MDF or Plywood?

[Steve Creameans]

Hello all. I've researched the forums for a topic that may answer my question but didn't really see one that applys.

I am building a work table for a craft room for the wife. I plan on using 3/4" MDF supported on 3 sides (left, back, right) by a 2x2 cleat screwed to wall studs. It will be 8' long, and 22" inches wide.

My question is, am I setting my self up for some sagging since I don't plan to use a center support, or will the MDF be rigid enough at that length and width to support itself ?

Thanks for any input!

[Russ Ambrose]

give the sagulator a look...it's helped me answer similar questions :)

http://www.woodbin.com/calcs/sagulator.htm

[Steve Creameans]

Thats a nifty tool. Thank you!

[glenn bradley]

I can only recount my experience. . . that will, in no way, be enought support for 3/4" MDF over that span. I would add a front support and center support. I have had 2 laminated 3/4" panels sag over a span of only about 20".

[Alan Tolchinsky]

If you build some storage underneath you won't have any problem.:) But seriously you need some "verticles" to support that, maybe three or four.

[Bill Huber]

I am with Glenn on this, there is no way that it will not sag like mad.

It wouldn't take much to make a frame and cross supports on 24 inch centers.

[Steve Creameans]

My other option (based on my abilities and wanting this to look presentable) would be to use a piece of MDF on edge as a center support screwed in from the top of the table down. I realize I would have to do extra finish work, but I'm not real concerened about that.

What, if any, material (budget minded) would work that span resting solely on the 2x2 cleats?

[Steve Creameans]

Thank you all so far for the input.

I considered framing it, but I am in a rental house, with really nice hardwood floors so Im trying to do the least amount of damage as possible. Me with a power tool in my hand is almost stretched to the limit.

[Lee Koepke]

I used plywood for a similar condition, a maple edge band on the front, and still added a brace in the middle.

It was a 'folding table' for her laundry room. My span was 6'.

It would be hard to justify an 8' span without some kind of support.

[Greg Funk]

A torsion box built with 3/8" skins on the top and bottom and 3/8" ribs would work over that distance and is not too difficult to make. The biggest challenge will be setting up a flat enough surface to build the box on. I built some 8'x3' shelves this way for attic storage and they are very strong. I use pre-finished 3/8" 2nds that I get for $20/sht for this type of work.

Greg

[Jim O'Dell]

What if you dropped down to 1/2" MDF and built a torsion box? Should control the sag. Would be a little heavier, though. I'm still not sure about an 8' span. Maybe a triangular support in the middle going back to the wall. Wouldn't impact the floor and would give you some more support directly tied to the wall stud. Jim.

[Joe Chritz]

The rear support will help some with the span but you will need something else.

The longest shelf supports you can find (like for laundry tables) should work and still leave it open underneath.

If that won't work then you will be into a torsion box or some kind of steel support.

Joe

[Steve Creameans]

Did some research on torsion boxes and I think that's way beyond my abilities.

How do you guys feel about creating a center bookshelf 20" wide, and 12" deep? I was thinking a simple "U" shape with a center shelf to keep it from racking and sliding it under the table flush with the front edge to close it off. Then drilling down from the top to secure it.

I know that would create some more finish work but sanding is something I have some experience in.

[Steve Creameans]

If you build some storage underneath you won't have any problem.:) But seriously you need some "verticles" to support that, maybe three or four.

I have no clue how I missed this one...

[Peter Quinn]

MDF $28/sht...sags///9 ply birch $45/ sht..doesn't sag...use plywood? Tortion box is a nice touch for airplane wings and assembly tables, I used a home cheepo luan hollow core door for my wifes's craft table...cost $15...hollow core door is a tortion box...hollow core door?

[Steve Rozmiarek]

Just an observation, the guy who built my hose, found some 1" MDF someplace, and made a bunch of shelves in the basement. They are attached to 16" center studs, every other stud, with a bracket every 32". The house is only 8 years old, so they haven't been here that long, and while I'm not sure what he had on the shelves, they sag bad. Average of a couple of inches I suppose.

MDF has a place, in cheap import furniture IMHO. I guess its OK for a few things, but never shelves, unless it is reinforced, which makes me think, why bother????

[George Bowen]

Just add corbels. I had a 20' counter top in my office built from 3/4" particle board laminate with corbels every 24" or so and it was very solid. No problem supporting a few hundred punds, and it was anchored into metal studs, wood studs would offor more support.

[Charlie Plesums]

Did some research on torsion boxes and I think that's way beyond my abilities.

What has you spooked about the torsion box? I wouldn't bet on 3/4 inch MDF, but I would bet on 1/4 inch plywood on the top and bottom, separated by almost anything that will keep them a constant distance apart... the farther apart, the stiffer the shelf. (A hollow core door is often made with corrugated cardboard holding the skins apart). I like things super strong, so I might use spacers perhaps 3/4 to 1 inch thick, in just about any pattern. The goal of the spacers is to keep the top and bottom a constant distance apart, so just glue them to the bottom, and when everything is in place, put a bead of glue on the top of all the spacers and drop the top on. If the top plywood has a bow, I put it on so that it is tight in the center and sticking up around the edges, then tack the edges down until the glue dries. Don't worry about interlocking the spacers to prevent side motion.

