I just started my BEng, so I actually don't feel ready to assess real cases.

Upper floor of my quite standard British terraced house. We would like to hide an existing drainage pipe which is currently running along the wall. To do that, I cannot find other ways but running it through the pictured floor joist (I already removed the plywood underfloor, as you can see). enter image description here Now, I am very wary of introducing possible structural weakenings when boring the joist: its sizes are 7cm (parallel to the floor) by 20cm (perpendicularly to the floor). Since the outer diameter of the drain pipe is 43mm (even less if use solven weld), I should be fine according to the information I found from multiple sources on the internet (maximum 1/3 of the height of the joist, hole centered and away at least 2" from the highest and lowest faces of the joist).

However, I'd really like to have further certainty for my specific case: maybe somebody who knows about these things could have a look at the picture and the data I provided and confirm this is the case? Alternatively, should I hire a (real:) structural engineer for a proper consultation on this single issue? Or would that be overkill?

I'll be glad to hear anything you might have to suggest/explain/add.


  • 1
    $\begingroup$ I'm inclined to feel it would be fine - "quite standard British terraced houses" are often thoroughly overbuilt (in a good way). This is not an answer, though, just my personal gut feeling. $\endgroup$ Nov 28, 2019 at 18:33
  • $\begingroup$ Not sure about in the UK, but in Canada you would need to have this type of work signed off by a building inspector, engineer or an architect. You want this done for liability reasons should things go wrong. Modification that are not signed off on also may void insurance, or decrease the value of the home. $\endgroup$
    – Forward Ed
    Nov 29, 2019 at 15:04

1 Answer 1


You need to try to stay away from the joist supports. Because the shear is max there.

I would keep at least 15% of span away from the ends of the joist.

Also, the notches in your photo to allow joist blocking need to be checked. Have they been over-cut, chipped corners, etc? is the wall double joisted?

If you add two metal straps, say 12" long with 10- 12D common nails near the top and bottom of the face of the joist over and under the pipe it adds strength.

  • $\begingroup$ Is there an easy way to know the span? Could you please clarify what is a double joisted wall? Thanks! $\endgroup$
    – anon
    Nov 28, 2019 at 20:08
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    $\begingroup$ @anon, 1- the span is usually from wall to wall in these types, or from wall to a row of columns. you can get a sense of the span by jumping up and down on the floor, where it feels hard there is a header or support under it so that is the end or beginning of the span. 2- Any wall resting not on the wall directly it must be supported by two parallel joists or a beam under it. This beam or double joist then supports the ceiling rafters intercepting it perpendicularly. Most likely it is OK. But since you have to open the floor, time to check up for any defects. $\endgroup$
    – kamran
    Nov 28, 2019 at 20:43

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