Quote Originally Posted by Rogue View Post
...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.