would a 80cm long 15-20mm oak board sag under body weight?

[Yrrol]

Sketching a bench. In the drawing, there's 80cm between the ends.

If I have a 15mm or a 20mm thick oak board, would it banana shape if someone sits on it?

 

[vojma]

Also depends on the depth of the board.

Ekskiva; glued board or solid?

 

[Yrrol]

30cm deep.

don't know the difference but hope to find something that doesn't look like planks/parquet... i.e. veneer board might be better.

 

[Fjelie 17]

One could be witty and say it depends on body weight. It might work for a smaller child, but with my 90+, I think it might bend. But it will probably hold. Can't you put a piece of the board on its edge right underneath? 50mm or so.

(Now, I'm more of a carpenter than a furniture maker, so I like to go a bit extra.)

 

[Krilleman]

I have built some things with IKEA's thickest oak countertop (3.8 cm finger-jointed)

But the last countertop I bought, to my great surprise (or not), is finger-jointed in two directions. Only the edges were "whole".



After all, it's the downside of buying cheap...

This got a bit OT here, but regarding the question about the deflection, I would think that 15mm is too weak...

 

[jac]

You can try to calculate it using this: http://www.woodbin.com/calcs/sagulator.htm

If I've quickly got the measurements and units right, a 2 cm thick board would deflect about 4 mm if you place 100 kg in the middle.

 

[Plutus]

If it is screwed at the ends it also affects, a loose board warps more easily.

 

[Tornhill 2]

If it is screwed at the ends also affects, a loose sheet warps more easily

It depends on what it is screwed into.
If it is screwed into two ends that cannot move (e.g. concrete walls), it will take MUCH more for it to deform. However, if it is screwed into a shaky wooden base, which is common for a bench, the difference is much smaller compared to a loose sheet.

/ Fredrik