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

Discussion in 'Remodel Forum & Blog' started by hd99flhr, Nov 1, 2010.

1. ### hd99flhrNew Member

Joined:
Nov 1, 2010
Location:
Virginia
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.

2. ### ThatguyHomeowner

Joined:
Aug 27, 2008
Occupation:
A bounty hunter like in "Raising Arizona"
Location:
MD
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: Nov 1, 2010

4. ### nukemanNuclear Engineer

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5. ### ThatguyHomeowner

Joined:
Aug 27, 2008
Occupation:
A bounty hunter like in "Raising Arizona"
Location:
MD
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: Nov 1, 2010