i have a problem with rain getting onto my garage floor the garage is made of pre-concrete slabs with a corregated roof( looks like asbestos) it is the wavy edge roof. the garage is detached and is at the back of the garden. although there is a slight slope away from the garage door the rain still finds it way in. I am wanting to make it watertight to store good stuff in, it does have a concrete foor but no dpc there. any ideas will be much appreciated.