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Chemist1961 02-10-2009 04:57 PM

Baseboard Joints /Butt or mitre
I am installing MDF baseboards in my family room and debating how to best do a single joint in a 20' wall. Anyone have any thoughts and past experience or Pro tips on keeping the joint invisible?
Will a change in humidity winter to summer cause any issues with MDF

DecksEtc 02-10-2009 05:18 PM

Mitre joint and dap.

Chemist1961 02-10-2009 05:32 PM

Do I bond the joint or just use dap to fill?

Willie T 02-10-2009 06:30 PM

Here's a link to another thread. Different installation, same idea.

Making a trim joint

Termite 02-10-2009 07:02 PM

You want what's called a scarf joint. It is essentially a miter that overlaps another miter. I'd highly recommend wood glue at the joint. With MDF, it is otherwise guaranteed to open up!

DecksEtc 02-10-2009 07:42 PM


Originally Posted by Chemist1961 (Post 228376)
Do I bond the joint or just use dap to fill?

Yes to the glue.

Also, it helps to shoot a nail through both pieces when they are in place.

sausagefingers 02-10-2009 11:00 PM

A good way to try and keep it from pulling apart is to splice it at a stud, so that you can nail both pieces together into a stud.

Bob Mariani 02-11-2009 07:45 AM

use a scarf joint where joints need to be strong and secure, the miter angle is usually very acute: 30 degrees or less. This creates a big overlap between the parts, thus giving the joint lots of mechanical strength, as well a lot of gluing surface area.

Chemist1961 02-11-2009 07:39 PM

Thanks guys. What about humidity changes. I have never had any mdf in the house....

Willie T 02-11-2009 08:02 PM

Normal humidity doesn't effect it too much, but direct wetting will. The junk swells when it gets wet.

Murphy 02-11-2009 09:50 PM

Dont use the spackle type dap on miter or butt joints on any wood transition. These joints will expand and contract with humidity. Always use caulk for these joints, interior you can use 20 yr. exterior always use 50 yr. Murph

Termite 02-11-2009 10:44 PM

I love working with MDF, but have regretted installing MDF casing, base, and crown in a number of rooms in my home. It is absolutely affected by humidity in the home (or lack of humidity). Moreso than wood trim in my opinion. I have no gaps in the joints of my pine and oak mouldings, but the MDF looks like hell 5 months out of the year.

Bob Mariani 02-12-2009 06:11 AM

Did you glue these joints? Of is it even a good idea to glue the joints?

TBFghost 02-13-2009 10:45 PM

I use a 22.5 scarf joint with LOTS of glue, MDF likes glue. I also put a backer behind the joint and nail to that...all my scarf joints look like the day i did them, the copes opened up a little this winter, but not too much.

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