Need suggestions for joining bench top to legs for wood movement

[photobug]

I am trying to finish a dinning room bench made from shoddy plans.  I just realized if I attach a 6 foot alder boards to these bench legs with pocket holes wood movement will likely affect the bench later on.

The leg structure is made of dimensional lumber and the top of the bench will be two 6/4 alder boards joined.  The pocket holes are already drilled into the stretchers.  How do I fix this to allow for wood movement differential of the legs vs bench top?

[Gary G™]

You could use wooden buttons or figure 8 clips and plug the pocket holes.

[fredhargis]

How wide is the top?

[photobug]

How wide is the top?

The top is not built yet but based on the two boards they will be 13-14" wide when joined.  The legs are 65" long.

Initially I was at a loss on how to make a groove for joining in the current 2x4 board stretchers but now I am thinking adding a piece of wood to the stretchers with grooves already in the added boards.

I am unsure if I could just add a strip of 3/4" plywood or does it need to be a hard or softwood?
How many screws over the length of 65" stretchers?

[Gary G™]

I have a solid wood console table I built about 15 years ago.
The top is 15” wide, 1” thick and 7-1/2’ long attached to the aprons with homemade wood buttons.
One on each end and 3 on each side.
It’s dead flat and has never had any problems.

[jteneyck]

Drill the pocket hole screw pilot holes all the way through, then widen them side to side so the screws can follow the wood movement.  Pretty common method for dealing with wood movement, used long before pocket screws were invented, but still applies.  

John

[Hank Knight]

I agree with John. A 15" wide board will move, but not a huge amount. Enlarge the pocket holes (or elongate them as John suggested). That should be enough to accommodate the movement of a 15" wide board.

[iclark]

I agree with John. A 15" wide board will move, but not a huge amount. Enlarge the pocket holes (or elongate them as John suggested). That should be enough to accommodate the movement of a 15" wide board.

The alder top is 6' long.
The dimensional wood stretchers are probably 5-1/2' long.

Without knowing what the dimensional lumber variety is, the difference in change in length is hard to estimate. Anyone have a good guess of the tree variety involved where the OP lives?

If the alder is quartersawn then the expansion difference between it and the longitudinal stretcher in the depth direction might be very small.

If the alder is flat-saw, then that might be a worst-case calculation.

Since this is for a dining room, it might be worthwhile to bring the base and seat-board into the house and let it acclimate for a week or 2 before attaching them together.

[jteneyck]

The alder top is 6' long.
The dimensional wood stretchers are probably 5-1/2' long.

Without knowing what the dimensional lumber variety is, the difference in change in length is hard to estimate. Anyone have a good guess of the tree variety involved where the OP lives?

If the alder is quartersawn then the expansion difference between it and the longitudinal stretcher in the depth direction might be very small.

If the alder is flat-saw, then that might be a worst-case calculation.

Since this is for a dining room, it might be worthwhile to bring the base and seat-board into the house and let it acclimate for a week or 2 before attaching them together.

Wood doesn't change much in length as it dries, almost zero. 

John

[iclark]

Wood doesn't change much in length as it dries, almost zero. 

John

Yeah, and I miss-typed when I said "longitudinal stretcher in the depth direction," I meant the stretcher for the base that ran in the depth direction: i.e.: cross-grain to the seat.

In the length direction, yeah, the change tends to be small, I just didn't try to figure out if it was negligible over 5' to6'.