Notch Underside of Joists

Feb 25, 2005 3 Replies
T

Hi all



I am currently renovating a back room at one end of which there is a bar (as in for serving drinks). Above the bar area is a lowered "ceiling" section, basically timber frame with wood cladding. An extension was built by the previous owner above this bar with an en-suite and associated plumbing. Removing the box cladding below shows that the plumbing has been carried out in plastic pipe which hangs below joist level into the space above the boxwork.


So, to allow for a level ceiling throughout the downstairs room, can I notch the underside of the joists for 15mm plastic pipes or will this introduce serious weakness?



The joists seem quite substantial (can't remember dimensions off hand) and span about 3m. Are there rules about notching underside of joists? I've seen rules for cutouts in tops of joists eg position and depth as a percentage of overall joist depth. I would consider putting metal straps across the notches to avoid any plasterboard nails hitting the pipes.



TIA



Phil


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I

Undersides is worse. Is it possible to do a hole through the middle of the joist, and thread the pipe through it?

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S

I'd think so - the load's from on top, the deflection puts the top in compression and the bottom in tension. A "notch" will open up more readily if the side it's on is being stretched rather than squashed, I'd guess; certainly, the round-portholes arguments (sharp corners = stress concentration = crack propagation) say that if you *do* notch, you're a

*lot* better off drilling a 15mm hole and cutting down to its sides, than just making two cuts and chiselling out the waste.

That'd be better - the phrase "neutral axis" leaps to mind (since the top's in compression, the bottom's in tension, stands to reason that a band in the middle is under not-much stress). Sparkies get told to drill through in preference to notching (but much more to avoid cables running within typical-nail-length distance than for structural reasons), to limit notches to 1/8th the joist depth and make them within 0.1 and 0.25 of the span (i.e. close to, but not at, the ends), while holes shoudl be on the centre line, no more than 1/4 the joist depth, and towards but not at the middle (between 0.25 and 0.4 of the span), and no hole within

3 diameters distance of another: hopefully those guidelines are relevant to this situation too...

HTH - Stefek

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C

If you do so, and it is the correct way to proceed, then use plastic pipework (unless gas!), as it is far easier to thread through than rigid metal and doesn't require accurate hole positioning, or multiple joins.

Christian.

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