cedar seasonal movement - outdoors

[Brian Kent]

Some friends and I are making cedar owl boxes for the summer school kids. All of the sides are vertical grain direction, so the floor boards will have to be parallel to 2 sides and perpendicular to 2 sides. We will use some kind of sliding joinery or elongated screw holes for the perpendicular sides.

My question:
What is the seasonal shrinkage for western red cedar (leftover fence material). It is dry now.

In this part of San Diego we get temperatures from 30° to 105°, long dry seasons and then about 10-18" of rain.

I assume we need room for about 1/2" of seasonal movement over 28" width. Is that enough? Is 1" allowance better? To be clear, the wood will be in the sunlight and will get direct rainfall.

Thanks all.

Brian Kent

[Larry Edgerton]

Hey Kent....

I always figure a 1/4" per foot cross grain to long grain with cedar. So you are about right on with your 1/2".

Another tip: Overcut your miters. In other words if you need a 45 degree, cut a 46/46.5 degree and match at the points leaving the back of the miter with a gap. It will keep it from opening up at the point as the cedar goes through its moisture changes.

See, free wood will get you into all kinds of free projects........

Larry

[Larry Edgerton]

After a moments reflection I think what you should do is divide the 28 inches into 4 parts and make 7" pieces of T&G and pin each one in the center so that the movement is taken down to 3 1/2" sections. Leave your walls dry, and assemble with the floor pieces wet/damp and you will not have any problems with movement later.

No wait!.......

Just kidding, Larry

[Brian Kent]

No wait!

Good idea, but the walls are already jointed, glued, trimmed and sanded together.

I think I will make a slot along the wall in question, not by cutting a dado, but by adding wood on each side of a slot, and let the floor float on those 2 walls.

[Rich Engelhardt]

cedar seasonal movement - outdoors
A lot....

These joints started out in late May nice and tight.
Cedar fence stock (new) - glued (TBII) and stapled (1 1/4" 18 ga) finished inside and out w/4 coats of spar varnish.

[David Helm]

I think you're over thinking it. These are owl boxes. Think like a carpenter rather than a wood worker. Don't use miters, they always open up even if overcut. Make butt joints and don't worry about movement.

[Brian Kent]

Here's the "architecture" I'm copying, except they used plywood. At least this proves the owls don't care much about warpage:

http://sportsmansparadiseonline.com/Live_Owl_Nest_Box_Cam.html

[Cody Colston]

Brian,

The tangential shrinkage of Western Redcedar from green to oven dry is 5%. That would be 1.4" over a 28" width. However, your wood will never see either state, green or oven dry, so it will never see that much movement.

Assuming a 10% change in moisture content, you will have only .11" of tangential movement...not enough to be concerned about.