Under-slate mortar in Victorian slate roof

[thesled]

As my other posts may hint at I've been poking around a lot in the loft of my Victorian(ish) end terrace house. It has a slate roof which is open at the back (no felt/sarking) so I'm looking straight at the underside of the slates.
There's lots of mortar on the battens and under the slates. Most of this is loose and quite a lot has fallen out just naturally and owing to various older roof repairs. Say about 30%. Of the remainder, almost all of it is loose. So almost none of it is sticking the tiles to the battens, if that was the original idea.
I can't believe those old builders would have gone to the trouble of mixing up all that mortar (by hand in those days) for nothing, so I'm wondering what it was really for. It might have some effect even now acting as a (rather loose) wedge between the tiles and the battens, which might stop the tiles moving a bit if it's windy.
It would be interesting to know why this was originally done, if it makes any difference to anything if it's not there, and if anyone ever replaces it these days.

 

[Lower]

It’s called torching, and was intended to stop wind blown snow coming up under the tiles.

I’d suggest leaving it as is until the roof needs replacing.

 

[thesled]

Brilliant. Well that is interesting. Number of Snow Days here this year - One. And I'm pretty sure it wasn't windy.
So I won't be too concerned. I guess the equivalent answer these days is sarking/liner.
Yes, I'll just leave it alone, although a lot of it has left/is leaving of its own accord.

 

[Deborah564]

You are right in thinking that mortar in the joints of a slate roof is not there just to act as a binding medium. It is also there to act as a sealant. The water coming through the roof is acidic. It eats away at any organic matter and can burn the steel in nails and so on. The mortar prevents this erosion by acting as a barrier. So, you can understand that if the original mortar was not there, it may have caused some problems for the people living in the house. Also, if you have some missing mortar, then it may have allowed some water to penetrate into the ceiling, causing damp problems. If you have mortar missing it should be replaced. If the original mortar was Portland cement, then you can look for a Portland cement based mortar, but it is not recommended that you try and mix your own mortar.