Wood Movement Issues in a Solid Wood Bench Seat

      Advice on how to allow for wood movement in bench seating in a commercial space. December 26, 2006

We are starting a job that requires some benches made of flat sawn red oak. The top of the bench (seat) is 24" wide by 3/4" thick, made up of edge glued boards. The top will then be cut with a 12" radius on either end, with about 5' of straight in between.

Here is my concern… The bench then gets a 6" skirt wrapped around it. The skirt was originally made of short radiused pieces all glued around the perimeter, but has now been changed (after insisting that the whole thing will break) to thin layers bent and laminated around the perimeter, with a small v groove in between the individual boards. The top panel has no room to move, as it is captured inside the skirt. Anyone have an idea how much the panel will try to move? The finished benches will be inside an airport in California, but built in Vancouver, Canada. Is this bench top going to self destruct over the change in seasons?

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
I'd glue the front long (edge) grain only. Stop at the radiused ends and screw and plug (through elongated holes) the sides. Design the back of the seat to allow for movement.

From contributor T:
If the board is trapped inside a wood frame, it will push this frame apart when it swells. I would suggest a veneered bench top and a durable finish like epoxy.

From contributor C:
The 6" skirt sounds like it will be strong enough. I would try to make them as a separate oval ring and stagger the breaks, since it's about 14-16 ft around the perimeter. The center panel would be stable from fir 1" plywood with 1/8" oak veneers v jointed and vacuum pressed. Pattern route your 5ft by 24" ovals, insert with good glue and dowels through the skirt and into the field panel at 6 to 8" centers. This should last a long time.

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
What is the MC at the time of manufacturing? In Vancouver, we would commonly see around 9% or 10%. What is the RH in the airport? We would commonly see 30% RH or 6% EMC. So, at the minimum, the oak seat will be shrinking in width about 1% or a bit more, which is 1/4". The skirt cannot stop this. In fact, the skirt will hold the outside, so that the shrinkage will result in a crack in the seat. One way around this is to fasten the skirt across the width of the piece using slotted holes and screws and this will allow the seat (and screws) to move (if the screws are not overly tight). In fact, this floating construction is commonly used for tables and so on. In any case, do not glue the skirt across the width of the seat.

If you do, then keep my name, because you will be needing a consultant to diagnose the problem. This work is how I make a living and I have plenty of business, unfortunately. If you do make this without special care, and the wood splits, it is possible to repair the crack using a spline. But then, when the seat swells during the more humid weather, you will see warp, etc.

From contributor I:
Solid wood is out of the question. If the ends were square, you could use breadboard ends with slip jointery, but there is no way to do this with radius corners and have it look decent when it moves.

The only real option here is to use veneer. I have used solid wood stairway landings in the past, but these allowed and directed wood movement to occur under a stair riser where it wouldn't be seen. To attach the glued up panel to the landing, I ran a 3/16th router bit full width from the bottom, then ran a metal cutting woodruff key cutter through the router cut. This created a "T" slot in the panel. I then drilled through from the top into the slot, slid washers under the holes, and screwed through the washers - plugging the holes afterwards.

This is a lot of work to do something that a reasonable person wouldn't do in the first place - but sometimes we have to please architects and interior designers who are not reasonable people. If your customers are not reasonable people and will not agree to veneer, then I suggest you pass on the job.

From contributor I:
I just realized that if you were to make the skirt proud of the top, then you could run dados into the skirt to accept dimensional changes. It doesn't sound to me that those benches are designed for the comfort of the people sitting on them, anyhow - so maybe the designers will go for that approach. How much more uncomfortable would a 3/8" or less rise around the perimeter be?

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