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Discussion Starter · #1 ·
I need to replace most of the 60-year old copper plumbing in my house, and have been considering PEX. Many of the lines need to be moved as part of a remodel, and as I pencil out how the new lines would run, I'm trying to decide if PEX is really meant to be run through holes in joists, like romex for example, or whether it should be placed beneath runners that span the bottom of the joists.

Expansion of the hot water lines is an issue, and it seems like there's not much room for expansion loops in the 14.5 inches between joists. On the other hand, getting the lines up between the joists and out of the way has its advantages.

I've looked all over for advice on this, but found nothing definite. Even a search of this plumbing forum for "PEX" returns no hits (??).

Any advice appreciated.
 

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No reason to not go through the joist. If this is just in a crawl space not a basement then just go under the joist.
 

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I assume you are talking about potable water, not PEX for heating. Potable water is rarely set hotter than 125 degrees F, occasionally I have seen 130 degrees F, versus hot water heat that can be 160 degrees, occasionally hotter. There is very little expansion of PEX at 125 degrees, so the hole through the joists can be slightly larger than the PEX, say 1/8 inch larger. If in any doubt, simply drop a short length of PEX into hot water, it will expand to whatever its final dimension will be very quickly, and you can test how large a hole you need.

As to the allowable dimensions for holes in joists, they are the same whether you are running wire, tubing, or just like to put holes in your joists. For 3/4 inch PEX, a 7/8 inch hole near the midpoint top to bottom of the joist should be OK, but you may want to use 1 inch to make it easier to pull. I have a fair amount of PEX run exactly that way in my basement. You can of course hang the PEX from the joists, or put it between joists if it goes that way, but if you are perpendicular to the joists and you want to hide the PEX a little, joist holes work well.
 

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Discussion Starter · #5 ·
I assume you are talking about potable water, not PEX for heating. Potable water is rarely set hotter than 125 degrees F, occasionally I have seen 130 degrees F, versus hot water heat that can be 160 degrees, occasionally hotter. There is very little expansion of PEX at 125 degrees, so the hole through the joists can be slightly larger than the PEX, say 1/8 inch larger. If in any doubt, simply drop a short length of PEX into hot water, it will expand to whatever its final dimension will be very quickly, and you can test how large a hole you need.

As to the allowable dimensions for holes in joists, they are the same whether you are running wire, tubing, or just like to put holes in your joists. For 3/4 inch PEX, a 7/8 inch hole near the midpoint top to bottom of the joist should be OK, but you may want to use 1 inch to make it easier to pull. I have a fair amount of PEX run exactly that way in my basement. You can of course hang the PEX from the joists, or put it between joists if it goes that way, but if you are perpendicular to the joists and you want to hide the PEX a little, joist holes work well.
I mentioned expansion loops only because I found several reports online of plumbers being dinged by the building inspector for not using them, even on potable lines.

I guess I'm still unsure of the relative pros and cons of drilling all those holes in the joists, versus running all those runners. Neither one sounds appealing. Was hoping someone who had done it could say which was preferable.
 

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Discussion Starter · #7 ·

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As long as you put the holes in the correct location, which is in the middle third of the joist (top to bottom), the holes hardly weaken the joists at all. This is because the middle of the joist is the neutral axis of the beam, and carries no load. A small hole centered top to bottom has a negligible effect on the strength of the joist. You can of course put the lines below the joist using hangers, but the advantage of putting them through the middle is they are less likely to be damaged by impact, and they do not reduce headroom.
 

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You can put a hole one third the width of the joist dead center without loss of strength. The top of the joist is in compression. The bottom is in tension. The middle is neutral. The Bang ins look nice, but not really necessary, except at the end of a run. IMO. 1/4 inch larger than the pipe OD seems fine to me. If you are going through the joists you are going to want t milwaukee right angle drill and augers with lead screw, or you will struggle. A piece of scrap 2x jiffy clamped to back up the hole allows the lead screw to pull right through the joist. Really cuts down on the sweat.
 

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Discussion Starter · #10 ·
Are those inserts required by Code for joist penetration? It seems to me that the smooth hole left behind by a hole saw would be no rougher than the inside of those inserts. Why not let the joist support the pex on its own?

That way a smaller hole could be used (?)
 

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Are those inserts required by Code for joist penetration? It seems to me that the smooth hole left behind by a hole saw would be no rougher than the inside of those inserts. Why not let the joist support the pex on its own?

That way a smaller hole could be used (?)
When I started apprenticing over 20yrs ago my jman handed me a hole hawg with a 1 3/8 bit(we call them water bits) and said go dill out the 3rd floor of that apartment complex. Been using one ever since.
The plugs are needed to avoid vibrations and abrasions. There are other styles too- like suspension clamps
I don't use them on every hole-
 
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