A random thought/observation.
In the shower this morning, I noticed the coped joint on the wood molding around the shower enclosure, not the first time. Then a thought struck me. We are told to cope inside joints because they will open up with wood’s expansion and contraction. But wood moves perpendicular to the grain, not parallel, and molding joints are endgrain joins. There IS no in and out movement, only up and down.
Have we been handed down a big load of hooey?
What does everyone think?
Chris
http://acornhouse.wordpress.com/
a hobbyist’s journey
a hobbyist’s journey
Replies
The movement that you are prevented from "seeing" with a coped joint, has little to do with the trim wood itself.
The trim wood, as you noted, will continue to move with changes in humidity, and corners that are coped versus mitered will make little difference in the visibility caused by that movement.
But houses move in lots of directions, and this becomes particularly obvious at corners. And, of course, the trim wood is nailed to the framing -- and it's the framing that is moving when the house "breathes".
Most times, the movement doesn't change the placement of the corner posts in the framing. Instead, the walls have a tendency to bow in or out just a bit. This has the effect of changing the angles of the corners.
And when you change the angle of a corner -- the angle between the two walls -- the trim corner-angles change too. And if those trim corners are mitered, the miters will open up (either from the front, or from the back).
And when mitered corners open, you see the gap. Even if the angle only changed by one or two degrees.
If, instead, the trim corners are coped, you are not likely to see the movement -- even though it is still happening. This is because the coped piece has been "sharpened to a point", and the point stays in contact with the non-cut piece.
And it stays in contact (depending on the experience and accuracy of the person doing the coping, and the profile of the trim) even if the miter angle changes by as much as five degrees.
And that's no hooey.
Chris,
Coping a corner to allow for shrinkage is applicable more in the case of say a door frame, whose inside corners are worked with a narrow molding (called a sticking, or a stuck molding) outlining the panel. In the case where (say a house door) the rails are fairly wide, a mitered joint on the stuck molding would be prone to opening up when the rail shrinks, whereas a coped joint allows the end of the coped member to "slide" along the stile's molding as the rail becomes narrower.
Coping a crown molding is preferrable to mitering not because of shrinkage, but because even a well-fitted miter joint tends to open up when the mold is nailed fast- the nails pulling each piece of molding tightly to its wall, have a tendency to pull the joint apart. Again, a coped join allows the mold being nailed fast to slide along the other, without a crack opening up. Another advantage is that the crack when it does occur, is in the plane of the wall-- that is, you cannot see into it as clearly as you can when looking into a gap in a miter, which is aimed out into the center of the room.
Ray
Ray
I'm going to remember that and repeat it slowly to the trim guys in my area. I am always getting the ribbing for coping all my crown molding and lots of base pieces. After a while, its not that much more time consuming. It sure looks better to me.
Furniture- it looks great when done carefully and it looks great in any season here in humidity land.
Coping is a good skill to master.
This forum post is now archived. Commenting has been disabled