Replies

1. 
   hurnik | Oct 15, 2005 05:06am | #1

   I'm not an engineer, but I would think that a 3' hole in a 2x10 is WAY too big for it to handle.

   Now, when you say 3" hole "through" the 2x10 you state it's 1' from the edge, but where is it in relation to the top/bottom of the 2x10?

   IMO, you're much better off doing one of 3 things:

   1) dropping the ceiling that's below that floor and run the waste line UNDER the joist (I had to do this, but it was right above the 1st floor tub, so it's not too ugly and I also had to go 5' this way otherwise cut through 3 joists)

   2) You MAY be able (again, I'm not an engineer) to "box" that joist (ie, you basically cut a section of the entire joist out and box around it with double 2x10)

   3) Either double up the joist, and/or I've seen metal "sheets" that have the holes in them already and you secure them to the joist (not sure if you need them on both sides) which will add the structural support you need. But I don't know if they go as large as 3". and the assumption is that you're not at the bottom or very top of the 2x10, but rather in the middle (height-wise).

2. 
   4Lorn1 | Oct 15, 2005 07:22am | #2

   General rule of thumb is 'The rule of thirds': maximum hole size is 1/3 the actual, not nominal, joist depth. This hole has to be centered in the joist side. Also the center third of the joist span has to be avoided when drilling and no more than three joists in a row are to be drilled at maximum size at the same distance from support.

   Applied to your case you have a 2 by 10 with an actual depth of something close to 9-1/2". You claim to be drilling a 3" hole. Cool enough if it is only a true 3" hole because three inches is less than a third of the 9-1/2" depth.

   On the other hand if your running a 3" drain line the actual hole size will be larger. Closer to 3-1/2". Which would by OK because 3-1/2" is greater than 1/3rd the depth of the joist. The joist would need to be 10-1/2" deep. A bit deeper than what you have.

   On the other hand the general rule is for holes drilled and no reinforcement added. If the hole is actually only 3' diameter and well centered your OK. If it is a bit larger your border line and reinforcement might get you by.

   You say your close to the hanger and that implies your not in the center of the span, where the stress is greatest. Avoid drilling too close, directly ajacent to the hanger and in crush zones where the hole would be over a support or between a load and support.

   The three joists in a row rule is only occasionally included in the general rule but it is said to prevent the concentration of forces across the joists. You mention only drilling one joist so it doesn't seem to apply.

   Frame carpentry used to be full of 'rules of thumb' and time tested proportions and ratios that, for the most part, worked well for generations. Later engineers and proscriptive codes got involved and it was clearly demonstrated that the old 'tradition' standards were sloppy. Sometimes too conservative and other times risky.

   The 'gold standard' is to consult with an engineer but if that hole is actually only going to be 3" in diameter you are, IMHO, ready to drill and not worry about reinforcing the joist.

1. 
   ron46 | Oct 15, 2005 04:13pm | #3

   thanks much for your info on my question. sorry it wasn't completely clear. first the bath is above the front foyer so no dropping down below the joists here. i'd checked in to drilling the joists and found the 1/3rule or 3 1/8inch diameter in this case. the center hole will be just above center to take in the slope of the pipe. i'll have to drill 2joists only, to get to a clear one, then will turn it and run it in the opening to the wall. here will drop down below the joist and box it in against the wall much neater. i'd heard of the metal plates you could use but think in this case that's to much,so thats where the plywood came in. for now there's only a vanity above this so i don't see a big problem but down the road who can say. so thanks again