I am pulling cable through my basement (old work electrical) and I have several cables I need pulled across the joists. To make it easy on myself, I’d like to open up a slot in the middle of the solid timber joist. The slot only needs to be about 1/2″ wide; from face to face; in the middle of the joist. How long can the slot be? The joists are 2×10, and the slots will be in the outer third of the span.
I’ve seen info on notches, and holes in joists, but never slots.
Thanks,
Tom
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Replies
Please don't do this. It will severely weaken the joists.
You'd end up with two pieces each having less that half the depth of the original joists. Since strength is proportional to the square of depth, you'd have two pieces each with under a quarter of the original strength, which gives you a total of less than half of what you should have. Worse than that, if you leave the end of the top piece unsupported, it contributes nothing to the structure, leaving you with less than a quarter of the original strength. Another issue to consider is that ripping lumber invalidates the stamped grade. Grading rules have to do with the size and location of knots relative to the whole piece, and changing the width like that can reduce the grade. A knot that is 20% of the width of the original piece would be 40% of the width of the piece ripped in half.
There has to be a way to run your wires without compromising the structure this much.
-- J.S.
In addition to the structural issue, you could run into a picky electrical inspector who would insist that you derate those circuits since you have many cables in a single "raceway" (heat dissipation issue). If you drill 3/4 inch holes (in the center of the joist, of course), you can easily pull 2 12 or 14 AWG cables in each (maybe 3 14 AWG cables).
Mr. r:
You don't run into raceway derating problems until you go longer than 24".
To the original poster:
How do you intend to make these slots? It's probably easier to just drill holes. Snap a chalk line and then use your combination square to locate a consistant starting point. Drill the holes with a 7/8" hole saw. When the point of the pilot bit breaks through, finish the hole from the other side to prevent tear-out. Chamfer the edges with your utility knife and sand flush. This is the accumulated wisom of the FineHomeBuilding forum.
~Peter, the former accidental janitor
Now that everyone is happy on the structural part, here's an elaboration of my point about derating the cables in that run due to heat dissipation: I have seen many electricians who, thinking it makes a neater job, drill a single line of large holes through the floor joists to the main panel in the basement and pull 10 or 12 home runs thru that one hole. Shouldn't the inspector rightly be concerned about heat buildup in the unfortunate NM cables in the center of that group? Now suppose the main panel is in an unheated drive-under garage and r-30 insulation is wrapped around all those conductors in the floor joist cavities. At this point we might not even have to call that inspector "picky" if he objected.
By the way, I suspect (but I obviously don't know for sure) that Article 100 defines raceways in a completely openended way just for a situation like this. ("Raceways include, but are not limited to, RMC, RNC, IMC, ...."). However, I'm not a pro electrician, so my argument will have to stand or fall on it's own merit.
PS: If you meant what you said about only 10 conductors, that's 5 NM cables (or less). they will easily pull thru two holes, and to make your slot you were going to bore two holes anyway (and then use a jigsaw!)
I remember in Rex Cauldwell's book that he was talking about cables being de-rated just for being flex-tied together in a job. So I guess it would again depend on the inspector.
In my area of the country, I notched the bottom of the joist (old growth full dimention 2x10) and put in a bottom steel plate. This satisfied the inspector.
I also ran cat5 through a 1½" hole in the centre of the joist. Passed.
What does your local inspector say?Quality repairs for your home.
Aaron the HandymanVancouver, Canada
Well, first of all, I wasn't talking about slotting the whole joist, only about a 3" section. Also, I was planning on doing this in the center of the joist, where little load is carried (the top is in compression, the bottom is in tension; the center is stable.) If the argument about a slot cutting the capacity of the beam by four is valid, then wouldn't it do the same thing when a hole is cut? The inspector suggested cutting 2-3" holes through the joists. I didn't want to do this because I know that would weaken the beam more in the parts that actually carry load. If I cut a 3" slot in the wood instead, then I would be removing less that half the wood than a 3" hole.
The reason I don't want to cut individual holes is that I will be pulling several wires around here, but I'm rewiring my house, so they will be done one at a time, and I don't want to worry about drilling holes close to a bunch of wires.
The point about derating the cables has been considered. I will only be running ten conductrors through this slot. If I have to run more, I will run them separately. Also, I don't think the derating would apply anyway, because they would be exposed to air on more than 75% of their surface area.
I'm not trying to cut corners, really I'm not. I just want to find a better way to do things than what I consider unsafe.
Tom
Big Country --
I think there's a little confusion about the orientation of your proposed slot. Is the long axis of the slot horizontal? If so, the stuctural impact is pretty much the same as a round hole whose diameter is the same as the height of the slot -- at least as long as the slot only a few inches long. And you're right, this horizontal slot will have less structural impact than a hole whose diameter is the same as the length of the slot. However, considering the difficulty of cutting this slot through the joists, the usual procedure is to bore one or more round holes.
