Loft conversion Purlin and new joists

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Hi, im in the process of planning my new loft conversion and have already had architects and structural engineers draw up plans which are not exactly what we had requested and im hoping someone could advise on some issues of disagreement.

1) we wanted to keep the loft Purlins in and remove the 2 props supporting it, and instead install the steel beams about 70cm directly below the purlins & support the Purlin with several 3x2 posts running off the top of these steels along the length of the purlin.

However the architect says the purling needs to come out and a box web structure built instead. This would restrict us greatly as we wanted that space beyond the purlin to remain as open as possible for storage.

2) we wanted the new loft floor to be suspended and totally independent from the existing ceiling, so that there is no noise produced for people in the bedrooms below when the kids are running around in the loft.

However the architect said this is not possible as the existing ceiling will sag and be unsupported (this makes no sense as its currently fine and a new suspended loft floor will not touch the ceiling so i fail to see why the ceiling would suddenly be unsupported) and has planned for the new loft floor joists to run alongside the ceiling beams and bolted together.

The loft space is large.
Party wall to party wall is 5.6 meters
Ceiling joists to ridge beam is 3.8 meters
There are two purlins on either side, 1 meter up and the second is about 2.4 meters up.

Any advise would be much appreciated
 
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It would be perfectly feasible to support the lower purlin with props off a steel beam below; in fact it would probably be more expensive to design and build some form of timber box girder.
You can support the timber floor clear of the existing ceiling joists, though whether the ceiling joists would sag depends on their unsupported span.
 
It would be perfectly feasible to support the lower purlin with props off a steel beam below; in fact it would probably be more expensive to design and build some form of timber box girder.
You can support the timber floor clear of the existing ceiling joists, though whether the ceiling joists would sag depends on their unsupported span.

Thank you for your reply.

In regards to the ceiling joists sagging, i dont see why they would sag if theres no weight being added to these? I'm not a structural engineer though so not really sure how a new loft may impact the existing structure. I was planning on leaving them as they are now, with the new floor suspended above these?
 
If the new floor is completely separate from the existing ceiling, and if you are not removing any existing support to the joists (sometimes they are supported mid-span by a timber runner supported by thin timber hangers nailed to the ridge or a purlin) then the ceiling will be OK.

Sometimes engineers point to the published load/span tables which will show that much larger ceiling joists would be needed for your spans. However, these tables allow for the extra live load when lofts are used for storage, which will obviously not apply in your case.
 
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1) we wanted to keep the loft Purlins in and remove the 2 props supporting it, and instead install the steel beams about 70cm directly below the purlins & support the Purlin with several 3x2 posts running off the top of these steels along the length of the purlin.

On ours the steel wasn’t quite under the purlin so a structural stud wall was designed to replace it. This was 4x2 at 400mm centres with structural ply fixed to one side. Bolted to the steel at the bottom and nailed to the rafters at the top.

I’m guessing your purlin goes all the way party wall to other wall?
 
Yes our purling goes from party wall to party wall. The main reason i dont want a stud partition type wall with ply board is we were hoping to keep that area open both for storage and as play dens for the kids, something they have been requesting
 
Ours has an opening but unfortunately the space is just about crawlable. A den would have been great!!
 

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