On 27 Mar, 12:18, Al 1953 wrote:
I have a garage with an old corrugated asbestos roof.

Read a couple of recent threads on asbestos and its hazards,
particularly for roofing.

The roof has a number of small holes where the rain gets in. Also, a draught gets in through gaps
where the sheets overlap. Would expanding foam be suitable to plug these
gaps and holes?

Squirty foam is great on the gaps in overlaps. However it has little
strength and very poor UV resistance, so it's not good for holes. If
you do use it for holes, you'll certainly need to cover the outside
with something opaque, such as aluminum tape. or even thick aluminium
foil (caterer's buffet trays, in my shed).

A better repair though might be more asbestos, such as offcuts of
other sheets.

A really useful tube mastic here is the black very slow drying
bituminous one from Wickes or Screwfix, sold as roof repair mastic.
It's sticky, flexible, and it just doesn't care about water. You can
seal wet boards, even while it's still raining.