# Groove on back side of door casing



## davej77 (Jun 30, 2010)

I would like to know the purpose of the groove that is found on the back side of commercially-made door casing.

I am about to make my own casing from 1x4 poplar and am wondering if I should cut a similar groove into my casing.

Thanks. I have enjoyed lurking on your forum before this post.


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## cabinetman (Jul 5, 2007)

There are a few reasons that are given. One is to stress relieve the wood. Another is to allow it to sit on surfaces that may not be totally flat, with the hopes the edges will be tight to the wall/jamb. And, whether it works or not is to provide some air. If it was just flat it may ride on high spots. The grapevine also says that it helps to stabilize the wood. 

I'm not sure what's actually true or not, but I machine the backs of my trim.


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## davej77 (Jun 30, 2010)

cabinetman said:


> There are a few reasons that are given. One is to stress relieve the wood. Another is to allow it to sit on surfaces that may not be totally flat, with the hopes the edges will be tight to the wall/jamb. And, whether it works or not is to provide some air. If it was just flat it may ride on high spots. The grapevine also says that it helps to stabilize the wood.
> 
> I'm not sure what's actually true or not, but I machine the backs of my trim.


Thanks for the advice. Do you use a stacked dado set to cut the backs, or is there a better way?


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## Ghidrah (Mar 2, 2010)

Pretty much all the store bought trim I ever saw was plain sawn and there's no guarantee what side of the stock the molding will be milled from, (convex or concave) side. 

The relief cut helps prevent splitting, most people nail trim at the edges, as the trim acclimates to the house it almost always shrinks. If it's nailed up when moist and shrinks as it dries it can split or tear.

Most casing problems come from plaster walls not drywall with joint compound unless the installer is a slob and doesn't now how to sand properly.


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## Hammer1 (Aug 1, 2010)

Primarily, the eased back allows the casing to fit tightly to the jamb and wall, since that transition is seldom perfectly flat. It can be done with stacked dadoes, router bits, shaper cutters. The last batch I made, I used the table saw and ran the casing askew to an old blade with a stock feeder, similar to the way you cut a cove shape. I thought it was faster and easier than other methods.


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