It may be hard to make the torsion box perfectly flat, but it is easy to make it stay as flat as it was when you built it, so just find the flattest floor and go for it. Without doing any engineering, I would probably do the spacers about every 6-8 inches in each direction.

Most of the load will be on the bottom skin, stretching it sideways, but if you are concerned about things falling on the shelf and punching through the thin plywood, make the top layer something like 3/8 inch plywood or MDF or whatever. That will give it strength from a surface attack (falling objects), but doesn't add much to the stiffness of the shelf.

[Steve Creameans]

It seems in order to properly build the torsion box I would need to find perfectly straight lumber or have the tools to plane it square for the grid.

Presently my tools consist of a few drills, a circular, and chop saw.

Would the torsion box, after the bottom skin and grid is layed out, then rest and be drilled into the cleats before the top skin is put on?

[Lee Koepke]

It seems in order to properly build the torsion box I would need to find perfectly straight lumber or have the tools to plane it square for the grid.

Presently my tools consist of a few drills, a circular, and chop saw.

Would the torsion box, after the bottom skin and grid is layed out, then rest and be drilled into the cleats before the top skin is put on?
no, you would build the box first, then install it. the previous description of a hollow core (standard interior residential door) is a good one. Imagine taking a door off its hinges, and laying it on top of your cleats, thats what the torsion box is all about.

If you are only doing about 3/4 thick for the spaces, you may be able to get away with 1x2 from the big box home stores.

[Dennis Parslow]

I can't stand mdf, so I am biased, but I would use plywood with oak trim. Buy a wide board of oak and rip it into strips. If you used say, 1" oak supports every 24" underneath, glued and countersunk, then you could use 1 3/4" oak to hide the front plywood and the supports (also glued, countersunk, and plugged. Strong, looks great, and you get to work with real wood.

[Steve Creameans]

I can't stand mdf, so I am biased, but I would use plywood with oak trim. Buy a wide board of oak and rip it into strips. If you used say, 1" oak supports every 24" underneath, glued and countersunk, then you could use 1 3/4" oak to hide the front plywood and the supports (also glued, countersunk, and plugged. Strong, looks great, and you get to work with real wood.

So if I am understanding. You suggest using the 1" oak for the cleats, glueing and screwing 1" oak strips to the top skin, then resting the outer 3 edges on the cleats, and coving the front with an oak skin? Would the supports butt up to the rear cleat? And would those need to be screwed in to the rear cleat as well?

[Steve Creameans]

Last question for you guys. Thanks for all of your input.

This is what I'm going with.

I'm using birch ply wood to make 2 18"w x 23d" 27"H boxes to use on the ends. Then I will put the 6' x 23" work top across the boxes. Will I still need a rear cleat?

To make the boxes I plan on clamping, glueing and screwing together. Can you drill then screw into the edges of ply without it crumbling?

Thanks again everyone!

[Lee Koepke]

you should be able to screw into the ply with no problem (as long as you are not too close to the edges)

mathematically speaking, you would have 5' left, either way, I woud still use a rear cleat.

depending on the deflection, you may still end up with a brace underneath. I would use plywood for the top, and at the very least use some scrap pieces about 4" wide to create a thickened edge around the perimeter. then attach a 1.5" hardwood edge band to cover the ply and give you some more strenghth.

[Charlie Plesums]

It seems in order to properly build the torsion box I would need to find perfectly straight lumber or have the tools to plane it square for the grid.

Presently my tools consist of a few drills, a circular, and chop saw.

Would the torsion box, after the bottom skin and grid is layed out, then rest and be drilled into the cleats before the top skin is put on?

Torsion boxes are often used when someone wants an absolutely flat surface that will stay flat with a load on it. Thus, many instructions for torsion boxes get really complex to produce the absolutely flat part.

I often use torsion boxes for the foundation of large beds (I have made multiple queen size bunk beds and even a queen size handicap bed). In those cases I have a wide span that I don't want to sag, but absolutely flat is not an issue. Construction is simple... as someone suggested, "1 x 2" lumber from Home Depot would give you a good spacer. I would just lay the completed box on top of the cleats, and not introduce any complexity for this simple shelf. When I make the beds, I do attach them from the inside of the box, but I also strengthen the spacers for that load.

To prove the strength of a torsion box, I built one 3 feet long, 4 inches wide, and roughly 1 3/4 inch thick, using 1/4 inch plywood (which is really well under 1/4 inch thick), with 1/4 inch plywood for the spacers about every 6 inches. Supported on the ends, it easily holds my weight (over 200 pounds). 3/4 inch plywood 4 inches wide and 3 feet long sags dramatically in the same test, and often breaks. The torsion box always wins!