# Wood Expansion of Slats



## jwshop (Apr 21, 2011)

I've looked at a lot of plans for furniture with vertical slats such as between the rails on bed headboards and footboards and from what I can tell, there is rarely any consideration for the expansion or contraction of the slats across their width. I'm planning to build a bed this way. If I fit 1 1/2 to 2 inch wide slats with 1 to 2 inches between into tight mortises on a cool dry day, am I risking problems of overstressing along the length of the headboard when the seasons change? Thank you in advance for your advice.


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## Big Dave (Sep 16, 2006)

In my opinion, no. Since it is going to be a bed it will be kept in the home which for most intents and purposes are cooled in the summer and heated in the winter helping to keep the humidity at a constant. Large swings in humidity is what will give you problems.


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## mdntrdr (Dec 22, 2009)

Cutting smaller tenons on the ends of the slats will also decrease any stresses caused by expansion/contraction. :smile:


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## Brink (Nov 22, 2010)

The wood will expand/contract a certain percentage. On a 2" slat, the amount will be so little, there shouldn't be a problem.

If you glue up a dozen pieces to make a 48" wide board, the total amount of movement becomes a big problem.

On a project like you mentioned, you can use a narrower tenon, or, make the mortis slightly wider than the tenon, and only apply glue to the center of the tenon.


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## cabinetman (Jul 5, 2007)

Big Dave said:


> In my opinion, no. Since it is going to be a bed it will be kept in the home which for most intents and purposes are cooled in the summer and heated in the winter helping to keep the humidity at a constant. Large swings in humidity is what will give you problems.


I agree. Slats that width aren't as subjective to E&C especially if they are thin. I wouldn't do anything different in figuring the M&T's. Go ahead and assemble.












 







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## TimPa (Jan 27, 2010)

the accumulative affect of many slats are just like one board, and will expand and contract. we use the rubber balls in the slots of small runs to pick up what they can. you may need more consideration if the slats are running the full width of head/foot board, deeper slot mortise for sure.


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## jwshop (Apr 21, 2011)

Thanks everyone for your expert advice. TimPa, you hit on my main concern - the cumulative affect. I'm not familiar with the rubber balls you mention. What are they, and do they go into the mortise beside the slat to cusion expansion across the width of the slat, or on top of the slat to cusion up and down? Also, if I use tenons on the slats, is a deeper mortise even a consideration?


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## Big Dave (Sep 16, 2006)

Anothe way to look at it is compare it to a wood floor which I do a lot of. Over a 20' wide room you only need to give it a 1/4" gap all the way around. In my experience expansion is usually never the problem unless you let the humidity in your house get to 60% or above which most people can't stand. Shrinking, especially in winter is a bigger concern because humidity can easily drop to 30% or lower. A humidity level of 40 to 45% is ideal. Great for all the woodwork in your home as well as your skin.


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## cabinetman (Jul 5, 2007)

jwshop said:


> Thanks everyone for your expert advice. TimPa, you hit on my main concern - the cumulative affect. I'm not familiar with the rubber balls you mention. What are they, and do they go into the mortise beside the slat to cusion expansion across the width of the slat, or on top of the slat to cusion up and down? Also, if I use tenons on the slats, is a deeper mortise even a consideration?


Here's my suggestion. You'll get a difference of opinion on forums, which can leave you in an undecided state. You never know who knows what. Anyway, I'm not saying I'm an expert, but I'll tell you what works for me. 

For those projects with a lot of slats, I machine a groove full length. Either the slat will fit into the groove as is, or the ends may have to be sized to fit. Then in between each insert a wood spacer cut from a single piece (so the grain is continuous). You can glue it all together.

With a beefy frame, I've done them with just gluing in the spacers, and I've done them with gluing all the pieces. I never had a problem with expansion from slats, even done with doing individual M&T's.












 







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## TimPa (Jan 27, 2010)

jwshop said:


> Thanks everyone for your expert advice. TimPa, you hit on my main concern - the cumulative affect. I'm not familiar with the rubber balls you mention. What are they, and do they go into the mortise beside the slat to cusion expansion across the width of the slat, or on top of the slat to cusion up and down? Also, if I use tenons on the slats, is a deeper mortise even a consideration?


I think they're called space balls, 0.260"dia that fit into the mortise on the long grain side of the slats. again we use these on short runs ~20-24". our slats are .25 thk x .75 wide. you'll not get much exp/cont along the length. 

i should've also said that it's not the contraction of the slats that ruins your project as much as the expansion, which can cause your joints to blow apart if not well done.


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