How to strengthen a 19mm oak panel

[ol_london]

Hi
I am making a basic desk out of a 19mm solid oak edge glued furniture panel. The desk will be 1500mm x 600mm. The legs are probably something simple like hairpins or metal box section with a fixing plate.

My concern is that if the user places any weight on the centre of the desk it may sag or even break as it’s only 19mm thick.

I am thinking of adding a few metal straps at maybe 500mm centres across the under side about 1mm thick (30mm wide, screwed full width) to cross brace the panel.
I’ve never used such a thin panel...will these braces be needed and if so is 1mm thick enough metal to do the job. Is there a risk of the wood warping with these straps on? Any advice please! Many thanks to all.

 

[Simon89]

I would think 1mm is far to thin, my 1m steel rule is 1.5mm thick and 40mm wide and is quite flexible

 

[ol_london]

I would think 1mm is far to thin, my 1m steel rule is 1.5mm thick and 40mm wide and is quite flexible

Thanks. There is a 4mm thick version too. 1mm is neater looking but no point if it’s useless.

 

[marcros]

when you say a desk, is this planned to be a sheet of material with a leg on each corner? if it is any size, I think that the aesthetic of this will look rather feeble, let alone the potential for sagging. If there is any weight on it, it will make the situation far worse too, it is surprising how much monitors weigh, for example.

I would look at creating a torsion box of sorts, or using the traditional methods with aprons.

 

[Rorton]

Yep as above, perhaps treat it like a table, so 4 legs joined by 4 aprons to make a frame, then the top can sit on this and be supported all round

 

[TheUnicorn]

As far as I can see even a 4mm strap really wouldn't be sufficent, you'd need to attach the strap edge-on to have any real strength, or an L shape cross section. that assumes that you are determined to go for hairpin legs, I think the traditional table construction with aprons might offer more support and do more to stop warping

 

[ol_london]

Thanks, all valid points. What about the straps causing warping.
I can’t tell if they will help to hold the wood together and prevent movement or make it worse.

 

[Dee J]

A thin panel and hairpin legs might be OK for a small low side table, but is too flimsy for a desk. Just the leverage of the leg length on short fixing screws is a fail point if anyone leans against it. Think it's best to plan an underframe / leg assembly in wood or metal that is a strong stable structure. All the thin top has to do then is lay on top, fully supported, with fixings which allow for the natural movement of the timber.

 

[ol_london]

A thin panel and hairpin legs might be OK for a small low side table, but is too flimsy for a desk. Just the leverage of the leg length on short fixing screws is a fail point if anyone leans against it. Think it's best to plan an underframe / leg assembly in wood or metal that is a strong stable structure. All the thin top has to do then is lay on top, fully supported, with fixings which allow for the natural movement of the timber.

Thanks, this is great advice.

 

[Jameshow]

How about at the back putting a decent length of timber say 4" deep. Perhaps an off cut if your oak?

On the front and side use an 3/4" inch square section to double the depth of the board. This will give the appearance of s much heavier and more expensive desk. Also I think you need stretchers going front to back under the top between the legs to stop it sagging there too.

Cheers James

 

[Sandyn]

I recycle old oak furniture with large panels and they are incredibly strong. I just tested a 900mmX220mmX19mm panel and loaded it with 62Kg. there was less than 2mm deflection at the middle. I wouldn't be concerned about the risk of the oak breaking, even with a 1500mm span, but if it was loaded excessively, there will be a deflection which may cause the thin legs to splay, then the problems would start. It sounds like a 'functional' bit of furniture if you plan to have metal box section legs?. If you were really concerned about it bending or breaking, I would make a metal frame of box section, or angle, but pay attention to strengthening the legs at the corners. Will you be welding the metal parts?
Flat metal strips will do nothing to strengthen across the width, unless it was a substantial chunk of metal. You need a box section or angle section to get a vertical component to add strength. There are many ways to do this with wood as well, as suggested in the replies above.

