We have a patio along a shared boundary wall that might, in the not so
distant future, fall over.

A wall further down did fall over and the time to clean up the bricks
etc was far less than it would have been to take down the wall by
hand, and of course, the cost to re-build was shared, whereas if I
take down the current leaner manually it won't be. So we wait.

Anyhow, the patio in question was getting into a bad state. Or at
least the last 4 meters of it was. It was constructed using, in
effect, another patio as a base, then some sand and finally very thin
tiles (less than half a centimeter thick X around 10 x 20 cm), many of
which were now broken, supported on splodges of sand-cement mix, which
had all but deteriorated in most places.

It's a heavy wall with a really heavy "capping". that means that if I
were to re-do the patio with some nice stone (for example) now, when
that wall comes down it'll damage it badly.
So I decided on a cheap repair option.

I removed all the collapsed area of the patio. Bagged up all those
flimsy tiles.
Removed the under-layer to the level of the "old" patio (which had
been made using concrete slabs on top of , well, just the soil)
Incorporated fresh cement into that old sand, (approx. 6Sd:1Cmt) mixed
thoroughly, returned to position and hammered it down using a purpose
built stomper.
Next day it was solid.
Then, getting 2 different types of sand and 3 different pigments I
proceeded to make my own slabs in various sizes, applying a key-stone
pattern, some with just one type of sand others, with a mix of the two
and same went for the dyes. 4 parts sand; 1 part cement

The snow has exposed a curiosity. I took a photo, but it doesn't show
it adequately.

Where the snow had settled on the "new" area, the layer is thin.
Where the snow settled on the old area, it's noticeably thicker and
showing more signs of melting. That includes the areas of "old "patio
which I left intact to "frame" the new one, where the "old" was still
intact..

Anyone know why that might be.