Backpointing/underpointing/buttering slate roof

[joarwel61]

Greetings experts!

This is my very first post and would be extremely glad if anyone can advise.....

My house, built in 1952 has a slate roof and the slates were underpointed with lime mortar. By now the mortar has cracked and bits have fallen off into the attic, exposing the underside of the slates and allowing wind and driving rain to penetrate. I've seen numerous adverts about the expanded polystyrene replacement, but am reluctant to resort to that at this stage.

I've surfed the web looking for anything about underpointing (including the other terms used for the mortar) but cannot find anything. I'd like to replace the missing mortar and a friend advised me to make up a mix of 1:3 sand & cement with plenty of pva and with handfuls of polysterene beads thrown in to (a) assist with insulation (b) to keep the mixture light (c) to ensure that it sticks and (d) to allow a bit of give with the minimal expansion/contraction of the slate tiles.

Anyone else had this problem and have resolved???

 

[HaydnExport]

The underpointing you are on about was put on to stop draughts before the invention of felt, even with it all missing you shouldn't have rain penetrating! It's called 'torching' or 'full torching' or 'single torching' or even 'half torching' depending on how exactly it is put on. it usually consisted of lime mortar and horse hair.

Whatever you do don't use sand and cement it will render the slates on your roof worthless.

 

[joarwel61]

Thanks Haydn

I'll explore the internet further now that you've kindly given me the proper terminology. Thanks for advising me not to use sand/cement. I may have to revert to the original 'recipe' for lime mortar, but I'm still tempted to use pva to help adhesion. I think I'll also use the polypropylene strands that screed layers use to stop it from cracking insted of horse hair.

 

[HaydnExport]

Glad to help

 

[Baildon1945]

The underpointing you are on about was put on to stop draughts before the invention of felt, even with it all missing you shouldn't have rain penetrating! It's called 'torching' or 'full torching' or 'single torching' or even 'half torching' depending on how exactly it is put on. it usually consisted of lime mortar and horse hair.

Whatever you do don't use sand and cement it will render the slates on your roof worthless.

I have a slate roof and a builder backpointed the battens using a sand/cement mortar 30 years ago. The mortar has over the years fallen off the batten and I have replaced it using a 3 to 1 mixture of sand/cement. I have noticed that in parts there is a lime mortar. I have read that lime mortar is slower setting and weaker, but does not shrink or crack. So have decided in future to use lime mortar. You say that sand/cement renders the slates worthless. I am only putting an inch of mortar on top of batten between slate and under batten for one inch. Will this have caused a problem? I do not force the mortar down the slate when putting mortar on top of batten,as I understand that this might cause a capillary action with water on roof.