Support for new floor joists in loft conversion short of supporting wall

[craig1234]

I am about to start a loft conversion and used my neighbours buildings regs as a basis for mine. I didn't want to use steel to support the floor so got the structural engineer to alter the neighbours design to use additional joists (20cm spacing instead of 40cm). At the time I asked him why he used steel but he couldn't remember (he is retired). However, today I think I found out why as when taking a closer look in my loft to locate the supporting walls (my regs are currently under review with the council), I realised that at one end, the existing joists are not actually resting on a wall plate but are nailed to the rafters (it's a Victorian house). The reason for this is that in the room below, the edge of the ceiling slopes backwards from the window following the external roof line so essentially the end of the joists are 50cm or so back from the external wall.
Obviously I can't have my new joists resting on the ceiling and I will need to consult building control but does anyone have any suggestions as to how I might be able to support the new joists? My thoughts are that I remove sufficient roof tiles up to where the joists are visible (I need to do some work on the roof anyway) and for each new joist I add additional timber in parallel to the adjacent rafter (nail the new timber and rafters together) and create a flat platform at the top onto which my new joists would sit. Essentially I guess I would be doubling up the existing rafters. Does this sound like a plausible option?

 

[frutbunn]

As a BCO I wouldn't accept unless the SE can provide calcs to prove it.

 

[craig1234]

I appreciate that BCO will probably require some calculations but in the 1st instance do you think that what I am suggesting is feasible?

 

[^woody^]

The concept is feasible, but whether it works in that situation will need working out first.

 

[frutbunn]

I appreciate that BCO will probably require some calculations but in the 1st instance do you think that what I am suggesting is feasible?

I'm sure its feasible, but probably involves a little more work than you than simply fixing an additional similar sized rafter.

 

[tony1851]

Wouldn't it be easier to form a binder of - say - two 225 x 50s screwed together, spanning from party wall to gable off h/duty joist hangers. Then use long-leg joist hangers to suspend your new floor joists as low as possible between the existing ceiling joists.

 

[frutbunn]

Wouldn't it be easier to form a binder of - say - two 225 x 50s screwed together, spanning from party wall to gable off h/duty joist hangers. Then use long-leg joist hangers to suspend your new floor joists as low as possible between the existing ceiling joists.View attachment 202523

It'll need substantially larger timber than that or steel,

 

[craig1234]

I like the idea of that Tony - it would definitely be simpler. I will run it past the boke who did my structural calcs.

 

[tony1851]

It'll need substantially larger timber than that or steel,

Not necessarily.
The normal live load for domestic floors is - as you will know - 1.5kN/m² (about 30lbs/sq ft in old money).
When doing calcs for loft conversion floor joists, it is perfectly feasible to assume a lower level of live load at the eaves-end of the floor, on the basis that it is impossible to load the floor in that position to the full extent normally required. Where headroom in a loft is less than - say 1m - it can be suggested that the floor will only be carrying a much smaller load in that area - say 0.75kN/m².
This makes a big difference to the sizing/spacing of the floor joists, and also the size of the beam supporting the joists; that's why timber (rather than steel) might be an option, depending of course on the span.

 

[craig1234]

I will take a look tomorrow but from memory today, I think the wall plate in the window section may be accessible so I might be able to make use of this and thus only require two relatively short "binders" to cover the sections either side of the window.

 

[^woody^]

Wouldn't it be easier to form a binder of - say - two 225 x 50s screwed together, spanning from party wall to gable off h/duty joist hangers. Then use long-leg joist hangers to suspend your new floor joists as low as possible between the existing ceiling joists.View attachment 202523

You can't hang a floor off a binder, that binder would need to be a beam spanning the outer walls and would need deeper timbers and/or a flitch plate if timber.

 

[tony1851]

You can't hang a floor off a binder, that binder would need to be a beam spanning the outer walls and would need deeper timbers and/or a flitch plate if timber.

You can suspend a floor off long-leg joist hangers - that's what they're for.
As for whether or not timber would do - see 2 posts above. It depends on the span/timber grade of the binder, and how much floor it is supporting. Looking at the pic of the OP's house, it doesn't look particularly wide - perhaps one room wide?
Subject to number-crunching, I suspect timber would suffice; might need two 250 x 50, or even one 300 x 100. But timber far easier to manhandle and install than a steel beam.
Wouldn't need padstones, either.

 

[^woody^]

You can suspend a floor off long-leg joist hangers - that's what they're for.
As for whether or not timber would do - see 2 posts above. It depends on the span/timber grade of the binder, and how much floor it is supporting. Looking at the pic of the OP's house, it doesn't look particularly wide - perhaps one room wide?
Subject to number-crunching, I suspect timber would suffice; might need two 250 x 50, or even one 300 x 100. But timber far easier to manhandle and install than a steel beam.
Wouldn't need padstones, either.

What you are calling a binder, is actually a beam and needs designing.

 

[frutbunn]

You can suspend a floor off long-leg joist hangers - that's what they're for.
As for whether or not timber would do - see 2 posts above. It depends on the span/timber grade of the binder, and how much floor it is supporting. Looking at the pic of the OP's house, it doesn't look particularly wide - perhaps one room wide?
Subject to number-crunching, I suspect timber would suffice; might need two 250 x 50, or even one 300 x 100. But timber far easier to manhandle and install than a steel beam.
Wouldn't need padstones, either.

Over the years I have seen a good number done with timber beams, but as you suggest the timber beam has been in the order of 300x100 dependant spans, loads etc, another option would be a flitch beam, but again it would require calcs. Personally I would use steel with a spliced joint.

 

[tony1851]

What you are calling a binder, is actually a beam and needs designing.

Yes, of course it would need calculating.
('Binder' is just an old term for a floor-supporting beam - ref. McKay).