You are right. The slot will run horizontally. like this
______
(______)
<--3"->
It really won't be easier, but since I can do this once, I won't have to go back and do this every time I wanna run another cable. Plus I don't have a fancy right angle drill.
Tom
Okay, we've got the basic physics worked out. But I'm curious -- how are you going to cut the slots? The obvious approach is to bore holes at the ends and connect them with a saber saw, but you've just said you don't have room to get a conventional drill in there. And if you do have room to get the conventional drill in there, why not just bore the holes and be done with it?
I didn't say I couldn't get my drill in there (actually, I haven't tried yet.) Its just that I plan on using a cordless drill and a spade bit, which will take much longer to drill a hole than a right angle drill and an auger bit. I'm not sure if I could get the auger bit to work with the drill I have and the joist spacing. Drilling two 5/8" holes with this setup and then taking out the middle with the jigsaw will probably be easier than drilling five holes.
As far as why not just drill the holes- I dunno, probably just to be different. Plus I think it will be neater, easier to pull through, and easier to add a cable to. The basement is just a spaghetti of wires, and anything I can do to clean up the new stuff, I'm willing to spend more effort doing.
In the end I might just end up drilling the holes, but this just seemed like a good idea to me, but I'd never seen it discussed anywhere, including various electrical wiring books. Thanks everyone for their input btw!
Tom
big copuntry ---
You wrote
"I didn't want to do this because I know that would weaken the beam more in the parts that actually carry load."
Even I as an engineer don't know that. Despite the claim that the direction of the slot matters, I don't know if that is right either.
The code allows a 3" hole. Put a nice round 3" hole in.I got the room built. How do I get outside?
I think that Big Country is right in that what weakens the beam most is the cut in the vertical direction. This is why those manufactured I-Joists work - a strong upper and lower flange with a flimsy osb core (which, according to the TJM site, you can drill in just about anywhere) As long as his slot doesn't exceed the maximum hole size, it should be o.k. But I think his slot will weaken the joist much less than drilling the whole hole.
Ben ---
If you read the directions for I-joists, they do not allow holes almost everywhere. There is a required spacing between holes. (Usually 2 hole diameters) There are required distances from supports. It is a lot more complex than one would want.
As far as slots versus round holes in solid lumber. The codes limit holes to "bored" or "drilled" holes. Holes must be at least 2" apart.
As I read the code: Slots are not allowed.I got the room built. How do I get outside?
Ben ---
If you read the directions for I-joists, they do not allow holes almost everywhere. There is a required spacing between holes. (Usually 2 hole diameters) There are required distances from supports. It is a lot more complex than one would want.
George-
Sorry, I was unclear. They allow placement of round holes almost anywhere in the web, provided that you avoid the ends, and don't make them too near the edge. Spacing has particular requirements. I was just making the point that the web isn't the important part in carrying the load - the flanges are.
As far as slots versus round holes in solid lumber. The codes limit holes to "bored" or "drilled" holes. Holes must be at least 2" apart.
But, if you look at the proposed slot, the widest diameter is the same size as the maximum hole allowed. big country's proposal would be the same as drilling a 3" hole and just leaving some of the wood in the hole. Take for example if he could somehow get a hole saw up there and drill the hole, and then take the offcut out of the hole saw, cut it into thirds, and then glue the top and bottom thirds back with epoxy. That should be allowed by code (although the gluing step would be unnecessary). The way he does it will also lead the more important parts of the joist intact, the ones that are in compression and tension to support the load. I think it's a neat solution.
As I read the code: Slots are not allowed.
I sure as hell would not do this without convincing the inspector first. Probably easier to just rent the right angle drill and drill 3 or 4 one inch holes two inches apart. I couldn't find any code that says anything about slots, just holes and notches.
Regards,
Ben
> I wasn't talking about slotting the whole joist, only about a 3" section.
Oops -- my mistake. I thought you meant that the slot would run for the enitre outer 1/3 of the joist. Sorry....
-- J.S.
You must be able to do it; the maximum hole is 1/3 the diameter, or about 3" in your 2X10. A slot would have much less impact than a big hole, because as you say, the load is on the top and bottom chords. Just make sure it's in the middle of the Joist where the loads are neutral (the opposite of notching, where you should never notch in the middle third of a joist (you shouldn't notch any time, in my opinion).
Or you could rent the right angle drill - sounds easier to me.
http://www.wwpa.org/pdf/A11.pdf
Pete - he's talking about putting a slot in the middle of the joist, just where you'd put a hole, not notching or ripping a joist.