Dandie, don't worry too much about it. Your hoosie ain't going to come down.
As Mr Ha asks - can we see a supporting wall underneath these joists running at right angles to them and very close to where the cut-outs are?
If 'yes', then
definitely don't worry; that part of the joists will largely be under a 'shearing' force, so is plenty thick.
If 'no' then still don't worry, although it's not at all ideal. The closer they are to the centres of an unsupported joist run, the more bending force they'll be under (with the 'missing' top edge in compression), so here they
can make a very noticeable difference.
But I suspect that, since you presumably didn't notice any issues with your floor until now, it isn't particularly springy?
Now that the plumbers have done the 'right' thing, I'd certainly sort these cutouts tho'.
The holes drilled in the centres was the correct (best) method, as these parts of the joists don't have any significant forces on it. Cutting the notches out of the tops
does weaken the joists, but they are usually 'oversized' anyway and can cope.But it ain't ideal.
However, it largely comes down to
where these cutouts are along the length of the joists; if they were anywhere near the middle of the joist runs, then that could significantly weaken the floor, and I think you could at least expect more springiness in it. If they are close to the walls where they are supported, then I doubt you'd notice any difference at all.
What to do? Sos's plan sounds good. Are the plumbers intending to do this for you, or is there a chippie also working in the house who will do so? The reason I ask this is, if the plumbers haven't said anything to you about how to reinforce these top cutouts, then why the hell did they not just reuse them rather than drill fresh holes?!
Almost certainly you'd have been fine to use them for your new plumbing runs, except with the addition of wooden blocks in between the pipes (but not too tight to them to cause expansion noises) to come up to the exact same level as the joist tops to support the floorboards.
If the plumbers are thinking "
We'll do it the best way and drill central holes and the cutouts can simply stay there 'cos they are not a problem..." then they are a bit silly and haven't done anything to actually
improve the situation here. In fact they've made it
marginally worse.
So, bottom line is that you will almost certainly be ok even if you leave it as it is. If the cutouts are near the ends of the joists, then that's especially the case - it should be fine. But I'd definitely do 'something' with them since the plumbers have gorn to the trouble of being pedants...
Certainly, if the cutouts are anywhere in the middle of the joist spans, then
super-definitely reinforce them.
How to? As Sos says, cut strips of 18mm ply to span over these cutouts extending by - ooh - around a foot further each way past the cutout ends. These strips could be the full height of the joists - in which case you'd have to cut slots in them so's you can slip them over the now-centred pipes - or (what I would do) is simply make them wide enough to fit from just
above these new pipe runs up to the tops of the joists. Basically so's they cover these cutouts with a nice overlap.
Then I'd personally PVA (use plenty - the joists have a course surface) them to each side of the joists and then fix in place using a half-dozen or so 1.5" screws driven in at even spacings. Once that sets, it'll be bludy strong. Obviously make sure the top edges of the strips are dead level with the joist tops so's the floorboards remain perfectly level.
You could even do this by cutting timber battens to fit
inside these cutouts. They'd have to be cut accurately, tho', with the tinniest of 'slopes' at each end, and a good tight 'interference' fit in the slots. Again coated in PVA and 'ammered into place. These top parts will be in compression, so it's important that the inserts are dead snug.
Personally, the ply method is easiest to get right.
(I'd love to know if the plumbers are going to suggest the cutouts are sorted tho'! If they
don't recommend you do this, then while the hell did they not just keep them rather than make the situation 'worse'!
