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Shelf Span...

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  • Shelf Span...

    Just getting into woodworking, so learning as I go. I recently purchased a TS3650 and R2930 (Router combo)...great machines. Never used a router before, but it turned out to be pretty easy...the LED light is great.

    Anyways, back to the reason for the post. I've working on building some bookselves that will have that "built-in" look, complete with crown/base moulding in a 9-foot ceiling room. The wood I have chosen is birch plywood and I've already purchased a small forest from HD. The span for the shelves will be 37 5/8. The sides of each bookcase is made from 3/4 BPW for the inside (routed for shelf standard) with 1/2 BPW glued to the outside. So we're talking 1 1/4 BPW sides with 37 5/8 span. My question is, will that span be too much to hold 3/4 BPW shelves? Would poplar shelves be less likely to sag? Since I'm using shelf standard to hold the shelves, should I make 37 5/8 cleats (not sure if that's the right terminology) that would be attached under each shelf?

    Again, I'm new to this, so I'm learning as a I go...


  • #2
    Re: Shelf Span...

    Hmm, why so thick on sides? You could of just used 3/4 thick ply for sides and dadoed a 1/4 deep and still had a 1/2 which is plenty.
    As far as shelves go theres several ways to look at it. One you can use the ply and put and edge band 1/2 thick by either 3/4 tall or taller and let it hang down below shelve to make it sturdier. You can make a hardwood shelves but not necessary unless maybe your putting the entire encylopedia brittanica on it.
    You can buy a piece of glass to go on top of each shelve also. 37 inches isnt too bad but inevitably with weight on it it will sag, the human eye can catch a sag up to a 16th inch I have read.
    I think the dadoe and a hardwood strip on the front edge will be fine.
    You can also bookmark this site.

    Also ply from the Borg isnt very good for woodworking. Its tends to err on the side of spltting and usually its not too flat. I used rarely for paint grade bookshelves.
    There is also different joints you can use for connecting the hardwood strip which I dont know alot about to strengthen it, I think its alot of extra work thats not needed in most apps.
    Also you can double shelves with maybe another1/4 sheet and make a strip 1 inch tall to cover them.


    • #3
      Re: Shelf Span...

      Thanks Woody!!!

      The hardwood strip is an idea I think I'll go with to strengthen it up...along with maybe glueing up another 1/4 BPW for even more added strength.

      In my infinite wisdom, along with this being my first woodworking project, I ended up trimming the 3/4 BPW with the dadoe cut for the standard shelving, 4 inches, so I don't have a full 96 inches....don't first project and I measured once and cut once, instead of measuring twice. So where the top of the bookcase will be, there will be a seam that will show on the outside. So I was going to glue a 1/2 BPW at full 96 inches to the outside to give me the height I need without having a seam show on the outside of the bookcase.


      • #4
        Re: Shelf Span...

        Okay, I'm going to fall back on some of my engineering background here, but bear with me. A frequently used rule of thumb for structural beams (your shelves qualify) is to limit the deflection to 1/360th of the span. For your 37-5/8 shelves that is nearing 1/8 in, which would be visually noticeable. I have run some simple approximate calculations for your shelves. A shelf load of books would cause over 3/16ths deflection - too much. To limit the deflection to an 1/8 (1/360th of the span) the load would need to be less than 40 lbs, less than half a fully loaded shelf.

        And as WWC said 1/16th would probably be visible.

        I recommend attaching an edging strip along the front that is around 1-1/2 inches high. This strip will greatly increase the stiffness of the shelf. It will also make the shelves look stronger and more finished. I've done this with shelves made from particle board and they are 43 inches long and holding up a load of books. The simple way to add this strip is to route a rabbet on a piece of 3/4 x 1-1/2 wood so that it fits tightly against the ply, then glue the edging along both mating surfaces and clamp them together. The groove depth can be anywhere between 1/4 and 3/8 in. deep. Make the groove a small bit wider than the ply's thickness, then after they are glued together use a flush trim router bit to trim the edging board flush. Then use a chamfer, or round over (my choice) router bit finish off the edging. The edging strip can be any type of clear, fairly straight grain wood. For a painted unit consider fir. If your staining you'll probably want to use birch to match the ply, but popular may work okay. (If you think these shelves may really be heavily loaded add a similar strip near the back of the shelf, this one only needs to be 3/4 high and can be glued, nailed, screwed to the under side of the shelf.)

        You may want to add a similar edging treatment onto the sides of the case.

        I also suggest you permanently attach (i.e. glue) 1 or 2 of the shelves to the case to make the case more rigid, particularly if the back is open. If you do attach shelves to the case I suggest making a bit of a tendon on the shelf's ends to fit into dadoes on the case's sides. These tendons can be as simple as a rabbet across the ends. The rabbet needs to only be about 1/4 deep and 1/4 to 3/8 wide and on 1 side of the shelf. The trick to a good looking joint is to install the shelf with the rabbet up so the shoulder of the rabbet covers the dado in the case side.

        Okay, I got a bit carried away here, but maybe some of this will help.


        • #5
          Re: Shelf Span...

          Thanks did help. Especially the idea of rabbetting the strip being used as the edging to the shelf (gives me a reason to buy another bit.. :-) ). I would have just tried to screw, countersink, wood filler, and paint over it. Again my first attempt at woodworking, so I'm a rookie. But you gotta start somewhere.

          Also the back will be enclosed with 1/4 oak ply, so I don't think I need to permanently attach 1 or 2 shelves, since the wife wants them all moveable.

          I plan on priming with Kilz primer and then paint white.

          When I get of these days....I'll post some pics...


          • #6
            Re: Shelf Span...

            I did some painted built-ins for someone. The span was around 46". All I did was use 3/4 paint grade maple plywood and glued a 3/4 x 1 1/2 strip of birch on the front edge along with 3 or 4 biscuits. I think it would have held me without deflecting too much (and I like that back bacon, eh).