3.5 meter floating desk help

Hi, I am thinking of building a floating desk along a wall on my flat.

I would really like it to be a floating desk along the entire wall which is around 3.5 meters.

I am thinking of using an oak countertop
65 or 72 cm deep and 28 or 38mm thick.
The weight for a 350x65x3.8cm counter too is about 55kg. It will only be supported on the 2 short sides and 1 long side along the walls.

My first question is:
Will this be strong enough to self support and not sag in the middle? And is the 28mm thick enough or will it need the 38mm?

Secondly regarding the actual attachment to the walls which are MDF with a timber frame. My idea is to have 2x6s along the 2 short and one long side directly bolted into the timber frame. Then the countertop would rest on that and be attached with pocket-hole screws or brackets

Would I need some shelf brackets as well? Especially to avoid sagging?

Well here is a simple calculation for mid span deflections which are governed by the stiffness of the material. The ‘strength’ of the oak is unimportant in this case.
Given density oak as 650Kg/m-3; Youngs modulus 11GPa, then
Deflection at middle due to self weight for 28mm thickness 1.7mm and for 38mm thickness 0.7mm

Adding a load of someone standing on desk the extra deflection would be for 28mm 4.8mm and for 38m 1.9mm
giving total deflections of 6.5mm or 2.6mm respectively.

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Thank you @frasco

I am trying to read and understand this but … :laughing: I am really struggling.

I guess that means the thicker one would be better?

Oops, the figures are wrong I forgot about gravity so the actual values should be about ten (9.8) times higher.
I made the conservative assumption of ignoring the support from the back rail.

I have a similar setup at home where I’m using two thicknesses of 19mm ply, but my span is only 1.5m.

I would go for the 38mm . Neither will break but the thinner one would sag!

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Ok. Thank you very much!

Nice to bring some engineering to this!

The back wall support should contribute a fair bit though?

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@Dermot I thought so, too

The problem is the fixity of the back rail. Since there is no support at the front edge, the top is acting as a cantilever. So the joint has to be stiff and strong. I use an oak back ledge firmly bolted to a brick wall with a lot of screws hold the top down. I’d be worried about the stiffness of the studding wall.

If I were to set my desk now, I’d stiffen the front with a length of 20mm square tubing bolted to the underside of the front edge (steel is twenty times stiffer than oak).

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The desk project is starting to shape up.

I used 4x10 cm beams to build a frame underneath and used bolts to fix it directly into the studs.

The oak top is 310x72x4 cm and is solid even if you add weight to the middle front parts which I am really glad about.
The top will be screwed to the beams underneath with heavy duty brackets.

I am hoping to add some hidden drawers underneath and some wireless charging points under the wood. Also need to cut out some holes for the cables to feed through.

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