and there's been no problems.

Answer 1

The boards shouldn't be attached to anything but themselves (glued & jointed together) & must literally float between the battens & rails. The battens, bottom & top rail should all be rabbeted to allow for full invisible movement of the boards, behind the battens & rails rabbets.

Answer 2

Short answer:

Put some splines in those corners! Based on the details of your situation, that is probably enough.

Long answer:

Wood movement can be a problem for frame miters because the boards will get wider and narrower with humidity changes. This means the 45 degree miters will try to stretch themselves to be slightly different angles. Here's what I would do:

1. Make sure the corners are solidly attached. Glue might be enough, esp. if you use glue size to seal the endgrain, but I would opt for a biscuit or a spline. If they are already together, you can saw one kerf in from the corner and glue in a spline that you won't see from the front. I've used this technique with an inexpensive ryoba (japanese saw) and a piece of veneer as the spline. The spline can be thin because wood is very strong in tension.

2. Avoid restricting movement with fasteners. It's easier to explain what you could do wrong: Lets say you put a screw through the top edge of the frame and into a stud. Then, if the board is 6" wide, you could put another screw 5 inches below it into the same stud. If you did this in a humid month, you might find a big crack down the middle in the winter because the wood shrinks, and the stud & screws are ripping it apart. So avoid putting multiple fasteners next to each-other in the same board. Instead, put all your fasteners along the inside edges of the frame, or all in the middle, or all on the outside...but don't mix and match. Also, nails will be more forgiving than screws, because they will bend a little with the seasons. If you use glue, same principles apply...one bead of glue down the middle is better than slathering the whole surface in this case. Google "cross-grain construction" for more info.

3. Avoid restricting the movement of any wooden boards you might be attaching it to. This might not apply to you if you are attaching to drywall, but if the wall was made up of vertical boards you might have another cross-grain construction problem that could be avoided by avoiding putting two fasteners next to eachother on the same board.