roof sealing

It happens that Paul C. Dickie formulated :

You do not need to employ specialised contractors to deal with white asbestos/cement corrugated panels, you just need to take a few basic precautions. Wear overalls plus a mask and keep the sheets damp. Get some bags from your council, or a skip for the job, depending on the quantity.

Are you sure it is the asbestos sheets which have become porous? Often it will be just the fixings which have lost there seal. You can buy mushroom headed nails with a rubber sealing washer, if that is the problem.

intending to tile, but wanted to waterproof in the meantime.

1. As already stated, as long as you take basic precautions and are aware of the risks involved, removing the panels is not difficult, and non-asbestos replacements are even available in the likes of B&Q warehouses these days.
2. I had 3 problems with my roof - the seals around the nails had gone, there were a couple of cracks in the sheets & it appeared to have become porous. I used roof & gutter sealant to fix the nails (removing them would have been difficult), roof & gutter sealant again for the cracks, with PVA on the underside, and waterproofing solution intended for bricks painted all over the roof.

So far, no leaks whatsoever over the past 2 years.

Alex