Stiffening Long Span of Plywood

[dlos]

Looking to fabricate a laminate countertop to "float" over a washer/dryer in a laundry closet. The top will span approximately 77" resting on cleats on 3 walls. In trying to keep this as light as possible, I'm planning to use 3/4" plywood but have not figured out the best way to stiffen the board to mitigate, or better yet, eliminate sagging. Doubling the plywood might help but will make it considerably heavier and I'm not sure it'll really do much with sag potential on such a wide span.

Further complicating the effort is that I want this top to be fairly easy to remove in order to service the machines, hoses, electrical, dryer duct. So keeping the weight down and ability to lift the top off is important.

There is only 1" space between the machines which makes adding any sort of leg difficult.

Some ideas I have for stiffening the plywood:

• A - using aluminum angle iron glued/screwed underneath
• B - steel or aluminum square tubing glued/screwed underneath
• C - plywood ribs with pocket holes (unsure of minimum depth for screws to work) - maybe 2" ribs joined to the underside of plywood would suffice?
• D - come up with plan to securely install a center leg/support between machines

Open to any suggestions or advice. Thanks!

 

[RickR]

I like to use a sandwich for long spans. Maybe 2 sheets of 1/2 ply separated by a core of 1x2. Screws and glue throughout.

 

[dlos]

I like to use a sandwich for long spans. Maybe 2 sheets of 1/2 ply separated by a core of 1x2. Screws and glue throughout.

That's a really good idea! Thanks, had not thought of going with different materials - stuck on "what I have on hand", but I think this might be a better way to go for sure.

 

[JNCarr]

Torsion box which is essentially what Rick is suggesting.

 

[bobby g]

I think torsion box is way to go. I made a thin one for my table saw outfeed table. I used a 1” square leg at the unsupported corner.

 

[junquecol]

A piece of steel bed rail, screwed about every foot to underside of plywood. Worst part is drilling holes in bed rail, as it's hardened steel . Screws need to be 5/8" long. I suggest using #8 sheet metal screws. Contact me if you need a piece of bed rail, as I rescue it from dumpsters. Handy stuff to have around for misc. projects.

 

[ssmith]

Use either steel or aluminum U channel to wrap the front edge. That keeps height gain from the stiffener to a minimum and may well hold with epoxy, so no need for screws. Of course, this will only work if the shelf is fairly lightly loaded.

 

[Rick Mainhart]

I would face the front edge of the plywood shelf with hardwood ... raised sufficiently to cover the raw laminate edge, and extending down as far as you can. This will stiffen the shelf as well as provide a nice finished edge. Poplar, maple or oak would provide a strong front edge.

Additionally, you can place a second piece half way back, spanning the width of the shelf, but short enough to allow the side support clearance necessary.

On both pieces, a gentle radius on any exposed edges will lessen the grumbles later when you remove and/or reinstall it. Less grumbles generally means less toss-induced repairs down the road! (Not that I have ANY experience in such repairs, mind you.)

Good luck with your project ... and post photos of your success!

Regards,

Rick

 

[mdbuntyn]

@Willemjm Any advise on this topic?

 

[chris_goris]

Torsion box for sure. "think" an interior door, not the cardboard ones made today per se but the generation prior, thin plywood facing with pine and whatever else they could find to space and glue the faces together, many times, cardboard tubing cut to thickness and glue in. Extremely light and rigid. on second thought, maybe you can find one of these and rework it into what you need, go look at your local Restore, they may have one!. good luck

 

[Gotcha6]

Torsion box for sure. "think" an interior door, not the cardboard ones made today per se but the generation prior, thin plywood facing with pine and whatever else they could find to space and glue the faces together, many times, cardboard tubing cut to thickness and glue in. Extremely light and rigid. on second thought, maybe you can find one of these and rework it into what you need, go look at your local Restore, they may have one!. good luck

If you are only going to use it for laundry (sorting & folding clothes, etc.) I would think a simple hollow core flush door would be sufficient. I use one frequently in the field as a plan table with favorable results. Deflection wouldn't be too much problem under medium loading. Try to find one you can cut down and reinstall the bottom stretcher on for length, perhaps at your local Habitat ReStore.

 

[Mike Davis]

If you go the torsion box route 1/4 or 3/8 plywood would be plenty and lighten the weight considerably.

 

[Willemjm]

@Willemjm Any advise on this topic?

The ability to easily remove and replace the top with free access to his laundry equipment really means something light.

Personally I believe the answer is in his last sentence.

Cleats on back and side walls. 3/4” Domestic veneer core ply, with 1 3/4” edging and a typical closet style middle partition using the same material.

Alternatively, the center support could be a simple bracket, available from Lowes or made from wood.

For us maximum unsupported span of 3/4 ply with 1 3/4 edging is 36”.

 

[tvrgeek]

One can imbed a steel bar on edge inside two layers of ply.
One can use a bracket on the top side.
Formica on both top and bottom will stiffen.
All plywood is not equal. BB is far stiffer.
One can make the front edge of angle iron.
One can weld up a very strong steel L lagged to the back wall using 3.4 steel pipe.
My shelf is just one inch SYP and does not sag. Might look at other woods as many are stiffer than ply.

 

[Gotcha6]

"There is only 1" space between the machines which makes adding any sort of leg difficult."

That should be more than enough to put in a wide stiff knee to the floor between them to help support the shelf in the middle.

 

[bowman]

1/4-1/2" aluminum angle stock, route a groove into the bottom of the shelf to inset one leg of the stock into the plywood. Drill holes into the angle to hold into place

 

[gamiller3rd]

Just mount an L bracket to the back wall between the two machines to support the center sag then hard wood edging along the front.

 

[Gofor]

Although 1" angle iron and aluminum will provide some support, my experience has been that both will , ocer time, sag significantly in a 6' or 8' length. If you can find a bed rail, they are much stiffer and harder metal, although you may need a cobalt bit to drill though it. 1 1/2" angle will sag less than 1". A center support coming from the top to the front edge will most likely elimiante any sag issues.

 

[JimD]

I think Dennis has given really good suggestions. A hollow core door will weigh less than a piece of 3/4 plywood. It is not very hard to remove the plywood skin from a piece cut off so you can glue it with construction adhesive back into the remaining piece of door - I have done it. Or you could make a piece for the edge from scrap. The advantages of this approach are weight, stiffness, and cost. Should look decent too. If all your source has are pre-drilled ones you could get a wider door to get a clear piece wide enough.

I think that will work without further stiffening but, if not, a leg of 3/4 inch wood between the machines should make it plenty stiff for clothes folding. It wouldn't even have to be fastened, just cut to length and resting between the machines.

If you really want to get scientific you can calculate the deflection of various alternatives, the equations are available on-line. Experience says a cheap interior door best fits your criteria, however.

 

[tvrgeek]

Although 1" angle iron and aluminum will provide some support, my experience has been that both will , ocer time, sag significantly in a 6' or 8' length. If you can find a bed rail, they are much stiffer and harder metal, although you may need a cobalt bit to drill though it. 1 1/2" angle will sag less than 1". A center support coming from the top to the front edge will most likely elimiante any sag issues.

A drop of oil on a sharp HHS bit is all you need. I have drilled literally hundreds of holes in old bed frames. I need to check Salvation Army store as I am running out. Made many a machine base, workbench and shelf of them. They used to be about $12. but even if three times that, way cheaper than buying the iron raw.

A closet center rod support bracket that uses a rod is only about 3/8 or less wide. Should fit fine. But again, my shelf has not sagged and it has been there since the house was built in '92.