Notching and Boring Floor Joists

By Frank G. Ross

It’s your house and you can do whatever you want with it, right?  Well, that’s true to a point.  You still need to follow some “rules” and “guidelines”, or in a word “codes.” 

A home inspection is not a code inspection, but there is some overlap.  Codes often deal with safety issues which do come under the jurisdiction of a home inspection.

Remodeling jobs often involve surprises.  Suppose you want to add SOMETHING (what?) that will require running electrical wiring perpendicular to several joists.  How do you know where to drill the holes?  What are the “guidelines” on hole placement?  (note here, click on this line of text to see a diagram that explains notching)

A floor joist spans the distance between two walls or a beam and a wall or two beams.  It is intended to support a load pressing down on it.  The pressure will be greatest in the middle of the span.  This means that the lower edge is being stretched while the upper edge is being squished.  The correct engineering terms are tension and compression, but it means the same thing.

When you cut away at the top or bottom edge of a structural member, you greatly reduce its strength.  And if you cut in the wrong place or make the notch too big, the joist can become bouncy or saggy.  Or, at the very worst, the cut section will be come an easy place for the joist to crack or fail completely.  You are better off drilling holes closer to the middle of your joists, to preserve the structural integrity of the members. 

The code requirements are pretty specific and will help ensure that your floor stays solid.  There are different sets of rules depending on the type of joists in the home.  Many homes have solid wood 2x8s, 2x10s or 2x12s.  Newer homes may have engineered wood I-joists or the less common open web or floor truss joists.

The requirements for the solid wood joists are the most complicated and can be lengthy to explain.  In this case, a picture is worth a thousand words.  Generally though, notches must be limited to 1/6 to 1/4 of the joist depth depending on the location on the joist.  No notches are allowed in the center third of the span.  Make sure the notch length is not more than 1/3 of the joist depth.  Holes must be at least 2” from the top or bottom edges of the joist. Maximum hole size is 1/3 of the joist depth.

For boring an engineered wood I-joist, ask the supplier to provide you with a set of hole-drilling standards to follow or use pre-drilled knock-outs if they are provided.  Notches are not allowed, EVER.

The floor truss joist rules are the simplest – no notching or drilling anywhere.  But since the design is an open web, this is not usually necessary anyway since the spaces between the web members provide ample options for cables or pipes.

Even with such specific guidelines to follow, I have, at times, seen situations when they were ignored.  Most often a plumber is the culprit, but do-it-yourself homeowner projects rank a close second.  As a home inspector, I will bring to a buyer’s attention obvious violations of these rules and then recommend further evaluation by someone more specialized. Consulting a structural engineer or the local building code official, who will have the final say on what is or is not allowed, can dispel any doubts.

Frank Ross is a Certified Utah Home Inspector for Pillar to Post Professional Home Inspection and a full member of the National Association of Certified Home Inspectors and a member in good standing with the Utah Better Business Bureau.  Readers may contact Mr. Ross at (435) 867-6400 or frank.ross@pillartopost.com