Loft conversion, floor joists and cutting a purlin...

[Cole Pearson]

Hi all,

This is my first post on here after reading A LOT of useful threads.

I have designs for a loft conversion where the joists (47x145) have been drawn to span just shy of 3m at 40ctrs. That span is from a steel beam in the middle of the house (dining room x kitchen knock-through) out to another steel, about 70cm off the external wall.

1) My first question is, if I do away with that second steel, the span would become 3.65m, too long for 47x145 at 40ctrs but would reducing that to 30ctrs, or even 25 or 20ctrs, be adequate?

My options for this seem to be either beef up the joists, use the steel or reduce the ctrs. Beefing up is not a great options as its a loft conversion and headroom is a premium, using the steel (in its designed position) means placing it onto a party wall with a wall plate and onto another internal masonry supporting wall right where an existing masonry purlin post is, meaning i would have to cut a hole into that purlin post. Hence i prefer the reduce ctrs option to increase the span without steel.

2) My second question is, IF i were to reduce to 20/25/30ctrs and that be adequate to solve the span of 3.65m issue, would those joists then form a strong enough structure upon which to run 2 sole plates (4x2s) across and build purlin support posts and wall to hold up the purlin of either side of a dormer?

The room is about 3m wide, with a purlin holding up 8 rafters. I plan to cut the purlin and 4 of the rafters out to form a gable ended, pitched roof dormer out to as close to the external wall as permitted dev would allow.

Id welcome anyones thoughts or experience with something similar.

Thanks,

 

[tony1851]

On 3650 span, 47 x 145 C16 would need to be at about 250 centres to work - deflection is the main issue.

Without knowing the load from the purlins, and where along the span the sole plates are to be, it's not possible to comment regarding their suitability, but on that span it would be highly unlikely.

 

[Cole Pearson]

Thanks Tony.

All the timber is C24 47x145. So at 250ctrs we’d be happy? What if I also doubled up?

The purlin is 700 along the span from the external wall, leaving 2950 before the slam reaches the steel over the kitchen / dining room knock-through. So I’d be running the sole plates at that 700 mark.

The sole plates would run the full width across all the 6x2s, and if I doubled them up and positioned at 250ctrs, those sole plates would span roughly 24 joists over 3000.

 

[cdbe]

Just out of interest, what is your objection to the steel? It can go further forward of the purlin prop (I'm pretty sure you can go closer than 700mm to the wall plate under PD). The purlin would usually come out in the converted area anyway, with the longer rafter span covered by deeper rafters bearing on a dwarf wall built off the steel.
Also, don't discount coming down into the bedroom below, some people seem to go to great lengths to save a bit of bedroom ceiling.

Also, assuming you have a steel supporting a first floor masonry wall above your ground floor knock through, has someone calculated that this is inadequate to carry a bit of loft conversion floor?

 

[Cole Pearson]

Hi cdbe, its a bungalow so the steel over the dining / kitchen knock-through is only supporting the loft conversion (which on that side of the property will be a little used guest bedroom) and the steel was calc'd as such.

Ive no objection to steel in general as ive already put one in where necessary, however this second steel would need to go under the purlin to support the walls either side of where the purlin gets cut for the dormer. The issue with that is it means that the steel needs to sit on the party wall and the wall where there is an existing masonry purlin support in the way. So without a lot of effort, steel is a much trickier option.

Timber is much easier to get where I want it so basically Im wondering if i double up the 6x2s and run them at 20 or 25 ctrs, will that be strong enough to build a partial purlin support wall, 70cms into a 365cm span.

Ive tried to draw what I mean but Im terrible. The "inside the room" drawing tries to show what I mean. The orange will be the sole plate run across and the green will be the remaining purlin, supported on that sole plate, after the middle of it is cut out.

 

[cdbe]

I did wonder if it was a bungalow.

I was suggesting the steel here, but if you can't get it in you can't get it in.

Maybe you could increase the depth of the rafters so they don't need a purlin (often necessary to fit the required insulation in any case)? I think it will help that the front wall of your dormer is (or appears to be) very close to if not on top of the front wall.

If your sole plate is on top of the joists it'll be above the floor across the dormer recess?

 

[Cole Pearson]

Yea I'm not sure I would even be able to get it in there, but I did also assume that the steel would need to be directly underneath the purlin. Heres another terrible drawing. This is basically what I imagined. a sole plate spanning the floor joists underneath whats left of the purlin after the middle being cut out, and another sole plate, closer to the eternal wall, as the base to the dormer gable wall.

Sounds like a combination of many things could be ok here. So if I double up every other floor joist, space them at 25ctrs, then bolster the rafters (there will only be 2 left on each side of the dormer) then perhaps ill have it covered. The weight of the dormer wall will be on the front sole plate, the weight of the dormer roof and side walls will be down onto the doubled up rafters which will be sat on the remains of the purlin which will be on a supporting wall spread across doubled up floor joists.

The dormer ridge beam will be on a joist hanger from the main ridge beam. That ridge beam will be supported by the stud wall im building on top of the internal masonry wall to form the loft guest bedroom wall.

 

[cdbe]

I don't know. To my untrained eye, having a big gap in the middle of the sole plate makes it worse because it's not sharing the load across all the floor joists. If you didn't have a roof at all and were building this from scratch (and you needed to have shallow floor joists) you wouldn't fit a purlin and a load of little rafters, you'd just use suitably sized rafters that didn't need additional support from the joists.

I think the doubling up and reducing centres suffers from deminishing returns and maybe just won't be enough.

Not wishing to get ahead of yourself, but as it's a guest bedroom have you considered a small en-suite in the other side of the roof while you're up there?

 

[Cole Pearson]

I'm hoping @tony1851 can chime in

My thoughts were that if the gap in the sole platewas only where there was no purlin to support, that would be fine as the remaining bits of the purlin is still supported directly below with the wall and plate. I cant see how a 2 foot length of purlin would be supported any more by the sole plate that extends a further say 4ft beyond the end?

I guess it sounds like the better option might be to beef up the rafters rather than the floor joists so the purlin can be done away with entirely. The rafters would take the load from the dormer cheeks and roof.

 

[tony1851]

Why does the sole plate go right across the opening where the dormer is? Wouldn't it stick above the floor, or have I misinterpreted it?

If the floor joists are C24, doubled-up, and at appropriate centres, it would be unusual to have them support the roof structure but it might work - with the emphasis on might!

You'd need to know the load from the purlin, and that depends on the slope and ridge-height of the roof, and the roof finish. Is there a second purlin higher up, or do the rafters go from the purlin straight up to the ridge?

 

[Cole Pearson]

Thats my bad drawing @tony1851 - there will be two sole plates as my latest image from my post at 3:40pm yesterday. One will be only spanning a few joists to support the remaining sections of the purlin - leaving the gap in the middle. The second will be spanning the full width and all floor joists to support the dormer gable wall.

Im starting to think that instead of beefing up the floor joists to support the span AND the purlin support wall and dormer, just beef up the floor joists for the span ONLY and then separately beef up the rafters to not need the purlin at all.