Drilling the bottom chord of a posi joist.

[ProDave]

I'n sure you will read the post title and say NOOOOOO you can't do that. But it's not quite that simple.

My posi joists run sideways across the whole width of the house, a total span of 12 metres, with two supporting walls.

At the very front and very back of the house, an "extra" posi joist makes the same "span" but it's not really being used as a joist, it's helping to support the walls of the upper floor. Here is a picture:

So hidden behind the Intello membrane are the front walls of the house. The posi joist is set roughly 2/3 above the wall and 1/3 overhanging in front of the wall. The upstairs wall rests on it in exactly the same way. (the intello membrane is the "tony tray")

So the joist is not supporting any weight in the conventional way a joist would, instead throughout it's length it is in compression from the weight of the wall above.

Now the problem I have is that it overhangs the wall by about 60mm.  I want to create a 50mm service void for pipes and cables. I have fitted one such batten to illustrate the problem, that the posi joist completely closes off the top of the service void so I can't get any pipes or cables through.

On the basis this is not really a "joist" the way this one is used, I cannot see an issue with drilling where required up through the bottom chord of this "joist" to allow cables and pipes through.

What do others say?

 

 

 

[Oz07]

Big coving to the ceilings! I'm sure worse has been done but are you best to speak to manufacturer

[Crofter]

Bit confused- is this joist at the same height as all the others?

Are you saying that the wall it supports is basically putting a shear load on each end of the joist, so there is not the normal compression/tension loads on the upper/lower flanges?

[ProDave]

Think of a "joist" that throughout it's length sits on the downstairs wall so is evenly supported throughout it's whole length.

Then the upstairs wall sits on top of this "joist"

It is not in any way supporting a mid point load and being subject to bending forces like a normal joist. In fact I don't even know why they used a posi joist in this position.

So instead of having the normal loads of a joist, that put some of it in compression, some in tension, some in shear, this one just has an even vertical compression load applied to it throughout its entire length.

Yes it is the same height as the others and the only reason for using a posi joist here is to ensure it is exactly the same size as all the others that are really doing the function of a joist.

[Crofter]

Ah OK. It's basically just a spacer, then?

Sounds like you could take some chunks out of it without worry, IMHO.

 

[TerryE]

I've got the same issue my PosiJoists.  I've attached a picture just before the lads slotted in this last joist on the 1st floor.  Internal walls and gable panels are 47mm or whatever lower than the side panels, so the joist top stringer sits on top of the gable frames but is level with the side frames.  The airtight membrane is taken up the internal wall and out to the external wall surface before the whole floor is floorboarded out (and so it also sits on the frame), and the membrane turned back in so the next storey's panels are dropped into position. 

The outer joists are hard against the panels. I am not sure what the structural / build rationale is here for positioning this just so, but as Dave says, they aren't load-bearing in the conventional sense as the floor boarding is full width so that it horizontally racks the whole structure.  I am just going to drill out mine where needed.  Luckily all of our wet piping is routed on the internal walls, so its only a case of the odd 12 or 15 mm hole for wiring.

[ProDave]

Yes just like TerryE's they are just like "spacers" He has an uncanilly similar arrangement to me, but I never got a picture of mine from above lke he did.

I personally can't see anything wrong with drilling them, I am not going to compromise their function as a "joist" as the edge ones are not "joists" but "spacers"

All I need is two holes for pipes, probably 22mm (cold water up from utility to rest of house, hot water down to utility) and a row of probably 20mm holes directly above the consumer unit for cables.  Everything else can find a different route.

 

[Jeremy Harris]

I can't see a problem at all with drilling vertically through the lower member.  There's no tension in that lower member as the joist isn't in bending, so there's only a compressive load through the webs to the bit of the lower member that's resting on the ground floor wall frame.  There will be vertical loads around the nail plates, transferred from the member above, so best to drill holes for cables, pipes etc away from those, but other than that it should be absolutely fine.

[ProDave]

Yes I was intending to drill "mid span" between where the nail plates fix to the bottom member, and not up in between the nail plates.