MDF Bookcases

[Dennis McDonaugh]

The latest issue of Popular Woodworking had a short article about using MDF for shelf construction and said a 32" shelf of MDF with a 1" hardwood edge would only support 90lbs. I just completed a set of built-ins for my office and I am beginning to have second thoughts about its strength.

Its three 32" high base cabinets 20" deep with two 8' high, 10" deep bookcases sitting on top. The center base cabinet is all drawers and 24" wide. Each side cabinet has one door and is 18" wide. The bookcases are 30" wide. The whole thing sits on a 2X4 base. Its painted so I used MDF instead of plywood for the case work. The counter top is two pieces of 3/4 MDF glued together.

I think the base cabinets can hold the weight of the bookcases since there are six vertical supports (each cabinet side), but I'm worried about the bookcases supporting 8' of books. The bookcases have rabbeted tops. The bottom shelf and one permanent shelf 15" from the bottom are set into dados. Four metal shelf supports are set into dados in the side of the bookcases. The adjustable shelves are 28.5" long and edged front and back with a a 3/4 inch thick, 1 1/2 inch wide piece of poplar. They will be supported by tabs connected to the metal shelf supports.

Does it sound like these shelves will be strong enough to support a full bookcase?

Thanks,

[Todd Burch]

Dennis, yes they do sound strong enough.

I typically would not install a bottom shelf on these types of bookcases, and just let the sides of the bookcase rest on the countertop. I toenail (plywood) or biscuit (MDF or plywood) them in place, if a built-in, and then wrap the perimeter with a small moulding.

The nice thing is that if your shelves DO sag over time, you haven't invested a lot, and you could always remake with a 2" front edging. I lean towards a 2" front edging for longer spans, but I over-engineer a lot too.

If you will be loading them up full with heavy books, a lot of magazines or encyclopedias, they will probably sag over time. You could put another layer of MDF, say 1/4", with a 1/4" spacer, on the bottom to create a torsion-box effect to help out as well. Here's a drawing showing what I mean... (I made the bottom piece of MDF transparent so you could see the spacers. You would want to put a spacer where your shelf clip sat too.)

[Dennis McDonaugh]

Thanks for the reply Todd. I guess only time will tell how well they will hold up. They are built in--attached to the wall like kitchen cabinets with fluted columns on the sides and a crown molding on top. I didn't like working with the MDF. Even with dust collection, fine dust went everywhere and you have to be careful when moving it because the edges really ding easily. I'll try to post a picture soon.