DIY Help - Floor joists in the loft.

It still seems a common but sad fact that most UK houses are built with a mostly useless space in the loft. If you want to use it then you will often have to carry out some often extensive remedial work to either strengthen it for use as a room and/or making space in the roof woodwork for what you want. Ironically, newer houses are worse than older houses in this respect since the newer cheaper to build trussed roofs often leave little or no room to use without major alteration. When you alter these roofs, you end up having to revert the structure back to an 'old fashioned' style with purlins (the lateral large joists that stop the vertical joists from sagging).
Anyway, lots of people ask about strengthening floors in the loft and when this is requried/what size timber etc. The science is quite straight-forward. The joist performs 1 or two functions (usually) the first is to support the imposed load of the room: dead weight is the joists, floorboards, fixings etc and the imposed load is the stuff we have in our rooms, whether people, baths etc. The second function that joists often perform, especially in the loft, is to act as a tie beam for the roof or walls so that the weight of the roof does not cause the bottom of the pitch to spread out (imagine folding a piece of paper in half and putting it on the desk like a tent, what happens when you press down on the top of it?). In downstairs rooms the joists will also stop walls from collapsing inwards which might happen if you took out all your floors without supporting the walls first.
Your first question therefore is what weight or force do the joists have to take in order that they don't sag excessively causing ceilings to crack. This is largely dependent on whether the loft will be used as a room (in which case it is the same as all other rooms) or just for some light storage (in which case if you keep the storage above about 3 feet of the supporting wall in the centre then you would probably get away with the existing joists - the sagging after all is largest in the centre of the joist span.
An Architect or Structural Engineer will be able to tell you what the recommended joist sizes are and these are related to the quality of wood (commonly called C16 - weaker or C24 - stronger) and also to the span that the wood has to bridge. If the walls are 2 metres apart then the wood will sag less than if they are 4 metres apart. Unfortunately now these tables are not publically available and are held by an organisation called TRADA (of which architects etc are members). You might be able to get a friend to look for you or ask at your library. These tables might say for instance that a piece of 50 x 220 timber in C16 quality can span a distance of 4.64 metres (or more likely you will have measured 4.5 metres and found the cheapest and smallest size of timber that meets this span as 50 x 220mm). Nobody is checking mm but don't take any big risks because the few more pounds of larger wood will be less than repairing your plastered ceilings when they crack or fall.
If you are planning to fit the joists yourself then you need to be comfortable working out the loadings of the roof (unless you are simply adding in the joists without touching the existing structure). In most cases, in a purlin'd roof, you will need to move purlin supports to fit in your larger joists and if you are making a room, you will also need to remove the tie bars, remembering that the roof can spread or collapse (very quickly) if it is not supported properly! Often you will need to do a bit at a time, taking out a single support and building underneath, adding a new support before removing any more. Depending on how well the roof is currently supported (i.e. it might be bordering on the dangerous) you might have to put in several temporary supports before you remove anything. The roof is heavier than you might think!!!
The Building Control department in your local council can advise on all these matters and they will take you to court if your roof alterations hurt anybody!