3/4 Plywood span?

[rob.nester]

I'm curious if anyone knows what the max span is for 3/4" ply? I'm designing a new desk for my office and I'm thinking that I want to just use ply (probably 3/4 maple or walnut) and I wondered if I could make it 8 feet wide or not. I'm attaching an image of the design and would love feedback. My initial thought is that the sides and top would be mitered together, with the back panel sitting in a 1/4 inset groove in the sides and top, although I might consider pocket screws in those areas which can't be seen (behind / along the drawers, etc).



As far as weight support needed: It'll hold two laptops and two 21" computer monitors, some studio recording monitors, a recording interface, etc...

Thoughts? Feedback?

 

[gdcarpenter]

I think your span is a stretch. If you could add a 'stiffening' ledger along the front edge of the desk surface, butting into the side hanging shelves/cabinets, that would go a long way to stiffening things up.

 

[cpw]

Check the Sagulator.

 

[Bill Clemmons]

Rob, I agree w/ Garfield. Even w/ the side drawer units providing some support for the top (assuming they are attached to the sides and not hanging from the top), you're still looking at a span of around 70", +/-. I would suggest either laminating two layers of 3/4 ply (1.5" thick) together for the top, or adding a stiffener along the front edge as suggested above.

 

[Mike Davis]

70 inch span is too much to support 30 pounds in center, will sag 1/4 inch.

With 2 inch strip support it is acceptable. Support can be in center or front.

actually the support at the rear may be enough to keep it from sagging.

 

[JohnnyR]

I wouldn't go with the mitered ends. Too subject to damage without a rounded or champhered (sp?) edge. I'd edge-band it with solid wood and you could just glue it or pocket screw it. Think about whether you have to disassemble it to move it or to get it into the room. The sides will probably support it if there is no lateral bumping but I might add some vertical support, such as solid wood of 1 1/2", tapered or curved, rather than just edge band, if for nothing other than aesthetics.

 

[rob.nester]

70 inch span is too much to support 30 pounds in center, will sag 1/4 inch.

With 2 inch strip support it is acceptable. Support can be in center or front.

actually the support at the rear may be enough to keep it from sagging.

That's why I was curious; would the rear support be enough? I don't necessarily mind doing a 2 inch strip at the front, but if I can avoid it...

I wouldn't go with the mitered ends. Too subject to damage without a rounded or champhered (sp?) edge. I'd edge-band it with solid wood and you could just glue it or pocket screw it. Think about whether you have to disassemble it to move it or to get it into the room. The sides will probably support it if there is no lateral bumping but I might add some vertical support, such as solid wood of 1 1/2", tapered or curved, rather than just edge band, if for nothing other than aesthetics.

All edges would get either an 1/8" or 1/4" round-over, so I'm thinking that would handle the the damage aspect?

 

[Mike Davis]

I recalculated with 24 inch width, 70 span, 3/4 plywood, the only option is fir, not attached at rear. With a 30 pound load at center total sag is 0.05 inch.

with hardwood ply it will be even stronger and attached at rear is stronger.

i think it is ok

could always add a strip if needed.

 

[chris_goris]

I recalculated with 24 inch width, 70 span, 3/4 plywood, the only option is fir, not attached at rear. With a 30 pound load at center total sag is 0.05 inch.

with hardwood ply it will be even stronger and attached at rear is stronger.

i think it is ok

could always add a strip if needed.

30# load is not sufficient for a weight calculation for this application. This needs to be sufficient to support the weight of a person leaning on it, or worse yet, sitting on it. This edge will need a full width (96" ) apron of substantial height, my guess is at least 5" to support that load. But Im just a mechanical engineer, what do I know?

 

[Rwe2156]

That's a lot of weight. I would consider laminate 2 sheets of plywood together.

If you want a pencil drawer or keyboard drawer (you're gonna want a pencil drawer) you can't use an apron.

Setting the back in 12" or so will add a lot of support.

I looked at my old Lehigh-Leopold desk. It has a 1 1/4" thick top. Weighs a ton but zero sag. Back is set in about 8".

 

[Mike Davis]

 

[KenOfCary]

If it were me, I'd double up and either put a 3/4" MDF sheet under the nice top piece or some other cheaper 3/4" plywood underneath. Glue them together and you have a very solid work surface. A little decorative edge with a nice round-over and it will just look like a 1-1/2" thick top. MDF would add a lot of strength and weight if that isn't a problem.

If you use pocket hole screws, I'd just put them on the inside of the skirt. Nobody will see them unless they crawl under the desk. You never know when you might have to move the desk where the back will be visible.

Just my 2 penny worth.

 

[chris_goris]

Mike you have to use the entire top width (96") in your calculation because it is only supported on the ends. 300# static load , in my opinion, is a bare minimum weight. There should be some safety factor incorporated in this as well at least a factor of 2, which will double the deflection

 

[JohnnyR]

All edges would get either an 1/8" or 1/4" round-over, so I'm thinking that would handle the the damage aspect?[/QUOTE]

Round over will expose plys. If you use 1/8 or 1/4" edge-band, you'll avoid the "picture frame" look but you also have to band the front edge. I wouldn't use adhesive edge banding for the front as it will be subjected to pulling off.

 

[rob.nester]

Mike you have to use the entire top width (96") in your calculation because it is only supported on the ends. 300# static load , in my opinion, is a bare minimum weight. There should be some safety factor incorporated in this as well at least a factor of 2, which will double the deflection
View attachment 18867

Chris,
Just curious, would the desk back not provide a ledger support?

 

[rob.nester]

All edges would get either an 1/8" or 1/4" round-over, so I'm thinking that would handle the the damage aspect?

Round over will expose plys. If you use 1/8 or 1/4" edge-band, you'll avoid the "picture frame" look but you also have to band the front edge. I wouldn't use adhesive edge banding for the front as it will be subjected to pulling off.

Oh I fully intend to expose the plys.... this is a desk for my usage and I'm thinking of going for an "industrial" look, not fine furniture

 

[chris_goris]

On that side of the desk, yes, but you have 2 sides to it. Set a sheet of plywood up ripped down the middle on 2 saw horses supported on the ends and sit on it. You can even support one side of you want, but it WILL deflect alot on the unsupported side youre sitting on. You can dramatically improve this by adding (or moving ) stiffeners like the end caps near the opening for the chair at 60 or 70"

 

[Roy G]

In my old office, all the cubicles had 8' long work surfaces that were 30" wide. They were MDF with lamination top and bottom. The surfaces were supported at the ends and with a L-shaped bracket in the center. You could stand on them with no deflection. Could you make up a support in the middle that would come off the back?

Roy G

 

[rob.nester]

On that side of the desk, yes, but you have 2 sides to it. Set a sheet of plywood up ripped down the middle on 2 saw horses supported on the ends and sit on it. You can even support one side of you want, but it WILL deflect alot on the unsupported side youre sitting on. You can dramatically improve this by adding (or moving ) stiffeners like the end caps near the opening for the chair at 60 or 70"

Gotcha. Question: Would the idea mentioned earlier, running a ledger between the drawer frames - say 2" high, provide enough support or would I need a full length wider ledger up there?

My goal with this piece is to keep as clean and simple as possible...

 

[CrealBilly]

Heck it's just plywood so you could add braces where ever you like. You could even glue in egg crate type bracing made of lap joints and be able to support LOTS of weight. It's not real wood so you don't have to be concerned with wood movement at all.

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