Keeping Joints Tight with MDF Crown Moulding

Gaps can appear at MDF moulding joints for various reasons. Here, installers give several suggestions for holding those joints tight. September 4, 2005

I have a customer who states that some MDF crown and base boards that were installed on the job have shrunk a 1/4". They have told me that most of the gaps occur in the middle of the run and not in the corners? What is the best way to fix this problem? Is there anything that they can use to fill these gaps that will not cause a problem later if the material decides to expand?

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor P:
I would first check to see if the crown is nailed securely since the gaps are in the middle of a run. MDF can droop or sag between nails if the nails don't catch wood when nailing, making it appear as if it has shrunk more than it actually has. I would check basically the same thing for the base. If it isn't nailed into the studs, it can move much more and much easier with differences in moisture.

Several years back, I started nailing MDF crown with hand nails rather than with a nail gun. I still use the gun in the corners to quickly secure the joints in place. But everywhere else, I nail by hand. Most all the crown we install is 5 1/4". I will drive a 16 penny finish nail through the center of the crown into the plate to draw it tight against the ceiling and wall simultaneously. I have not had a single problem with shrinkage since I have been nailing it this way, rather than bottom nailed to wall and top nailed to ceiling. And I always take the time to locate studs and nail the base to the stud rather than just cross nailing as I have seen numerous so called trim carpenters do. Once it is securely nailed in place, use a good quality adhesive caulk (not construction adhesive) along the edges. You shouldn't have any more problems.

From contributor J:
This is coming from a wood finisher, but using panel adhesive and finish nailing to hold it in place until adhesive sets-up will do the trick. Use colored Dap to fill as it will expand and contract as needed. Tint it yourself with water colors.

From contributor B:
I would suggest cutting loose a 1/4" sliver of the moulding (if you still have some of it), tap it in with glue, and then sand it smooth.

From contributor T:
If you want a good method for splicing crown so this won't happen, try doing this when you splice butt joints. Take a little piece 1/4 inch luan about 2"x6" and 1/2 pin it and glue it to the back of the crown and create a tongue back letting it stick past the end of the crown. Now put some glue on the tongue and your ready to slip it in and pop it in place. Then brad it to the other piece after it's in place. Sand it after the glue dries and you will never get called back again on a splice.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
When splicing a joint, instead of straight cutting (butt jointing) try angle cutting the joint at a 45 degree angle so that the face overlaps and use adhesive and a nail. This way the joint will never gap.