Question about structural integrity of loft conversion.

[adamthekiwi]

Hi All,

My partner and I have done some renovations to our 1880s "colony" flat in Edinburgh, including complete redecoration of the existing loft conversion (no dormers) - consisting of 2 bedrooms and a bathroom. In doing so, we replaced 3 of the 4 very small Velux windows with larger ones - this was covered by architects drawings and approved by building control.

We also, however, removed the short vertical walls (around 90cm high) that ran from the pitched wall/ceiling to the floor from roughly underneath the windows. We had intended to submit revised drawings for approval but completely forgot to get it done. These walls were light studwork, haphazardly nailed together and similarly nailed to the floor (not to the joists), covered with plasterboard. Their removal gives us lots of extra storage in one bedroom and makes the other feel much larger.

The BC inspection revealed this change and the officer (who was very nice and helpful) said that we would need to submit new drawings, which our architect has now done. The council have just got back to us and requested structural calculations to prove the soundness of the loft conversion.

They are, of course, completely within their rights to do this, and I understand that the work has to be judged by today's standards. I just wanted to get a feel for whether we should be worried about this? Can such an insubstantial wall add meaningful structural integrity? Assuming it has made a difference, or that we need to add more strength, how would we go about this?

Ascii drawing:

/ /------ / / / / /| / | / |

[RubberBiker]

Step 1 - check the original structural calcs, and see if the removed portion of stud was figured in them (it may have only appeared in the drawings, and not the calcs).

Step 2 - if it wasn't, you're home free - if it was, you'll have to re- run the calcs, no two ways about it.

Step 3 - if you're lucky, the rafters (on which the velux's sit) will still pass the re-run clacs as they are (as they will to some degree have been overspecified up to the next regular timber size).

If you're unlucky, extra support will be needed. Possibly just a triangular infill (i.e. the triangle in the plane of your diagram) under the rafters either side of the veluxs will suffice.

[Steve Firth]

Short answer yes. Such structures can be very important. A similar wall on the third floor of my current home is what stops the roof truss from collapsing under the load of the tiles. It may not be necessary in your case but the BCO is correct to point out that you need to have the calculations performed. I suspect he's also concerned tha who ever installed the wall may have removed structural parts of the truss to do so.

[John Rumm]

Yes it can. Its a popular misconception that stud walls can't be load bearing, when in fact there is no reason why a stud wall can't be - and often they are in situations such as you describe.

You may find for example that originally the rafters were supported mid span by a purlin of some description[1] and this was removed to make more space. Its function however would have been replaced by dwarf walls that prop the rafters and transfer the load to the new loft floor.

[1] Assuming it has made a difference, or that we need to add more

Being realistic - by putting the wall back.

The alternative would be to increase the size of the rafters to compensate and that would at a minimum require removing all the internal boarding on them. Fitting deeper ones would obviously be difficult, although you may be able to double up each one - or possibly make it a flitch beam with an additional rafter and steel plate bolted to the side. Basically alot of work. YOu would probably then need to reinsulate to current standards.

A structural stud wall I built on mine for exactly the purpose described:

[Harry Bloomfield]

[e-mail] explained :

Was there/ is there a large wooden beam above the stud wall running horizontally right angles to the roof timbers - or in other words along the full length of the loft?

If not, then the stud wall might have been providing some additional support to the roof timbers, bracing between these and the loft floor.

[Roger Mills]

Or even if there *is* a beam (purlin), it might be of a smaller section than would be required to support the roof rafters unaided - and might rely on the stud wall for extra support. If this is the case, you'll need either to re-instate the stud wall or to support the rafters by some other means - such as a steel purlin running between the end gable walls.

[adamthekiwi]

No, no purlin (at this height).

I guess so, although I cannot believe (given the standard of work in this stud wall) that it was constructed with support in mind. To describe the stud work as haphazard would be charitable.

I think there is a purlin at ceiling level, with horizontal joists (supporting the ceiling, forming the floor of the uninhabited loft space above) running from one side to the other (the dashed horizontal line near the top of the primitive ascii diagram I originally posted). This would be about halfway up the height from the bottom of the pitch to the roof apex. I'll check.

Don't know if it make any difference, but the joists and rafters are pretty substantial, at around 170mm depth.

Thanks to everyone for lots of useful information.

Cheers - Adam...

[adamthekiwi]

out a photo and post it, but essentially it was 45mm x 45mm vertical members nailed (badly) into the side of the rafters and dropping to floor level.

Cheers - Adam...

[John Rumm]

If you are lucky they were just battens inserted to give some thing to fix the plasterboard to.

[John Rumm]

It does in the sense that the deeper the rafters, the longer the unsupported span they can manage - and by implication the less likely the need for a supporting dwarf wall.