Purlin Removal or Alteration

[delta9]

Hi

Looking for some help and ideas! 😀

 

We are currently modernising a 1960s chalet type house (similar to the one pictured but not the actual house and no chimney present).

 

Planning to install 150mm PIR insulation in the existing and new rafters and also planning on gaining a bit more space by removing the existing stud walls and installing ashlar/dwarf walls, with some wardrobe space behind these ashlars.

 

The issue is the purlin is hindering the plans as it's sticking out a bit too far, ideally I think this would be best removed or perhaps there is another way (i.e. install a flitch beam 95mm x 75mm underneath existing purlin, then cut the existing purlin in half so it becomes 2 x 95mm x 75mm beams) but not sure if this can be done...

 

I know in typical loft conversions, steels/flitch beams would be installed underneath the ashlars, but again unsure if this would be necessary in our case.

 

Any ideas on how we achieve what we suggest? We are following the rules by the way, house is subject to building control approval for structural alternations.

 

I've not contacted any structural engineers yet (it looking like I need to), just going through some ideas I can then put forward to him/her.

Thanks for any help / ideas

 

[Temp]

I suspect the roof membrane isn't vapour permeable so will need 50mm ventilated void under. I would try for more than 150mm insulation if possible. We wish we had put more insulation in our 1.5 storey house.

 

Check 190mm is enough for floor joists with a 6.5m span. I don't have my old TRADA tables with me right now.

 

 

[delta9]

Cheers Temp for the advice,

 

Good point on the 190mm floor joists 👍 Will try putting more than 150mm insulation if possible thanks 👍

 

Yes, that is correct with the felt roof and we did plan on the 50mm air gap (cold roof).

 

Been reading/thinking since posting yesterday, I think the only way to achieve our plans is by removing the purlin completely and beefing up the existing rafters.

Question is, do we actually need steels/flitch beam underneath the new ashlar wall?

 

As you mentioned with the 190mm joists spanning over 6m, if these were not up to to spec, I suppose the solution would be to double these up (say every other joist) and in turn would these then be able to support the new ashlar walls 😬

 

Cheers

 

 

 

 

 

[Temp]

I checked..  My TRADA span tables are out of date but still provide a guide. They say 195x44 joists are limited to about 3.5 to 4m span. The table doesn't go as high as 6.5m. The max is just under 6m and suggests something much bigger is needed, perhaps more than 235 x 89mm. I believe the latest version allows slightly greater spans but not that much greater!

 

Do they really span >6m or do they rest on interior walls downstairs? 

 

 

[Temp]

I think given there are several potential issues (rafters, joists, purlin, dormers?) I would get an SE involved. Have him do a site visit and give you an initial opinion of the work involved and a quote for the drawings and calculations needed for Building Control Approval. 

[delta9]

Ok great thanks 👍

They don't span >6m, they are resting on ground floor walls as you say, so they span about 3m + (sorry for the confusion)

 

Yeah it looks like I need help from a SE 👍

Cheers