Broadback expressed precisely :
My daughter has purchased a house with a sectional garage, I think the make
is Compton. Anyway, it gets into the garage three ways.
1) Through the sides of the door.

Probably only when the wind blows the rain. There are supposed to be
thin narrow strips along the sides which help prevent this. Check they
are fitted or fit some.

2) Past some roof nails (these have rubber "top hats" on).

The nails may have been partially pulled out by the wind lifting the
panels - such that there is a gap rather than the top hats sealing
tight. You might try replacing the nails with stainless steel screws so
this provides a tighter seal.

3) The most by water running off the roof at the rear of the garage then into
the gap between the corrugated roof and wall.

That sounds as if there is not enough overlap of the roof at the rear,
to throw the water clear? Have you thought to try fitting a gutter to
catch the run off?

Often the leakage problems will due to the rain water which falls on
the exposed edge of slab, making its way under the garage sections and
onto the floor. A triangular dam of mortar all the way around the base
along the inside of the wall usually fixes this. It needs to have a gap
between wall and mortar, to allow the walls some movement. A bit of
thin ply or similar laid vertically against the wall, then removed just
before the mortar sets hard works well to provide the esential gap.

Clean and dampen the floor before you start, to get a good bond.

Any suggestions please as how best to deal with this?
Many thanks


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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org