Mains, PSUs and other goodies.

[irving2008]

...stuff...

I assume that if the joists are too small there's nothing I can realistically do, short of miniaturising the build or rebuilding the attic?

Not at all, theres a few things you can do, sadly none of them very easy. The absolute simplest solution is to increase the size of the joists by bolting another alongside it. This is what you do in bathrooms under the bath. Its not ideal as a 4 x 4 still doesnt have the resistance to bending that a 10 x 2 does. Another, better, option is to add a second joist on top of the first a 4 x 2 on top of a 4 x 2 will make a huge improvement, however it does mean that you'll have a step up into the room, headroom is reduced and inward opening doors have a bit of an issue - but this is whats often done for loft conversions.

Another, more expensive option, is to bolt some steel angle alongside the joist. Obviously this has better bending resistance than wood so can use a much smaller cross-section, but it weighs more so some calcs need to be done about the overall floor weight and wall strength.

[Jonathan]

I notice irving's specified the loads in Newtons per square meter, so surely if you can spread the load by placing the benches on their own 'joists' running perpendicular to the existing joists the area and therefore number of Newtons supported is increased, perhaps enough? You'll probably be tripping over them all the time though...

[Rogue]

Another, better, option is to add a second joist on top of the first a 4 x 2 on top of a 4 x 2 will make a huge improvement, however it does mean that you'll have a step up into the room, headroom is reduced and inward opening doors have a bit of an issue - but this is whats often done for loft conversions.

...surely if you can spread the load by placing the benches on their own 'joists' running perpendicular to the existing joists the area...

They say that great minds think alike, but then again they also say that fools seldom differ. This is the first thing that occured to me when I read the post. Can the joist be on top of the floorboards? Basically creating a raised platform at one end of the room? Hopefully Jonathan and I have great minds but I must admit I'm not doing our side any justice at the moment.

Edited to add: Ahh, I guess you mean the extra joist being lined up exactly on top of the old joist, not perpendicular to it? Still, same question applies!

[irving2008]

They say that great minds think alike, but then again they also say that fools seldom differ. This is the first thing that occured to me when I read the post. Can the joist be on top of the floorboards? Basically creating a raised platform at one end of the room? Hopefully Jonathan and I have great minds but I must admit I'm not doing our side any justice at the moment.

Yes in theory, but you'll need to support the joist at the wall at each end using joist hangers and not just terminate it at the wall. Although just having an additional joist on the floorboard will improve the 'bending' aspect by spreading the load, it wont reduce (indeed it will increase) the shear force at the wall and the underlying joist could still fail with the load. The correct solution is to lift the floorboards, add new joists, cutting them back into the wall and then board over with 18mm OSB which will be a lot stronger than the old floor boards. The new joists need to be securely attached to the old ones at no more than 600mm intervals, either drilled and screwed through into the old ones (BIG screws lol) or you can use retaining straps screwed to lower joist and taken over top joist in a notch and back down to lower joist, tensioned, and screwed through into both joists. Tensioning the straps needs some sort of tool...

Incidentally the minimum joist size now acceptable for loft conversions and new builds is 9 x 2 but 10 x 2 is usual as this matches the 10"/250mm insulation depth requirement and allows direct boarding over.

[Rogue]

Yes in theory, but you'll need to support the joist at the wall at each end using joist hangers and not just terminate it at the wall. Although just having an additional joist on the floorboard will improve the 'bending' aspect by spreading the load, it wont reduce (indeed it will increase) the shear force at the wall and the underlying joist could still fail with the load.

Which realistically means "no" in this scenario. Fair enough.

The correct solution...

...sounds painfully expensive and simply not possible at this moment in time, certainly. Hmm.

Incidentally the minimum joist size now acceptable for loft conversions and new builds is 9 x 2 but 10 x 2 is usual as this matches the 10"/250mm insulation depth requirement and allows direct boarding over.

I think the first step will to be to work out the joist size and then plan around that in terms of what can be safely done. That might scupper things more than anything else.