Joints for Large Window Casing

      Trim installers discuss ways to reinforce the corner miters when casing out a window with wide material. May 6, 2007

I've got a 4000 sqft house to trim. Nice big casing, 1 1/4 x 5. These casings have plenty of meat for biscuits or big pocket screws. For the last 5 years my trim carpenter friend has been using big screws from the head casing across the miter. He thinks they hold better on the big casings versus the biscuits. You don't get the teeth marks from miter clamps as well. Anybody using big screws instead of biscuits?

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor B:
The part of the miter that usually opens is the inside. That is hard to hold with screws. The biscuit should be placed as close to the inside (short point) of the miter as possible. On a 5" casing I would use two. As far as the miter clamps, back out the middle two screws on both jaws. Now you will only have two small holes on the edge of the casing. You will already have 20 + nail holes to fill on the face of the casing. Nothing pulls the joint together like the miter clamps.

From contributor D:
The miters will open on the inside if the material dries after the miter cut. It will open on the outside if it swells from the time the miter is made. 5" wide is too wide in my neck of the woods - this would be done in two pieces.

We always laugh at the newbies that come to town selling big casings by saying it is all "just one piece - half the labor." Then the builder buys a bunch and the carpenters do their best, but all the miters open 1 to 6 months down the road.

From contributor B:
In our part of the country (N.E.), we use a lot of 1 x 4 casing. Never have a problem as long as the wood is dry and the house conditioned (a/c summer and humidifiers in the winter). We biscuit and clamp everything.

From contributor L:
We screw almost all casings together. For thin casing we set the saw for slightly less than 45 degrees. This causes the legs on a door casing to spread out about an inch. When this casing is nailed to the jam, the spread puts pressure on the inside of the miter, while the screw holds the outside of the joint. With wider casing, you have to use very little, if any, spread. A joint that is screwed, even if only on the outside, is going to be tighter than one that is just pressed together.

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