Support for Wide Shelf? - Question of the Week

November 7, 2006 | Filed Under Blog, Question of the Week 

Hello fellow wood butchers! This week’s Email Question of the Week comes from Steve.
He writes:

“I am building a large bookcase and don’t want to add any more support than necessary. Everything will be made from 3/4″ plywood. My plan is to mortise the shelves into the sides of the cabinet and screw the shelves to the back panel. Will that be enough support for a 76″ long shelf?? Will I have problems with sagging??”

And here was my reply:

“Hi Steve. That is one heck of a wide bookcase!! You are right to be concerned about sagging over such a great length. Screwing the shelves into the back is a great start and will provide a significant amount of support for the shelf. It just might be enough, depending on how deep you plan to make the shelf.
Whenever I have to decide on how much support to give a shelf, I always start by checking The Sagulator. This is a great online resource for calculating how much deflection to expect using different materials under different loads. It spits out a number and you simply decide if you can live with that much sag. The author of the website gives a very helpful tip: “The eye will notice a deflection of 1/32″ per running foot.”
Unfortunately, The Sagulator has no way of calculating the additional support you plan on giving the shelf.
As I said before, I think the screws in the back might be enough. But when in doubt, you should err on the side of caution. I would recommend attaching a 1 1/2″ piece of trim, either solid wood or ply, to the front of the shelf. This trim piece will give the shelf a lot of extra support, and you can be confident that the shelf will not sag. And if you use solid wood, it give the whole bookshelf a more “finished” look.
Good luck with the bookcase and be sure to check out The Sagulator.”

Marc

Comments

2 Responses to “Support for Wide Shelf? - Question of the Week”

  1. Greg on November 27th, 2006 7:11 am

    I would suggest to Steve that he make his shelving unit into three sections if visual appeal is important in his project.
    I built an entire hallway of floor to ceiling shelves and having them roughly two feet wide looked great. If he does face frame them then having two vertical stiles for support and dividing the shelves into thirds visually would do the trick for strength and aesthetics.

  2. Howard Rosenberg on November 30th, 2006 4:43 pm

    I’ve made bookcases in different widths with no supports between the gables.

    The widest one I’ve made has 86″ wide shelves and exhibits zero sag.

    I make my stuff entirely out of MDF and never use screws or nails.
    As you probably know, MDF has no grain and can’t support its own weight.

    The trick is to use housed joints - such as dadoes and grooves.
    The dadoes hold the board in place and prevent sag.
    They don’t need to be all that deep - 1/8″ deep is fine - but with 3/4″ plywood, you can easily go 1/4″ deep.

    I route dadoes and grooves into the gables and back for the shelves to fit into.
    You’re best making a jig of some sort that will let you route to the exact thickness you’re dealing with - there are many variances in 3/4″ plywood.

    Seeing you don’t want to introduce supports between the gables this is the only way to diminish/eliminate sag.

    BUT - you’re gonna need another refinement.

    The shelves might sag front-to-rear.

    You’ll need to introduce a a face-frame on the shelves themselves to maximize rigidity.

    This can be a pain using plywood but you can do loooong mitres on an accurately setup TS.
    (It’s obviously more forgiving using MDF and painting it)
    But just set the blade for an ACCURATE 45deg and just glue the bastards together.

    I’ve done this on about 20 different bookcases without nails.
    Works every time.

Got something to say?





  • What's New???

  • Check this out!!

    Loading the LumberJocks Widget

  • Extra Reading

  • Google


  • SuTree Videos

    Find free how-to videos
Close
E-mail It