Posi-joist installation & cable support.

1Justin · Nov 3, 2014

I have a customer costing up a new build using steel herringbone composite timber/steel type "Posi-joists" for first floor and flat roof areas. I called up the joist manufacturer and was amazed to hear that no-one had EVER asked them about installing electrics between, and support thereof.

He said the metal struts were cut VERY sharp.

I'd expect to probably clip direct one or two cables to the side of the wooden part, but closer to the surface than I’d like, (where 50mm doesn't apply to ceilings, but good practice does). - More cables wouldn't work.

It won't look nice, but cheap and cheerful. - Perhaps timber laths fixed centrally to the steel with tie-wraps would solve is most easily? Or maybe a split flexi conduit hung from the struts, - or is that a crap idea?

Who has worked with these? - Did you do it more elegantly?

guest119 · Nov 3, 2014

Any photos?

1Justin · Nov 4, 2014

From the manufacturers website
I guess threading them inside between the metals would be the place.
Otherwise I think I'll fix a ply web to the side and screw cable clips to that.
I seem to have answered my own question.. Aren't forums great!

[ElectriciansForums.net] Posi-joist installation & cable support.

davesparks · Nov 4, 2014

Whack a bit of tray or basket through them

Johnuk · Nov 4, 2014

we work on house with them, there great saves drilling, to be far you can run them as normal, the metal is rounded off

1Justin · Nov 4, 2014

Johnuk said:
we work on house with them, there great saves drilling, to be far you can run them as normal, the metal is rounded off

Thanks Johnuk.
Well the pic seems to show the metal to be rounded, I'm only reporting what one manufacture told me (but I think several companies make the same stuff, maybe some have different machines).

Rgds

driverman · Nov 5, 2014

1Justin said:
I have a customer costing up a new build using steel herringbone composite timber/steel type "Posi-joists" for first floor and flat roof areas. I called up the joist manufacturer and was amazed to hear that no-one had EVER asked them about installing electrics between, and support thereof.

He said the metal struts were cut VERY sharp.

I'd expect to probably clip direct one or two cables to the side of the wooden part, but closer to the surface than I’d like, (where 50mm doesn't apply to ceilings, but good practice does). - More cables wouldn't work.

It won't look nice, but cheap and cheerful. - Perhaps timber laths fixed centrally to the steel with tie-wraps would solve is most easily? Or maybe a split flexi conduit hung from the struts, - or is that a crap idea?

Who has worked with these? - Did you do it more elegantly?

Just done house the same........I asked on here also. I used cables ties. If you clip to the actual joist you are less than 50mm from ceiling board. You could if you wanted use off cuts of cable instead of cable ties.
Compared to traditional house, you wont use many Tower clips, unless you use them behind dot n dab. I used metal capping. cable ties the way to go. 1 packet of large size
I'm referring to clipping the cables parallel to the joist. No problem at all going through them.

1Justin · Nov 5, 2014

Hi Driverman.
So no sharp edges in your experience, even if you went in between the side webs and along the joist? That's good.
I would probably also plan to fit some webs or tray to go across where span would be too large for most cables unsupported. (Or allow them to drape to the ceiling for support which is after all what we mostly must do on retrofit, but doesn't seem nice for a new build.).

driverman · Nov 5, 2014

1Justin said:
Hi Driverman.
So no sharp edges in your experience, even if you went in between the side webs and along the joist? That's good.
I would probably also plan to fit some webs or tray to go across where span would be too large for most cables unsupported. (Or allow them to drape to the ceiling for support which is after all what we mostly must do on retrofit, but doesn't seem nice for a new build.).

I would be very careful if you intend going between the Z metal parts, they can be sharp. also pick what part of the joist your going through so as to avoid the sharp edges whilst pulling in lengths of cable. I tended to install cables where I wanted them, then used cables ties to fix them against the metal part of the joist which is not sharp. That way they are out of the way from fixing material from above and below. Also allowing enough room for the insulation (if being installed) to go above the cables so as not to encase them 100%. The joist were 600mm apart.

elliot123 · Nov 5, 2014

I've worked with these on a new build, the ones that they had used didn't feel sharp enough to damage the cable maybe if went on a angle or something along those lines

1Justin · Nov 5, 2014

driverman said:
I would be very careful if you intend going between the Z metal parts, they can be sharp. also pick what part of the joist your going through so as to avoid the sharp edges whilst pulling in lengths of cable. I tended to install cables where I wanted them, then used cables ties to fix them against the metal part of the joist which is not sharp. That way they are out of the way from fixing material from above and below. Also allowing enough room for the insulation (if being installed) to go above the cables so as not to encase them 100%. The joist were 600mm apart.

Thanks.. If clipped to the metals where not sharp, and that's easy, then good. Which presumably means you supported cables crossing the joists? 600 is I think what I have here, but is a bigger span than recommended for most unsupported cables.