Why drilling and notching rules matter
I once worked alongside a plumber
nicknamed “Ethan the Terrible.” The
name had nothing to do with his
skills; he was one of the best
plumbers around. But if a floor joist
(Fig. A)—or anything else—stood in
the way of his pipes, he’d break out a
saw or drill, then slash or bore away
until he made room. Fixing the
ruins—at least for us carpenters—
was the “terrible” part.
You can’t simply cut, notch and
bore through structural members
and expect your house to remain
strong and your floors flat and solid.
Building “from scratch” with smart
planning can limit the need for
most notching and boring. It’s
during remodeling—when you
have to run wires, pipes, gas lines
and ducts through the joists
already in place—that it becomes
important to know the rules about
tampering with joists.
Plenty of trial and error plus a
healthy dose of engineering and
testing have gone into the official
rules in the building code. Here
we’ll examine the rules.
How joists support floors and ceilings
Joists—the horizontal members
that span two walls and/or
beams—have to carry the weight
of the walls, people, furniture,
appliances and other stuff we place
on them. When a load is applied to
a joist, the wood fibers along the
bottom edge go into tension and
those along the top go into compression
(Fig. A). As long as the
top fibers and the bottom resist
these stresses, the joist will do its
job of keeping your floor strong,
straight and solid. But when you
notch or drill a joist, you cut some
of those fibers and reduce the
joist’s ability to withstand compression
or tension. And if you
create a hole or notch too big or in
the wrong place, you seriously
weaken the joist, making it
bouncy and saggy, and in a worst-case
scenario, give it an easy place
to crack and fail. In bathrooms
and kitchens, water makes a beeline
for these low spots, promoting
rot and worsening the situation.
And while catastrophic
failure is rare, the sag and bounce
you experience every time you
cross a weakened floor is irritating. Plus, walls crack,
doors stick and marbles roll under the dresser. That’s the
real reason you want to follow the notching and boring
guidelines.
Figure A: How Joists Work
When weight is applied
to a joist, the top edge
goes into compression
while the bottom edge
goes into tension.
Improperly sized or
placed holes and
notches weaken the
joist, make it bouncy
and saggy, and provide
an easy place for it to
crack and fail.
Follow the rules to keep your floor solid
You can notch and bore joists without sacrificing critical
strength, but you must follow the rules.
If your home is more than 20 years old, your floor joists
are most likely solid wood 2x8s, 2x10s or 2x12s. The notching
and boring rules of thumb for solid lumber are shown in
Fig. B. Codes don’t directly address how many holes and
notches you can cut in a joist. One rule of thumb is to provide
twice the distance between holes as the diameter of the
largest hole. Also, notching the top of a joist weakens it less
than notching the bottom, and you should avoid locating
holes (and notches) near loose knots.
Engineered wood I-joists have begun to replace solid wood
joists during the last 20 years. If you’re remodeling or
building with I-joists, your lumber supplier can provide
you with a set of hole-drilling standards to follow. The
guidelines for these are shown in Fig. B.
For those working with the less common open web or floor
truss joists (Fig. B), the guidelines are simple: You can’t notch
or drill them anywhere. Period. The cool thing about these,
however, is that the space between cross members is usually
big enough to accommodate big pipes, even ductwork.
The rim joist (Fig. B)—the framing member that runs
around the perimeter of your house and that the floor joists
butt into—often gets riddled with large holes. This space is
the logical exit point for dryer and furnace vents, big electrical
service wires and fireplace fresh air intakes. Since the
rim joist is continuously supported by the walls or foundation
it rests on, the strict notching and hole-boring rules
don’t apply. Just don’t create a large hole or notch directly
under a group of studs that support the end of a beam, or a
window or door header above.
When in doubt, consult a structural engineer or your local building code official, who will have the final say.
Notching and boring rules
Figure B: Notching and Boring Rules
Engineered I-Beam Rules
- You can never, ever,
ever notch or bore
through the top or bottom
flange or chord.
- You can drill 1-1/2 in.
holes anywhere in the
web. In fact, most have little
perforated knockout
holes that can be punched
out with the swing of a
hammer.
- Drill large holes toward
the center of I-beam joists,
not toward the ends.
- Leave at least 1/4 in. (or
the amount specified by
the manufacturer) of web
between the edge of the
hole and the flange.
Joist Boring Rules
- Holes must be at least
2 in. from the top and
bottom edges of a joist.
- Maximum hole size is
one-third of the joist
depth.
Joist Notching Rules
- The maximum depth
of a notch at the end
of a joist (where it
rests on a wall or
beam) can’t exceed
one-quarter of the
joist depth.
- Maximum notch
depth in the outer third
of a joist is one-sixth of
the joist depth.
- Limit the length of
notches to one-third of
the joist depth.
- No notching in the
middle third of a joist.
Floor Truss Rules
No notching or boring anywhere!
Good planning means less notching and boring
If you’re building an addition or a new home, you can avoid
most notching and boring by planning ahead to provide
adequate pathways and space for pipes and ducts.
For plumbing clearances, follow the guidelines
shown in Figs. C and D. The place where
you’ll be most tempted to create oversize holes
and notches—under the bathtub—is the part of
a floor you can least afford to weaken. A cast
iron tub, with water and occupant, can weigh in
at more than 800 lbs. If you need to cut through
a joist, firm up the floor as shown in Fig. E.
To minimize problems with ductwork:
- Position bathroom vent fans so the ducts can
run parallel to the joists—right up to where they
exit the house.
- Make sure not to install joists so they run parallel
to, and directly under, walls where you intend
to install ductwork.
Finally, if all else fails, drop your pipes
and ducts down below the joists and box them
in with wood-framed soffits. They’re least
obtrusive when run along an outside wall or
center beam.
Clearance for bathtub P-trap
Figure C: Joist Clearances for Bathtub Drains
Leave at least 6 in. of clearance to one side and 3 in. to
the other sides of the drain to provide room for the P-trap.
Joist clearance for toilets
Figure D: Joist Clearance for Toilets
Leave at least 3-1/2 in. of clearance from the
center line of the toilet to the nearest joist to
allow room for the waste pipe.
Doubled joists create space
Figure E: Create Space by Doubling Joists
If you need to remove part of a joist, double the joists to each
side, then add doubled joists between them to carry the load of
the severed joist. Use joist hangers for all connections.
