Post joists?

[junglejim]

My posi joists are joined with cross members like the attached. I’m wondering if can drill through those or take out to run MVHR ducting along the length? The open web is great for running across the building but I also need to go parallel to joists. Any advice appreciated

 thank you

[crispy_wafer]

I’d get in touch with the supplier, they should be helpful in advising your options with your specific joist layout/size/depth..

[Tony L]

The posi joist mfrs call those "cross members" strongbacks.

[Gone West]

I had a similar problem at my last self build. I cut out the section of strongback, where I needed access and replaced it with a noggin top and bottom between the metal web joist flanges.

[Oz07]

I've also cut sections out before and noggin between

[Gus Potter]

On 10/08/2026 at 14:57, junglejim said:

My posi joists are joined with cross members like the attached. I’m wondering if can drill through those or take out to run MVHR ducting along the length? The open web is great for running across the building but I also need to go parallel to joists. Any advice appreciated

Very good question, looks like you are on the ball!

 

Below is a typical Strongback detail:

 

 

If you want to make a hole for a duct in these you'll likely have to add some plating detail or the like. It all looks quite doable..if your ducts are not too large. 

 

Can you dimension up your sketch / photo and mark on the hole and diameter you need to make? 

 

Plating with ply each side with glue is an easy option if your duct is not too large a diameter. There are plenty ways of getting around this, but we are looking for the simplest and cheapest option.

 

I say glue as if you just use nails when you calculate the forces you often need lots of nails and the nail edge and end distance just takes loads of time to detail. The nails are just used to clamp ply until the glue cures.