MDF or Plywood?

[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!