Hole Diameter for running cable

As others have pointed out, the strength of the joist is not affected by drilling holes in the proper locations. Assuming the structure is sound to begin with, if you follow the guidelines of the IRC which I posted earlier, you won't need to shore up the joists at all.

That's a very different problem -- a very significant one if you happen to be abouard that ship (:^)) -- which doesn't relate to your issue.

True, but drilling a 2" hole or even multiple 2" holes in the joist does not reduce the cross-section of the framing.

It helps to remember that the purpose is to reduce the flexing of the beam (assuming the holes are not at the very ends of the joist).

The material at the top of the beam resists flexing because it resists compression. The material at the bottom resists extension (stretching).

There is a plane in the middle that is neither stretched nor compressed -- that is the material that does not add to strength and is just added weight.

If you remove that material, you don't increase flexing of the beam. (This assumes a homogeneous material. If you have big knots or other weak spots, there can be shear failure.) So boards that extend a two hole diameters to either side of the hole should be ample using three screws per side (assuming steel or plywood that won't split.) But as others have written, if the hole is in the center of the beam, little strength (~= resistance to bending) is lost.

... Marc _F_Hult

It helps to remember that the purpose is to reduce the flexing of the beam (assuming the holes are not at the very ends of the joist).

The material at the top of the beam resists flexing because it resists compression. The material at the bottom resists extension (stretching).

There is a plane in the middle that is neither stretched nor compressed -- that is the material that does not add to strength and is just added weight.

If you remove that material, you don't increase flexing of the beam. (This assumes a homogeneous material -- no big knots or other weak spots.) So scabbed boards that extend two hole diameters to either side of the hole should be ample using three screws per side (assuming steel or plywood that the screws won't cause to split.)

But as others have written, and as explained above, if the hole is in the center of the beam, little strength (~= resistance to bending) is lost.

... Marc Marc_F_Hult

Agreed - but this lumber is low quality with knots and other imperfections that could easily contribute to an overall weakening of the joist. I suppose as every arch has its keystone, every beam has a spot where a given hole can cause the most structural damage. Since timber was so precious in the early years of WWII the wood in these houses is clearly substandard, something I learned, of course, only after we moved in. The roof boards were severely cupped and the roofer said he believed they had not been kiln dried. There was no building paper used to underlay the floors and the walls above weep plaster dust into the basement through the subflooring joints.

Given that those are the shortcuts I can see, I'm going to brace the vacuum cleaner pipe holes - and I may only brace the joists where the pipe hole would run too close to a knot above or below the hole. I've been trying to line up "the best shot" for the pipe but a path that avoids all knots in all joists doesn't seem to exist.

-- Bobby G.

Too late now re: Hayden! It's do or die time. The joists aren't even close to 10" - they are 8" of green, knotty, splitting WWII ration-book lumber. Bracing will take me a couple of hours extra, at most.

-- Bobby G.

You don't need to restore the joist strength for 2" holes. Just keep them away from the center 1/3 of the joist and at least 2" apart from each other. Also, stay near the vertical center of the joists.

Beware. Hayden can sometimes be a royal pain when you need repair parts or warranty service. We dropped them after a couple of run-arounds.

If the joist isn't otherwise damaged, ignore the gut feeling this time. 2" isn't a large hole in a typical (10" or larger) floor joist.

Many communities now post the IRC code online. Google a question such as "what is the maximum hole diameter in a floor joist" and you'll get a lot of opinions plus several municipal code websites with the standards.

Bobby,

If the central vac system will have only one run crossing the floor joists, there's another solution that you may want to consider. You could simply run the line along one wall at ceiling height in the basement and box it in with drywall. This would shorten the ceiling by 3" along the one wall -- possibly not much of a loss. If you don't drill holes in the joists you won't need to brace them.

Both my house and the Liberty ships were built at the same time during the same material shortages. I'll bet the Liberty shipyards got the better timber. But my point was that engineers design things mostly to operate within expected environments. What might be sufficient for most uses:

1) Everything except rough seas in extreme artic cold for the ships, and 2) Everything but an extreme snow load for my house

Now I'm pretty sure my basement's not filled with 40' waves at 40 below zero (although it does smell a little like cod after a ten-year flood) but the analogy still holds, at least IMHO.

Overengineering is why I *used* to like Compaq PC's so much. They used connectors rated for 10's of thousands of insertions instead of just thousands, at least in the early PCs, and that gave them an "edge" in reliability that other machines of the era could not match. There was a price premium, but it was worth it.

-- Bobby G.

I have seen this posted several times and it could be misleading to the reader or it is wrong.

By "center 1/3 of the joist", I am assuming that you really mean the center

1/3 of the span where the stress is the most.

The holes SHOULD be in the center 1/3 of the joist, measured across the width of the board. As others have pointed out that is where there is very little stress on the board.

---- The origonal poster has also commented on knots being a problem. This is for him. A lot depends on the condition of the knot and it's location. A knot that is not loose or cracked is much less of a problem than a loose or cracked one. The tight knot will still disrupt the stress pattern but much less. Knots that are towards the top of the joist are in compression. I that location, they are less of a problem. A loose or cracked knot at the bottom edge of the joist is in tension and is almost equivalent to a notch there.

For strengthening the areas that you are unsure of, I would recommend the spike plate material that is used in building truss framing. It is strong and does a good job of marying the stress to the area under it with many "nails".

I assume we're talking about a joist like this with O representing a hole:

________________________________________________________

O Here O Not Here O Here

O Definitely not here! ________________________________________________________

There are knots on the top and bottom but I think I've mapped out a path that avoids any knots that are on the bottom of the joist. As you point out, that's where a fault can produce the most structural weakness.

That sounds like a good idea. I am just not sure how easily I can punch a

2" hole in something like that. I've got loads of plywood scraps, a hole saw and lots of woodscrews.

-- Bobby G.

I believe some information given in this thread may be incorrect.

According to the Boise engineered Wood Products Specifier Guide at

holes are allowed in the center of a span but not near the ends. The Guide has tables showing the minimum distance a given size hole can be from an end of a beam.

That document is for engineered beams, which are a different animal. Any cutouts near their ends will weaken that flimsy chipboard center section. Those beams have die-cut knockouts over their entire span for running wires. The limitation refers to larger cutouts, such as needed for ductwork.

Jeff

Yes.

I wasn't thinking of having the plate through the hole. For the same reason that the hole along the centerline of the joist doesn't weaken the joist much, the plate won't strengthen it much at that point. I was thinking about putting it on the bottom edge below the hole. I would only do this if you needed to put a hole close to the bottom or really close to a non-solid knot on the bottom edge.

Unless the plywood is really strong, my own feeling is that it won't help much.

Jon,

That is a requirement for Boise's engineered joists. They're not actually prohibiting holes "near the ends" but rather in the load bearing area. The intent is probably to maintain maximum compression strength at the point where the heaviest loads (i.e., walls and upper floors) are found. Boise actually permits up to a 1.5" hole anywhere along the span except at the bearing points as long as you maintain separation.