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Thread: Drilling through joists

  1. #1
    DIY Senior Member nelsonba's Avatar
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    Default Drilling through joists

    I need to run 2" ABS through several joists. Assuming my 2x8 joists are actually 7.5 inches tall, this allows me to drill holes 2.5 inches in diameter (1/3 joist). Is this big enough to accommodate 2" ABS?

    If so, assuming it needs to slope at 1/4" per foot, I figure my longest run could be 4 feet between joists. Here is how I came up with that number.

    7.5" joist.
    Can't bore holes within 2 inches of top or bottom leaves 3.5"
    Hole is 2.5 inches.
    3.5 minus 2.5 = 1 inch of slope to work with.
    1 divided by 4 is 1/4"

    Sound right?

    My joists are 16" o.c. Does this mean 3 joists would be the max I could drill through? (48" divided by 16" = 3)

    Is there anyway to extend this? 4 feet is not nearly long enough to get my shower drain to the main stack. It's probably closer to 8 feet.

    Thanks

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
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    How long are your joists, and what is directly above?

    Holes for an 8' run of 2" pipe are above the usual limits there, and I would definitely not do that under a high-traffic area. Personally, I would use 1-1/2" pipe with minimal slope, but that kind of work-around might not be acceptable in your situation.

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    DIY Senior Member Livin4Real's Avatar
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    I don't think building code allows a 2.5" hole in a supporting 2"x8". I realize it says not within xx amount of edge but that doesn't mean you could cut a 4" hole in the joist and it be code. You should look up what percentage of the width of the joist can be drilled. If you do it your way your looking at less than a 2"x6" support for your house. I realize it may not be aesthetically pleasing to run it under the joists instead of through but it would be safer.
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    Moderator & Master Plumber hj's Avatar
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    Default holes

    If you do it your way your looking at less than a 2"x6" support for your house.

    Not a good engineering assumption. Heavy equipment trailers have holes drilled all along the rails but that does not mean they could have used narrower rails, or that their's are weaker because of them. And trusses have a lot more "hole" than they do wood.

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    DIY Senior Member Livin4Real's Avatar
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    Your missing my point. I'm not saying holes in material are bad, I'm referring to the percentage of the hole size in relation to the width of the material. 2.5" of 7.5" would be taking 1/3rd of the width of the joist in that particular place thereby reducing the load rating considerably, then factor in three in a row and you significantly reduced strength in that location. Heavy equipment trailers are steel and have totally different characteristics than wood, apples to oranges. Truss systems don't have holes drilled in the actual material, are engineered and reinforced with plates, again apples to oranges.
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    In the Trades Bob NH's Avatar
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    The strength of a joist depends on where the material is removed as much as how much material is removed.

    If you put a 3.5" diameter hole in the center of a 7.5" deep joist, the property that defines the strength in bending will be reduced to 89.8% of the original bending strength, or about 10% loss of strength.

    Here is a solution for you.

    1. Lay out your 2.5" holes so that the highest one is centered 1" above the centerline and the lowest one is 1" below the centerline, and the others are spaced between in a straight line. Bore the holes with a hole saw. Be VERY careful with alignment as it will be a close fit. The edge of the hole in the worst case should be 1.5" from the edge of the joist. You will probably need 2 5/8" holes to get the pipe through.

    2. Get one 8 ft 2x4 for each joist that you are boring, at least a pint of carpenter glue (the yellow stuff), and about 150 Torx head screws, 3" x #8. The Torx is because they are going to be tough to drive and Phillips is hard to hold in a tight space.

    3. Cut the 2x4s to 4 ft lengths and notch each one smoothly so it will match the hole edges for each joist. The easiest way to notch them will be to clamp a piece of waste stock against the edge and use a larger hole saw to cut the notch. It is OK if the notch is longer (the larger hole saw) as long as the depth is correct.

    You will install them as follows:
    a. Hole 2" or less from edge of joist - Both reinforcements on the smaller section; one on each side of the joist.
    b. Hole more than 2" from both edges of the joist - one reinforcement above and one below the hole, on opposite sides of the joist.

    The reinforcements above the joists will be against the subfloor and the reinforcements below will be flush with the bottom of the joist.

    Install the reinforcements with glue on both surfaces (joist and reinforcement) and screws about every 4" with an extra screw near each end. Drill pilot holes through the reinforcements before you install them and put the screws through to just flush at the point before you put them up. It will be a lot easier to use the driver.

    The reinforced joist will be stronger and stiffer than the original.

    It is possible to design a reinforcements for NOTCHED joists, which are a lot easier to deal with. However, it requires more care during construction.

  7. #7
    Jack of all trades frenchie's Avatar
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    Quote Originally Posted by nelsonba View Post
    I need to run 2" ABS through several joists. Assuming my 2x8 joists are actually 7.5 inches tall, this allows me to drill holes 2.5 inches in diameter (1/3 joist). Is this big enough to accommodate 2" ABS?

    If so, assuming it needs to slope at 1/4" per foot, I figure my longest run could be 4 feet between joists. Here is how I came up with that number.

    7.5" joist.
    Can't bore holes within 2 inches of top or bottom leaves 3.5"
    Hole is 2.5 inches.
    3.5 minus 2.5 = 1 inch of slope to work with.
    1 divided by 4 is 1/4"

    Sound right?

    My joists are 16" o.c. Does this mean 3 joists would be the max I could drill through? (48" divided by 16" = 3)

    Is there anyway to extend this? 4 feet is not nearly long enough to get my shower drain to the main stack. It's probably closer to 8 feet.

    Thanks




    Wouldn't pass code here. Holes have to be less than 1/4 of the joist. In a 2x8 joist, 7 1/2"... the biggest hole you could make would be about 1.75.

    Personally, I'd run it under the joists, and build a soffit around it if it's in a finished space. Or else I'd call an engineer.
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