ok, this may be a muppety moment, but why would it be an issue? aren't the joists in the loft holding the roof up? thats pretty heavy
In more detail - the sloping timbers under the roof (the rafters) come together at the top (the ridge) and at the bottom generally rest on a wall plate (a block of wood at the top of the wall) - they are joined together at the bottom by a
joist that goes from one side of the roof to the other.
If there is no floor built into the roof when the house is built, all this joist has to do (apart from support a ceiling attached to it), is to stop the roof spreading - it certainly isn't strong enough to support a normal floor.
Older houses (especially Victorian) were over engineered and may have joists that are strong enough - more modern houses won't. You can of course measure your depth of your joists and look up to see if they are strong enough - but remember wood from before the 1940's will be stronger (because is grew more slowly) than more modern timber.
Putting in the new joists, or more usually steel beams (RSJ's) to support new joists is what makes up most of the expense of a loft conversion.