Steel goal post clearance to top of rafters

[nghakrmyk]

Hi there - I’m looking for some guidance on an extension that is ongoing. There has been a deviation from the original plans and I’m unsure if the builder’s / architect’s solution is going to address the issue (or if I’m asking the right questions). 

I am extending a Victorian semi with an apex-type wraparound - a steel goal post is to support the existing rear wall. In the drawings / structural calcs, the goal post beam was not cranked and it was intended the top of the beam would sit below the bottom of the rafters. Cold bridging at the top of the steel would have been managed, as I understand, as we’re using a ventilated warm roof. However the builders have now put the steels in, added the rafters and the top corner of the steel beam (where it meets the steel column) is 10mm below the top of the rafters. I had to raise it with the builders (not sure why it wasn’t spotted) and we have now been trying to find a solution. We are also limited in the extent to which we can add height to the roof due to existing 1st floor windows - this gap (ie from the top of the rafter to lowest window cill) is currently c.178mm. Lastly, the roof is setup at a 16 degree pitch. Not much flex as the builders don’t want us to go lower as we’re putting normal veluxes in.

 

A picture is attached which shows the steel corner pinch point.

 

The architect’s suggestion was to add a batten (eg 38mm high) to the rafters to allow for c20mm PIR over the pinch point and a 28mm air gap to the vapour membrane. Then counter battens + battens for slates. This leaves c.80mm to the window cill for flashing - TBC if we could possibly use thinner battens to make it 90mm. I’m concerned 20mm PIR is not much of a solution and that 28mm is not sufficient, but absent changing the beam to a cranked beam so it sits below the rafters, there don’t seem to have many options?

 

Any thoughts would be much appreciated. 

[nod]

The easy option would be to pull that insulation out a wrap and tape the steel with multi foil 

[Russell griffiths]

I would get two cans of illbruck fm330, squirt it in to every little gap and fill it proud of the rafters, then when it’s gone of cut it flush. 

[saveasteading]

Jam all the gaps with PIR and squeeze in foam, as above. 20mm will make a big difference, and the extra heat loss will be a few £ a year.

10mm tolerances are acceptable in house specifications... prob no blame.

[nghakrmyk]

Thanks all.

 

nod - if we used foil, my concern was that it would only be 10mm compressed at the pinch point so not offering much protection. However, there's an air gap benefit ie if we still used a 38mm rafter, we would then have a 38mm air gap (vs 28mm air gap and 20mm PIR)? Not sure if that would be a big difference.

 

Russell / saveasteading - I suspect this is what we'll go for so pack in PIR and foam as much as we can. My concern was really (i) damp / condensation and (ii) minimum gap to the cill, but the immediate reaction suggests I'm maybe over-thinking it?

 

If not clear from the picture, the steel column is not inside the insulation anyway and the column/beam join is not thermally broken, so question how much of a difference this will make in any event.

 

In terms of the measurements, it's a c.150mm difference to the plans so a bit more than I'd like.

 

 

[saveasteading]

2 hours ago, nghakrmyk said:

maybe over-thinking it?

I don't think so. It's great to think it through. You are now the world expert in this issue.