We are converting our loft. The main floor area was built to building regulations standard and passed in 2005. We now need to make sure the rest of the floor - a smaller area which will form the landing where the spiral staircase will access the loft - is also to building regs standard. We also plan to have a small shower room in this space. We need advice on the size of joist necessary, whilst retaining the maximum head height possible. Thank you.