Notching Rafters for wires to res

Question

Is it safe from a structural standpoint to put small notches ( 1 inch deep x 3 1/2 in long at the deepest/longest point, not the whole area) in 5ea of the 2x6 bottom chords across attic area that run from front of house to back of house to allow wires running though the area to lay so i can put plywood pieces down and use area as storage and not have the plywood sitting on wires?

Answer 1

No, it's not. You'll weaken the rafters. Edge cuts are particularly harmful to structural members.

If anything, you would rip a whole bunch of shims, (say, 7/8" thick), and lay them across the tops of the rafters, omitting them where the wires are. Or sister the rafters.

• However, this would create a second code violation: wires too close to the floor surface, and vulnerable to being nicked by nails. This can be remedied by putting steel "nail plates" above the wires to guard them from nails up to 1-3/4" long. However it wouldn't be enough to protect only above the rafters, because the cables are being drawn taut between rafters, so they are vulnerable there too, especially near the rafters.

  • Stapling the wires to the rafters and using sufficiently thick shims would take care of that.

You're also not allowed to cut the -- well, hold on. You're not allowed to cut the cables and then make a desperate, taut splice with what little length remains. You are allowed to cut, reroute and splice additional length in the middle. However all splicing must be inside junction boxes*, and the boxes must remain accessible forever. I recommend large-ish steel boxes, because if you do have a bad connection that arcs, the steel won't melt and burn through, and will distribute the heat around the entire box, reducing the chance of any point turning hot enough to ignite building materials.

Steel junction boxes, plus non-flexible metal conduit (EMT, RMC or IMC) are a great way to make the "shim" strategy actually work. Now the shim needs to be no thicker than the pipes, i.e. 7/8". These tough pipes are their own nail plates, taking that problem off the table. Inside the conduit, you'd use THHN/THWN-2 individual wires; up to 4 circuits per conduit pipe. The pipes are also their own grounds, meaning no need to push ground wires through the conduits. Most circuits only need 2 wires, and the 1/2" trade size of these pipes can support 9 or 10.

• There's no profit in using 3/4" pipe. You could use 1" pipe and put up to 10 circuits in a pipe, but the wires in the pipe would need a wire size "bump", 14 AWG cable -> 12 AWG THHN, 12 AWG cable -> 10 AWG THHN. In those cases I prefer to lay two or three 1/2" pipes, especially if bending is involved.

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* If you ask how to splice without a junction box, the Home Depot clerk will cheerfully press one of these into your hands. No mention will be made of the fact that these are illegal, except in very particular applications. These are the same dim-bulbs who will cheerfully recommend a NEMA 10 dryer outlet "since it's universal".

Answer 2

Some people are lucky. I suspect you’re one of those...

You say it’s a 2x6 bottom chord of truss that extends across your house. If so, the member is in tension and a small notch will not matter, provided: 1) the span is not greater than 50’ or 40’ in a heavy snow zone, AND 2) you do not overload the new space.

1) Bottom chords of trusses are in tension only. So, notching the member does not affect the “bending moment” because it’s not in bending. A 2x6 bottom chord is extremely large. Most are 2x4’s. Taking a notch out of the 2x6 will not cause a failure unless the truss is over stressed.

The bottom chord could be over stressed if there is an extra long span, extra large snow load, or an extra large floor load from above, or a superimposed load from the roof.

2) Ceiling or attic spaces are designed for about 10 lbs. per square foot. When you add plywood to the top of the 2x6’s and start storing things on top, you’ve changed the space from attic to living space. Living spaces are suppose to be designed for 40 lbs. per square foot. So, you can do what you want, just don’t overload any one area. (Store Christmas decorations not stacks of books.)

Btw, a picture of your attic would help confirm my answer.

Answer 3

Without knowing any details of the structure, it's safer to add 1" thick strips (with gaps for the wires) to raise the floor slightly than to notch the (lower truss member or joist) since that may already be marginal at carrying the load.

You should use protective steel plates over the wire gaps to resist anyone nailing into them later on.