Simple question - plywood tabletop

sarkozy

I've got a pair of Ikea Lerberg trestle table legs. On top of these, I want a natural birch plywood top. The table depth must be 64cm max. The length can be much longer.

It's a semi-temporary table. I haven't really done woodwork before. I don't have tools (but could possibly borrow them).

I want to know: how long could a table could I get away with using, e.g. 15mm furniture grade birch ply without needing to get into bracing underneath?

If I need to do this, what's the best way that keeps it simple for someone like me? One thing I've seen is to get two differently sized sheets, gluing and screwing the smaller one underneath giving the appearance of a single sheet.

As for cutting, I could get a sheet pre-cut at Milofsky's/Woodworkers?

Another plan I have is to create a very simple set of shelves using simple boxes so perhaps some surplus could be used for this.

Advice please! I'd like to get on this ASAP!

recipio

Anything in 15 mm is going to sag - you would need to double up the sheets to 30mm at least and even that would only take a light load before deflecting.
I'd suggest you buy a cheap flush door ( which won't have any lock mortices cut in it ) and this will be strong enough to take most domestic loads. Not as nice as birch ply but a lot easier.

sarkozy

I don't mind doing a bit of work. Think this would work? (My table would be smaller): http://ashleyannphotography.com/blog/2014/08/26/diy-ikea-lerberg-trestle-leg-tables/

Could I strengthen a single sheet underneath by screwing in some batons in a certain formation, like this, for example? http://assets.curbly.com/photos/0000/0016/0812/DSC05027_large_jpg.jpg?1341600603

jack of all

You could glue and screw some narrow strips (3- 4" wide) in matching ply to the underside of the top, just around the perimeter with some intermediate ones along the length also. In effect your table top will look twice as thick, be more rigid, but not so heavy as a full thickness top (if you get me?). Flush door would be an easier option but I like the look of plywood veneers on the edges.

sarkozy

Thanks. I was considering this as a possible solution:



Given the thin trestle profile, I'm trying to avoid the top looking thick/doubled-up. Due to the narrow space the table needs to fit into, neither flush doors nor Ikea tops will fit snugly enough to get maximum use of the space, which is why I'm approaching things this way.

Would the above approach work? What if I replaced the full area covered by the batons with a single sheet of glued and screwed ply?

jack of all

Your idea would be fine, but I think I'd have 5 "C" intermediates as opposed to 3. I would prefer this to replacing the full area with a single sheet- it's a waste of material and adds unnecessary weight but obviously is less work.

recipio

Screwing and glueing batons to the underside would certainly work. If you are screwing into 15mm ply you need to be very careful with drilling holes as you have little tolerance - better use 18mm if possible. It is the glue, not the screws that adds stiffness to the ply. Also, standard sheets are 1220 wide so you will probably only find a I/2 sheet ie 610 mm wide assuming you want the grain to run lengthwise.
Lastly, remember you will be lifting them ! A baton/ply construction is certainly going to be lighter than two 15mm sheets glued together.
If you had the tools and inclination you could construct a torsion box - two skins of 6mm ply glued over a grid of batons ( flush doors use a paper 'eggbox' construction ) but that's getting a little technical !

sarkozy

Thanks, guys. I think, at a push, a 610mm deep desk would do me fine for now. Apart from it being a much lighter construction, it'll work out cheaper. I hope to get busier with woodwork over the coming year, and hope to see my ambition and skills increase gradually, though I'm not planning anything complex.

Would this be an alternative arrangement to give extra strength? (A couple of lines are missing - the trestles would rest in the 'C' shape top and bottom at a particular point).



What measurement of batons would I need given that I don't want them to be seen?

endacl

sarkozy wrote: »

What measurement of batons would I need given that I don't want them to be seen?

How tall will the people who might potentially see them be? And how far will they be standing from the table?

Trigonometry time!

jack of all

recipio wrote: »

It is the glue, not the screws that adds stiffness to the ply.

But glue lines can creep over time so screws help too, no?

sarkozy

How tall will the people who might potentially see them be? And how far will they be standing from the table?

Trigonometry time!

Ha! Let's be reasonable! The underside might be visible briefly from the half-landing, if the door to the room is open, but approaching the room, entering the room, etc, average height adult using the desk. It's a box room, so not much chance of getting lower angles of view!

But I'd like to keep the profile as thin as possible.

jack of all

I think the middle long rib is unnecessary and would prefer the short intermediates. As for width of the ribs I think 70mm is good, no trigonometry required just judge by eye, whatever looks good! If using thin material, as recipio has said be careful not to puncture the top sheet- one way around this would be to drive your screws in at a slight skew angle (stronger too and gives a little more purchase for the screw as depth of penetration is increased slightly.

recipio

jack of all wrote: »

But glue lines can creep over time so screws help too, no?

I think you mean PVA glue ? This is often quoted but to be honest, in 25 years of woodworking I've never seen it.The function of the screws in this instance is to hold the battens while the glue sets.
Sarcozy, if the battens are to scale I think they are too thick. You are adding weight for little gain in strength. Something like 3"x1" in softwood would do. I know its a little tricky to drill vertical holes in 3" wood - put some tape on the bit for depth and stand a trisquare next to the hole for guidance.

sarkozy

Whoa, whoa, you're saying the batons underneath need to be fixed on the narrow edge? Can I not fix them on the wide edge so the supporting frame underneath is only 1" deep?

bonzodog2

Take a 3x1 plank 6 ftlong, you'll easily flex if a good bit in the flat direction but not the other. I'd use 3 lengths of 2x1 with 4-5 noggins between each pair, offset. So it looks a bit like 2 ladders next to each other with the rungs not next to the other one's

recipio

sarkozy wrote: »

Whoa, whoa, you're saying the batons underneath need to be fixed on the narrow edge? Can I not fix them on the wide edge so the supporting frame underneath is only 1" deep?

Eh.......no. My hunch is that even with glueing , a 1" batten is going to deflect.
You might just get away with 2x1" on the narrow edge. Again , 3 cross battens would do as there is almost no crossgrain deflection. Have a look at the 'sagulator' - a site that will calculate this for you.
I appreciate its a temporary setup but its human nature to plonk heavy items on any horizontal surface so you need to make it as stiff as possible.
Its extraordinary how the human eye picks up slight sagging of any horizontal line.