We are having a fairly large built-in bookshelf (w cabinets in bottom) made for our livingroom and the guy making it recently mentioned that he was considering mdf for the back panel (the rest of the bookshelf is plywood). Is mdf ok if it is just the backing? The is going to be a 90 inch wide unit, 8ft tall. We hope to have this stay strong and in good condition for many years. Just trying to understand if this is a bad place to use cheaper materials or if it is ok?
Also, the 90" unit is made up of three columns of 30" wide shelves. We had imagined that he would have constructed 3 different segments -basically three different bookcases of 30" each and then attached them together. But instead he has started it as one huge unit – a 90" board all the way across the top and bottom. (One unit for the top and one unit for the run of bottom cabinets.) He is adding vertical plywood panels going down at 30" intervals inside the 90" frame/box that he has made. Aside from my confusion about how he will attach fixed shelving at the same height throughout the unit, I am also wondering if the unit will be much weaker and less stable if it is made as one 90" piece versus three separate smaller units attached.
I kniw he's done other carpentry projects but I don't think he's ever done something like this before so I'd like confirmation that this all sounds ok (and advice on mdf backing).
Thanks!!!!
Built-in bookshelf questions
carpentry
Related Solutions
Since you're putting a gable roof on it, I can only assume this is being built for storage purposes, not for aesthetics.
Building it with a glulam ridge beam would be more difficult, but it would be much stronger. The main issue with a ridge beam design would be lifting the beam into position. If you can manage that pretty easily, the rest would be a breeze. I'd put some ceiling joists up at either end, lift one end of the beam over one of the joists, pushing it far enough to clear the ceiling joist on the other side, like so...
B
B
B
JttJttJBttttttttttttttJttJttJ
S B S
S B S
S B S
bbbbbbbbbbbbbbbbbbbBbbbbbbbbb
B = ridge beam
J = joists at the ends of the barn
S = studs at the ends of the barn
t & b = top and bottom plate
... and then lift the bottom end of the beam up with some ropes. The joists will need some strength to support the ridge beam, and you'll need to be able to stand on them, so I'd use three 2x6's @ 9" OC on either end (where the outermost "2x6" is actually the top-plate of the walls on either end of the diagram) with some 5/8ths plywood on top of them. I'd toss in some temporary studs at the center point to make sure the joists hold up to the weight of two people and the ridge beam. Once the beam is in place, put in the ceiling joists and plywood on top of them to make walking around easier (and to create an attic for extra storage,) then put the rafters up.
Grab some metal brackets for the joists, rafter heels, and rafter/ridge connections (it'll cost you an extra $125, but it will make the structure a lot stronger) from a HI store and your little barn thing will be able to withstand some nasty winds.
You could simply make the shelves at the corner "L" shaped, like this.
Another approach would be to make a corner unit, at an angle to the two wall units. Like this.
A smaller corner unit, is also an option.
Though all of these options are dependent on your personal preference.
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Best Answer
First off, please clarify your questions in a list, it'll help sorting through your narration! Anyway, I'll take a stab at what I think your questions are:
MDF for backing: MDF's weak point in this installation would be in its tendency to sag when supporting weight if it were to be used as shelving. As a backing it is perfectly suitable, provided it's joined properly (using pilot holes, the right screws, etc.).
are 30" segments ok vs. individual 30" carcasses? Tough to say without knowing anything about the joinery. I'm not sure why you question the ability to maintain consistent shelf height though - that's the easy part (just cut the "wall" segments to the same dimensions.
In general, built-ins are a low-risk proposition - woodworking joinery is really meant to maintain joints subject to complex and changing loads (imagine rocking on the back two legs of a 4-legged chair). The static loads associated with built in shelves (vs a movable bookcase), even large ones from many books, are less of a concern. Actually, my main concern with this is your friend's design of the shelves to minimize sagging - make sure he has taken that into account. A helpful tool for that is: http://www.woodbin.com/calcs/sagulator.htm