What mix mortar for ridge tiles on 1904 slate roof

I would recommend doing 3 soft sand to 1 cement.

I would recommend a Mortar mix ratio of two building sand, one sharp sand and one cement for this type of application. This will provide a good solid Weather tight base For the ridge tiles on this property to sit on.

I would recommend a ratio of 3 building sand to 1 cement. I’d also give the underside of the the ridge tiles a coat of neat PVA. This would ensure excellent adhesion.