Beware the Kerf
Kerf: A notch, gash, opening, slot, channel, or slit that is created in any material by the act of cutting or sawing that material. NB - ANYTIME you cut or saw material, there will be a kerf – even when cutting paper with a razor blade.
The kerf is responsible for many of the frustrations experienced by the do-it-yourself carpenter. It is also the culprit that can be held accountable for many an incorrect project materials list. And finally, the oft overlooked kerf has many a project error attributable to it. The kerf is a fact of life – something that simply exists, must be understood, and must be properly accounted for and dealt with. Failure to do so facilitates a downstream flood of errors and problems.
In Section 1 of Diagram A three views of a generic saw blade are shown. The first is a side view, the second is a front view in the scale of the first, and the third shows an exploded view of one kind of saw tooth profile on a blade. Note that the cutting surface/tooth is wider than the blade itself. This is common in saw blades. It is the width of the cutting surface that creates the kerf – NOT the width of the saw blade itself.
Section 2 of the diagram allows us to see how neglecting to account for the kerf can cause problems when creating the parts list for a job, and also lead to errors in the job itself. The piece of lumber shown, which we'll call a 2" x 4", is 8' (96") long. My job calls for 16 pieces of 2" x 4" x 12" in length. It seems that we can get those out of two of the 2" x 4" x 96"s: 16x12"=192", and 2x96"=192. But once we allow for the 3/32" kerf, we find that we're going to come up short of lumber – and/or have short-cut pieces. We'll have to cut each 2" x 4" x 96" seven times to get our eight pieces of 12" lumber, but with a 3/32" kerf, we're going to turn 21/32" of lumber into saw dust – leaving us well over ½" short of the required length for the job. So, if we want to get all 16-pieces of 2" x 4" x 12" and have a saw with a 3/32" wide cutting tooth, we'll need three 2" x 4"s – NOT two.
Some do-it-yourselfers, mostly novices, attempt to avoid the effects of the kerf by cutting on the center of their marking lines. All this does is yield slightly shorter than specified pieces of stock – an unacceptable road to remedy, to be sure. This leads us to another common mistake: marking all 7 cut lines at one time, before you start cutting. Though it can be done, it is almost impossible to accurately make all 7 cut lines before you start sawing. The reason? The kerfs. Measure, cut, measure, cut, measure, cut… Then you should have 7 of your required pieces sawed to the right length, and in the instant example, have one leftover piece of 2" x 4" stock that's 11-11/32" long.
A good rule of thumb to remember about the kerf is: the smaller the work, the more critical the kerf becomes. When you're cutting the edges of the final row of 4' x 8' roof decking, 3/32" is not really much to worry about. Conversely, if you are doweling together some pricey pieces of quality hardwood to make a custom cutting block, 3/32" is a great deal.
Establish routines for precision sawing. Always mark the line on the 'keeper' side of the board (the side you'll use for your job). After you make your cutting mark, measure it again to make sure you made the line precisely where it belongs. Then always cut to leave the line – but just barely. Remember the Carpenter's First Rule: measure twice, cut once.