re-roofing

i have a bungalow 9x13m, which i am going to re-roof to give me a better pitch which will give me more space in the attic for bedrooms, as opposed to if i had straight converted it.
Anyway i plan to keep the original celing and ceiling joists and floor and re-roof between. This is so that we can all continue to live there during the construction.
Question is how do i keep it dry? I can see the ceiling being exposed for a good few weeks, and that my only option is scaffolding up with a temporary membrane sheeted roof. How much would you budget for something like this, or does anyone have any other ideas for me.?
Many thanks.

roofed scaffold is the answer a normal tin hat scaffold for a semi would probably run in at close to £5k in my area,probably half that for a bungalow.

you will probably require planning permission to alter your roofline.

Thanks Chappers
Was maybe thinking this forum was a bit of a clique.
I have planning thanks, just trying to budget and i was thinking around 5K so thanks for the confirmation.