Can loft boards be nailed directly onto joists or are stilts needed?

there is no problem nailing to joists. the problem is what are you boarding for ..if its storage then generally no problem but you must make sure you dont put too much weight. if its for living then see a structural engineer and he will tell you if you need to replace joists. Stilts are used to raise height not for damp. You should have no damp in your loft anyway

stilts are good if you want to add extra insulation other wise for storage purposes fitting them directly onto joists is fine ..dont nail them as that might cause damage like popping holes or cracks to ceiling below ..we always screw them down plus easy to take up if needed

Presuming your loft is in your house (not outbuilding/garage) you will have ceilings below. The suggestion is that if you have ceiling attached to the bottom of the joists and then floorboards on the topside of them, there is a closed cavity created in between. (Which obviously may contain your insulation). And if you are not insulated between the rafters, you just have tiles and membrane in the roof so that loft space will easily get cold. Where the rooms/hallway below your loft will be warm from your heating. So there is possibility of condensation where the cold meets the warm. If you have these cavities, then moisture may build but not escape as there is no air for it to evaporate. It will be minimal but may build over time and it will depend on many other factors as to how bad it may get. But this is the theory. So some suggest that if boards are raised with gaps at the end of the floor boards, it gives this condensation some airflow. The main use promoted for "loft legs" is to allow for the 270mm of insulation noted as Building Regulation in a normal domestic property (if laid in the ceiling joists). As if the 270mm insulation is squashed it does not provide the thermal rating it is there to create. You can mix that depth between the rafter insulation and the ceiling/flooring joists... i.e. 100mm (standard height for some joists in modern new builds) in the ceiling/floor & 170mm in the rafter insulation... something like that anyway... which means that you could probably board directly on to the joists with insulation in-between... however, the above theory of condensation then applies.... Hope that helps. Good luck with it Best Nick Oddjob Does Odd Jobs