Replies

1. 
   calvin | Jan 20, 2013 06:32am | #1

   james

   There are shear values for drywall screws, but beats me what they are.

   Drywall screws come two ways (three if you count self tapping for fastening to metal).  Coarse and fine thread.

   If your sides are 3/4" thick, then 1-5/8's are a bit short.

   Shelf units have been made with several different kinds of material-solid woods, plywood, mdf, and particle board are a few.

   Millions of shelves have been fastened to the sides using drywall screws.  Some  pulled apart, some have split the shelves and come apart and some have failed because the material couldn't hold the shear weight and the shelves went down leaving the screws in place.

   Not many have had the screws snap.

   The dado certainly takes some time depending how you do it, but it sure eases the mind knowing the weight is held by material rather than a couple screws into relatively thin material.  Add glue in the assembly and you can eliminate the screws if done right. 

   Do it your way and add a back-fastening that to the shelves, will help divide the weight by more fasteners.  Won't do much for the front edge, but will certainly help.

   Add a face frame that lips over the shelf a half or 3/4 and run a fastener through that into the shelves for added security.

   If you use a face frame, hang it over the inside enough to add 1/4 " plywood "panels" under each shelf, effectively building up a dado.

   The important thing  if this is wall hung, don't use drywall screws to do that.

1. 
   jameskim | Jan 20, 2013 11:00am | #6

   thanks!

   thanks for all the advice!  makes good sense.

2. 
   DanH | Jan 20, 2013 09:32am | #2

   Generally, I prefer using deck screws to drywall screws in any situation where the screw will be under load, and. more recently, I generally use GRK "structural" screws (which look pretty much like deck/drywall screws but which have actually been tested for strength) for such cases.

   But, if these screws are being driven through the side and into the edge of the shelves then they're basically acting like pegs, and the shear load on a single screw is at most 100 pounds or so (presuming you're not storing your hoard of gold bricks on the shelf).  So pick the screw based on whatever color screw head will look best.  The plywood shelf will split before the screw fails.

3. 
   DanH | Jan 20, 2013 09:34am | #3

   Of course, another option is to drill holes in the side from the inside and use shelf pegs.  Templates are available to make this job relatively accurate and non-tedious.

   (For tall shelf units you may need a fixed shelf in the middle, though, to keep the sides from spreading.)

1. 
   gfretwell | Jan 20, 2013 10:36am | #5

   A piece of pegboard makes a decent jig for drilling the holes for shelf pins. You can usually buy a 2x4 piece pretty cheap at the BORG if you don't have any laying around. Use a brad point bit to start the holes and it won't eat up the pegboard. Just be sure you have it squared up well and mark the rows of holes you are using. I mark both sides and use the jig in the same orientation from side to side (same edge in back and bottom). This is a throw away thing. After a few uses the holes will still get wallowed out. but good for a guy building a cabinet or two.

4. 
   junkhound | Jan 20, 2013 10:28am | #4

   First, the correct terminology for drywall screws is "Piffin" screws. <G>

   2nd, as C. said, most faiures are of the wood, not the screw, hence use of 2" vs. 1.625" is advised. Offset the screws slightly so they are below the centerline of the horizontal members.

   Unless you are using oak, hickory, hard maple or similar, skip the predrilled holes.  Pine, spruce, or other softwoods work better with NO predrilled holes for keeping the shelves aligned.

5. 
   florida | Jan 20, 2013 12:14pm | #7

   Why screws at all? I've built dozens of bookcases and never used a screw.  I glue and nail the carcass together using 2" finsh nails

   and a nail gun. I drill pin holes in the side and use shelf pins so the shelves are adjustable but I've also used finish nails to nail the shelves on. Never had a shelf fail yet.

   But, to directly answer your question. Assuming your sides are 3/4"  the 1 5/8" drywall screws and glue will work just fine. The load is in shear and you couldn't put enough weight on a bookcase shelf to shear even a drywall screw.

6. 
   sapwood | Jan 20, 2013 01:20pm | #8

   An even faster way to build utility shelving (for books, tools, boat parts, etc) is to use pocket screws. The pocket holes are drilled into the upper surface of the shelves so they angle down. Start your assembly at the bottom. Use spacers to locate subsequent shelves. No glue required except for the back if you wish. Gun nail or staple the back on. 

7. 
   xxPaulCPxx | Jan 20, 2013 03:22pm | #9

   So many screws, so little time
   If you are using screws for anything, be aware that they are specifically engineered to do one particular task, like holding onto a deck board or attaching two pieces of sheet metal together. Drywall screws are frequently misused just because there are so many of them just lying around.

   If you are going to use a screw to hold a shear load, don't use anything smaller than a #9 size. I differ from the other poster who didn't like predrilled holes - I think they are necessary most times just to ensure the screw drives in perpendicular to the work. Size the predrill hole to the thickness of the center section where the screw threads wrap around. You don't have to drill the whole way in softwoods, just 1/2" is all that's really needed.

1. 
   DanH | Jan 20, 2013 06:10pm | #10

   In the case of plywood shelves, the pre-drilled holes are necessary to prevent splitting the workpiece.