Hello all. I read the recent thread about notches in joists with interest and would like an opinion on a job I am currently doing. I'm in the middle of fitting a new bathroom suite and upon pulling up some of the floorboards, I can see that whoever did the original plumbing cut notches in the joists to cross the bathrrom with the pipework. The notches are probably 10-15% of the depth of the joists, right in the middle of the span. I need to run a couple of new pipes along the same route for the power shower supply and was wondering if I should just widen the existing notches or whether I should drill holes near the middle of the joists? Would widening the notches weaken the joist further, or would it remain the same as long as the depth of cut is not increased?
Widening the notches is probably not much worse than having them there in the first place.
Wood is too rigid to be able to see how this works in practice.
Try some other construction material, such as toast.
Make a beam out of toast, long enough that it sags a bit under its own weight.
Now try cutting notches and stuff out of it.
Consider the floorboard beams. Assuming that they are equally loaded, they will take on the form of as segment of a circle, rather than a completely straight beam. This is not visible usually.
Imagine continuing this beam out till it forms a circle, loaded outwards all the way round.
As you increase the load, the beam sags more, and the radius of the circle gets less.
At some radius, the difference in circumference between the inside and outside of the circle exceeds the amount that wood can stretch or compress without breaking.
If you cut notches out, instead of the neat symmetrical circle, you get a point which bends more than it normally would, and sticks out, indicating that it's more stressed, as you effectively have a strong beam with a weak one in the middle. Here is where it will fail first.
Increasing the length of the notch does not make that much difference, as if the weaker segment is going to fail, it will.
Attic beams are in most cases, unless the nouse is very old probably going to be more of a problem. In practice, I suspect that knocking an inch off most modern build beams would not cause immediate structural failure in more than a very few cases.
(I am not a structural engineer, merely having done some physics. If your house falls down as a result of this advice, I reserve the right to point and laugh)
Our house has lots and lots of notches in the floor joists, made by various plumbers and electritians before we bought the house. Some are uncomfortably deep. As I've rewired and replumbed, I've made sure that any joists which look to have been compromised are sufficiently reinforced by screwing the floorboard firmly across the notch.
Wood normally fails at a knot, even a beam with deep cut notches in it will more than likely still fail at a knot, so providing you don't cut notches near large knots I would suggest you will not weaken the beam at all
and in the words of Ian Stirling.....> If your house falls down as a result of this advice, I reserve the right to point and laugh
Provided you are not increasing the size of the notch too much then all will be well.
In fact, if your alternative is to drill holes through the joists, and those holes (even small ones) are anywhere near the notches, then you will actually make matters worse, since you will be giving a 'route' through the joist for any potential break to occur. Far better just to have a slightly larger notch, in that case.
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