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Thread: Drilling through four 2x8s? Can it be done to run a drain line

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  1. #1
    DIY Junior Member hd99flhr's Avatar
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    Default Drilling through four 2x8s? Can it be done to run a drain line

    I am remodeling a bathroom on the top story of my house, where the bathroom overhangs the first level by 18". The floor joists in house run east and west, but they installed 4 2x8s (east/west) to support the joists (running north and south) for a portion of the bathroom. I am wondering if I can drill a hole through the 4 2x8 for an 1.5" drain line? The hole would be within the first 1/3 of the overall span of the 2x8s.

    Thanks in advance.

  2. #2
    Homeowner Thatguy's Avatar
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    Holes in the centerline, the "neutral axis", shouldn't affect the joist deflection.
    For other locations you need to find a reference that shows how much E, the modulus of elasticity, is affected by the hole and then do the tedious but not difficult calculations for deflection.
    Normal values of E for this wood range from 0.8 million PSI to 2.2 million.

    I'll see if I can scrounge up my Strength of Materials text.

    There are also patents on a metal sleeve that bolts to joists and allows you to remove a large chunk of the joist cross-section. It works on the principle that steel is about 18x stronger than fir.
    Last edited by Thatguy; 11-01-2010 at 11:45 AM.

  3. #3
    Nuclear Engineer nukeman's Avatar
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    ... or you could just use a guide like this:

    http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

    For 1.5" Sch 40, your hole will probably be 2" or so. As long as the hole is put in the right place, you should be fine.

    If you are talking about going through the cantilevered part of the beam, please avoid doing that.

  4. #4
    Homeowner Thatguy's Avatar
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    Here's another way.

    Assuming a 10% reduction in beam strength is "down in the noise" have a bunch of people stand over this composite 8x8 beam so that you can measure deflection due to this concentrated force.

    Drill a hole that is 1/3 the area of a 1.5" hole, which is sq rt of [(1.5^2)/3] = 0.87" in diameter. Recheck your deflection with the same concentrated load and estimate what lost area would give you a 10% loss in strength.

    Based on your estimate, enlarge the hole to 2/3 the area of a 1.5" hole, which is sq rt of [(1.5^2)/(3/2)] = 1.22" in diameter. If this larger hole still results in significantly less than 110% of the original deflection drill your 1.5" hole.
    Last edited by Thatguy; 11-01-2010 at 02:37 PM.

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