Support needed for MDF shelving

[Chuck Burns]

I am building some built in bookshelves. I usually use a polycore plywood but it seems there was a bit of misunderstanding and my order came in as Cherry veneered MDF (3/4 for the sides and shelves and 1/4 for the backs). I'll be spanning 31" to 34". With polycore, hardwood, or even pine I would usually put a face of 3/4 x 1 1/2 to 2" on the front of the shelf to prevent sagging. I did a search for MDF and read a good bit about it sagging, even under it's own wt. Should I put a reinforcing brace across the back of the shelfs too or will I be OK with just the front? I've got plenty of extra Cherry on hand to do the additional support if it's needed.

Obviously I haven't used much MDF in the past so I've got a bit of learning to do. I've got some Confirmat screws on order from McFeelys and to get started bought some PB screws from a local independant lumber yard. Their main sales guys says he sells a bunch to local shops that do a lot with MDF and they're happy with them. I guess I'll know soon enough. I'm going to compare both in some scrap MDF with an inch pound torque meter.

[keith ouellette]

I have use mdf for shelving but not for anything fancy. The veneer should help stiffen it but i would put a face front and back. If the depth is more than 18" I would put one in the center. Mdf will sag under its own weight. With books...?

[Jim Becker]

For that span, I'd probably do something to support the back, too, either solid cherry or a small angle iron painted to "disappear".

[Wes Bischel]

Chuck,

Do a search for sagulator it will give you a good feel for the sag. I would add support as well.

Wes

PS can't add a direct link since it is on another forum site I believe.

[Dave Falkenstein]

How about gluing and stapling the back to the shelves? That's what I do, along with a 1X2 hardwood face frame, which is also glued to the shelves.

[Dennis Montgomery]

I do a lot of work with MDF. Confirmat screws work good in particle board, but not MDF. Screws in MDF are a hit and miss proposition. The percentage of splitting is so high that I only use biscuts with MDF. When I have long span shelves I face each shelf like you mentioned but I attach the face with biscuts. Anything up to 36" I face the front edge. If its wider, I will face front and back edges.

[Chuck Burns]

Thanks for the replies guys. The bottom two shelves are going to be fixed so I'll support them from the back as Dave suggested. The others are going to be adjustable so that won't be an option with them. If I had used 1/2" for the back I could have drilled 1/4" holes for shelf pins in the back.

That sagulator program is pretty slick; thanks for pointing me to that. Figuring just the MDF i come up with .45 deflection. Figuring just a 2" piece of solid cherry in front the deflection is only .03. So a I'll probably do a piece of cherry on back as I plan on retiring in this house, the kids are gone and this will be my study for a good number of years.

In thinking about this, after the fact as usual, I could have made the shelves as little torsion boxes. A 3/4 x 2 cherry perimeter with a rabit top and bottom for a sheet of 1/4" cherry ply. Maybe 1 cross piece in the middle. It would look good and might even be cheaper. Though with 60 shelvs it would be a good bit of work.

[Rob Will]

I would laminate a piece of 1/4" ply to the bottom side of the 3/4 mdf. That way, you end up with a 1" thick shelf that has ply on the "tension" side. A bit of glue shrink and you have a miniture version of a pre-stressed bridge beam. :cool::D

I would also add a 1-1/2" or 1-3/4" x 3/4" band at front AND rear of each shelf. I think 2" may look heavy (?)

Good luck!

Rob

[glenn bradley]

MDF at that span, I would brace front and rear.

[Dave Falkenstein]

Thanks for the replies guys. The bottom two shelves are going to be fixed so I'll support them from the back as Dave suggested. The others are going to be adjustable so that won't be an option with them....

Chuck - Do you REALLY need adjustable shelves? Every adjustable shelf cabinet or bookcase I have ever used was adjusted once and stayed that way for the life of the cabinet. Since this project is for your own use, can't you plan the shelf heights in advance?

[Chuck Burns]

Chuck - Do you REALLY need adjustable shelves? Every adjustable shelf cabinet or bookcase I have ever used was adjusted once and stayed that way for the life of the cabinet. Since this project is for your own use, can't you plan the shelf heights in advance?

I thought about using all fixed shelves. The units I built in the past were adjustable and I did use them. My wife thinks units like this look better if there are some "things" displayed rather than all books. And things have varying heights. And I will often insert an angled shelf or two for a dictionary or album.

[Chuck Burns]

I would also add a 1-1/2" or 1-3/4" x 3/4" band at front AND rear of each shelf. I think 2" may look heavy (?)

Rob, I think you are probably right about the 2" looking a bit heavy. I've always done 1 1/2 in the past. I might try to just bring the 4/4 lumber down to 7/8 as opposed to 3/4 for a little additional strength.

And I'll do as most have suggested and run a band front and back. Better safe than sorry; and I hate re-doing things.