 

[ol_london]

I recycle old oak furniture with large panels and they are incredibly strong. I just tested a 900mmX220mmX19mm panel and loaded it with 62Kg. there was less than 2mm deflection at the middle. I wouldn't be concerned about the risk of the oak breaking, even with a 1500mm span, but if it was loaded excessively, there will be a deflection which may cause the thin legs to splay, then the problems would start. It sounds like a 'functional' bit of furniture if you plan to have metal box section legs?. If you were really concerned about it bending or breaking, I would make a metal frame of box section, or angle, but pay attention to strengthening the legs at the corners. Will you be welding the metal parts?
Flat metal strips will do nothing to strengthen across the width, unless it was a substantial chunk of metal. You need a box section or angle section to get a vertical component to add strength. There are many ways to do this with wood as well, as suggested in the replies above.

Thanks, really helpful. I’m not making the legs myself. Yes box section feels like a safer option than hairpins. The straps idea was a belt and braces in case the panel staves split, just to help hold them together to stop a collapse! Just being a bit paranoid but I like the idea of a few metal straps across the staves to help them stay tight.

 

[ol_london]

How about at the back putting a decent length of timber say 4" deep. Perhaps an off cut if your oak?

On the front and side use an 3/4" inch square section to double the depth of the board. This will give the appearance of s much heavier and more expensive desk. Also I think you need stretchers going front to back under the top between the legs to stop it sagging there too.

Cheers James

Thanks, I am looking for a thin minimal effect hence not wanting aprons, stretchers and all that traditional support.

 

[Ollie78]

Thanks, I am looking for a thin minimal effect hence not wanting aprons, stretchers and all that traditional support.

Those things are there for a good reason.

I can suggest one way to " add lightness " to a design in to bevel the edges on the underside.
If you put stretchers further in, and add a nice mellow bevel this can give a lighter appearance while actually being strong.
Look at good Danish style mid century modern stuff, they did this a lot.

Ollie

 

[ol_london]

Those things are there for a good reason.

I can suggest one way to " add lightness " to a design in to bevel the edges on the underside.
If you put stretchers further in, and add a nice mellow bevel this can give a lighter appearance while actually being strong.
Look at good Danish style mid century modern stuff, they did this a lot.

Ollie

Thanks, nice idea.

 

[recipio]

Really, any sag, even under a heavy load is a design failure. Did you try www.sagulator.com to calculate it ?
One idea that springs to mind is to run a few saw kerfs under the table top and glue in slips while clamping the table top to a flat surface. A torsion box gets its strength from the glue lines, not the materials themselves. ! Never tried it so it would need testing on some scrap.

 

[ol_london]

Really, any sag, even under a heavy load is a design failure. Did you try www.sagulator.com to calculate it ?
One idea that springs to mind is to run a few saw kerfs under the table top and glue in slips while clamping the table top to a flat surface. A torsion box gets its strength from the glue lines, not the materials themselves. ! Never tried it so it would need testing on some scrap.

Thanks. A tiny bit of sag would be acceptable under heavy load for this. I’m not looking for rock solid from a 19mm panel but it needs to be strong enough to for day to day normal use.

 

[Jameshow]

You could router out 6mm grooves in the underside and and glue in 6mm carbon fibre rods.
This way the oak is in compression and the strong carbon is in tension.

Cheers James

 

[ol_london]

You could router out 6mm grooves in the underside and and glue in 6mm carbon fibre rods.
This way the oak is in compression and the strong carbon is in tension.

Cheers James

Great idea. Routing inside and close to the edge but stopping 20mm short maybe?

 

[profchris]

Be careful about cross-grain reinforcement. Your top will expand and contract across the grain by several mm because of humidity changes, and rigid reinforcement means the stress has to go somewhere - usually by the whole thing cupping up or down! So cross-grain reinforcement needs to allow for wood movement.