MDF Shelf Sag

[Steve Mellott]

How wide can a 3/4" x 12" MDF shelf be in a bookshelf (without a face frame) before you have to worry about sagging? What if you double the thickness of the shelf so that it is a total of 1 & 1/2" thick?

Thanks.

Steve

[Jamie Buxton]

Don't use MDF. It isn't as stiff as plywood, so it sags. Worse yet, the sag increases over time. Plywood doesn't do that. And MDF is nasty to work with; it generates fine dust which hangs in the air, and is greasy-slick on the floor.

Production lines use MDF because it reduces their cost of materials, and that's the dominant part of their budget. For small shops, a small change in cost-of-goods is insignificant. Using better materials and building better products is what's important.

[Joe Chritz]

www.woodbin.com/calcs/sagulator.htm

About 33 inches with a 25 lb load. Depending on the use that isn't very much.

Edit: fix the link

Joe

[Prashun Patel]

With due respect, I don't agree with blanket statements against MDF. At short spans for paint-grade things, it works fine.

Contrary to a lot of prejudice, I experience MDF to machine exceptionally well. There's no grain to contend with so x-cuts and routing are easy. Working with a lot of MDF does shorten the blade life, though.

If yr cutting mdf inside, make sure you arrange over-blade dc. That IS nasty, but easily contained with a shopvac hose clamped next to blade or sharkguardish guard.

I'd keep spans less than 32" and I'd cleat them.

[Dave Burris]

Would it be possible to add a 1/2" hardwood edgeband to the front of the shelf to give it some rigidity? I used a poplar edge on some adjustable shelves I make a few years ago for a bookcase and have yet to see any sag from the MDF. Not sure if this would work in your project. Just a thought ...

[Dave Falkenstein]

I have made lots of bookcases with MDF and 1X2 hardwood on the edges to stiffen the shelves. If you will be painting the bookcase, use poplar for the edging. I would put a hardwood edge on MDF regaerdless of the width of the shelf. If the shelves are fixed, dado them into the sides and staple a back to the shelves, in addition to the 1X2 edge. If the shelves are adjustable, use 1X2 on both edges. Plywood works well also, but still needs edging to prevent sagging, in my experience. In addition to sag perevention, a 1X2 edge on shelves simply looks better, IMHO.

[glenn bradley]

I have used MDF over spans of as much as 14" without sag. I have had 1-1/2" MDF sag over a span of 18". I have side and back supported 3/4" MDF with a 3/4" oak trim piece in front that has done OK but it is not holding a full shelf of books.

Bookshelf is a general term. If you plan to fully load the shelf with books, I would choose a different material but MDF could be reinforced to work.

[Wes Bischel]

I would second Joe's link to the Sagulator. I've used it for a few shelves and it has been a good reference.

Wes

[Charlie Plesums]

I would not use MDF for shelves, although I use it for other things. I do use hardwood on the front edge for stiffness, even with plywood.

But to answer your question, if you have 2 layers of material, it will be twice as stiff as one. But if you have 2 layers glued together thoroughly, so it becomes one piece, it is 8 times as stiff (third power of the thickness)

[Ben Cadotte]

If your willing to use thicker MDF, I would suggest you use the 3/4" and nail the back real well and use a 1 1/2" board across the front. This way your supporting it front and back. We have several of the inexpensive put together shelves and it seems we have to flip the shelves over every couple months, and there's really not much weight on them (1/2" though). These shelves are only 12" deep and 21" wide.

I use MDF for quite a few things. It has its usefull purposes. But I don't try to use it beyond what its good for. Unless it was for a garage shelf or a quickie shelf I would recommending using a decent plywood for the shelves though. I have been getting poplar ply sold in 18mm that is really good stuff for $39 a sheet. Its 11 ply material and has 2 very good paintable surfaces. Made a few shop projects with it and have a couple sheets to do some more. Would be really good shelf material.

[Pat Germain]

Gee, I would think doubling up 3/4" MDF would make for one beefy-looking shelf. Also, if you're using twice as much material, aren't you kind of negating the cost savings of MDF?

You did mention the word "bookshelf". Does that mean you'll be putting books on the shelf? If so, I would also advise against MDF. I have no problem with MDF. It's great for what it's good for. I do not think it's good for bookshelves.

In fact, one of the reasons my wife doesn't mind me buying expensive tools is I can mak her bookcases with shelves that don't sag at all! She really hates the department store bookcases that don't hold any weight without sagging. And boy, can she load up a bookshelf!

[Steve Mellott]

Thanks very much for all the responses. The Sagulator seems to be a very nice reference source. Thanks again.

[Howard Rosenberg]

1
- MDF shelves withstand sag easily
- best method involves a housed joint

2
a
- best housed joint = least-sexiest joint = dadoes and grooves
- dado the back and gables
- you'll eliminate more than 85% of the potential for sag
b
- add a face frame to the front
- you'll eliminate 100%

3
- I've made what seems like millions of MDF bookcases using the above techniques
- all of them varying lengths
- all with zero sag

4
a
- I've even made them with adjustable shelves around 90 inches long
b
- same techniques plus a couple of variations
- but NO SAG - whatsoever

HTH
Howard