Proper thickness for shelves?

[Charles Cooper]

Hi all,

I am new to wood building and I just received my Kreg DIY kit.  For my first project I want to build a CD storage rack.  It is going to be 4ft tall by 6ft wide.  I plan on using plywood to build it out of.  My question is how thick of plywood do I need for a 6ft wide shelf to prevent too much sag or would particle board or some other material be better for the shelves?  I am unemployed right now so I’m trying to keep cost to a minimum.

Thanks for your help,

 

Charlie C

[Don Foley]

Charlie,

You do not want to substitute particleboard, unless its covered on top, bottom and face with laminate.  Unlaminated particleboard is weaker than plywood.  I have added some illustrations I downloaded from an article on shelving that may prove interesting.  The first one deals with distance between supports for shelving using various materials. The second illustration shows the relative rigidity of various hardwoods, and the third compares load carrying capacity of various building materials versus an average hardwood.  This is the illustration I referenced when I suggested not using particle board unless it was laminated.

Hope this helps, Don

[Don Foley]

Continuing on the above comment, I have two other illustrations depicting how to strengthen shelves or how to reduce the likelihood of a shelf sagging.

Don

[Russ Haynes]

CDs are heavy.  At 6 feet long, if the shelves are only supported on the ends of the unit, it's going to sag over time.  The thicker the shelves, the less likely they will be to sage, but the more expensive and cumbersome the project will become.  You may want to consider adding two more vertical supports to break the case into three 2' wide sections.  You can get away with thinner shelves if they are only 2' wide.  But if you're going to use plywood and then load them up with CDs, I suggest following Don's advice and using solid wood front pieces turned on end for your edge treatment.  That should really help bowing over time.  You can attach the edge pieces with pocket holes in the bottom of the shelves.

[Jay Boutwell]

Hi Charles.   I have read the other posts related to your question and I agree with them.  I would like to add that a span of 6 feet of 3/4 inch thick shelf is much too great to expect a shelf to remain free of any center bow without any mid support regardless of how many reasonable sized stiffeners that you place under the shelf.  A distance of 4 feet is stretching the distance if you expect to add any weight exceeding 25 to 30 pounds.  Of course the amount of dead weight that the shelf will support will depend on the type of wood or plywood you use and the way the grains run.   I plywood it will depend on the number of plies and the type of glue and the type of and condition of the plies as well as the pressing of the plies together.

 A much better plan is to include a mid support on the front and rear by using a stile done by building the case using a face frame method.   If you are attaching the shelves using pins (Pin bored sides) or making the shelves permanent by screws or dados the face frame method will work.  In the face frame plan simple include a mid stile in the face frame and pin bore the back of the mid stile just like you would do for the sides. 

In either case where or not you use a face frame method or the frameless the width of this unit will require a middle fixed shelf to prevent the case from racking.  you can control the rear of the case by insetting a 1/4" plywood back and do this by cutting a rabbit in the case members on the rear and if using 3/4" ply, (which you should be using) cut the rabbit at 3/8" and 1/4" deep.   Before cutting the 1/4" deep rabbit consider this and cut the rabbit to 1/2" deep.  This will allow you to attach a piece of 1/4 thick hard wood or solid wood to the rear and glue this member in place at midway.  I would make this piece about 1 1/2 to 2" wide.  Glue this to the plywood.  The purpose of this it to give you a method of pin boring or attaching the shelves at mid point on the rear just as I have suggested to do to the front.

I jumped around some but back to the fixed mid shelf.  At a point of about mid way you should attach a middle shelf at the ends of the case and at the mid stile and also at the rear now that you have a method of attaching them, (through the plywood back and into the 1/4 solid wood behind the ply back.)

If you do not the case will be weak at the front and can be racked over time from normal use unless the shelves are all fixed shelves.

Something to think about is the physical appearance of the case that is 4 feet high by 6 feet wide.  It will look a lot better if it is face framed and a mid stile used as it breaks up the wide width making the case look more uniform.  Now another thing to think about:  As the case is going to be four feet high that means that there is going to be a flat surface on top of which is ideally correct for added stress and this will come from it being used for a surface to set items on and or not out of reason for someone to sit down on it. (people do strange things and do not think before they act.  Cars hoods and fenders are not made to lean on or sit on but it happens).

If this is face framed and build correctly it will be very strong and will not rack and or fail from normal and abnormal use.

Some thing I recall seeing in one of the posts showing the illustrated photos of shelves with cleats.  One was a shelf of which had a rear cleat attached to the rear of the shelf.  Although it does make it stronger it would be much stronger if the cleat was mounted under the shelf member and not attached to the rear.  It would also look better as it would have no joint showing from the top of the shelf.

Need more help or additional explanation of what I am posting feel free to contact me.