Crumbling stone walls. Advice sought.

[Ted_Vinyl]

Have an old stone cottage in West Yorkshire.

Some of the external walls are crumbling in the lower level courses. There are little scree slopes of dust and flakes at the bottom of the walls

I have no idea what is causing this. Various helpful friends have suggested masonry or mortar bees, poor quality stone, physical erosion from gritting. And probably aliens.

The crumbling is only in the first 3-4 courses and never affects higher courses.

Looks like this.


Currently somewhat mystified.

 

[Jahouli]

I may be wrong, but it appears the lime mortar isn't old old, if you know what I mean, so may be from a more recent fix to an old problem? There's a chance the mix was too weak or not cared for appropriately to allow proper curing. Whatever the cause, once erosion begins to occur it can accelerate due to an increase in voids in the wall etc. as can be seen here.

I had a similar situation with my last house - stone wall (limestone in my case) abutting a tarmac pavement, splashed by passing vehicles, no dpc. Hard cement pointing in places with mortar bees in the soft lime.

The bottom of walls take a bit more of a battering due to rain splashing off the ground, evaporation of moisture coming up from the ground through the footings, and often salts splashed by passing cars.

Repointing in an appropriate lime mortar should be a priority to arrest further deterioration. Consider the mortars ability to remain breathable but resist capillary action, as not all lime mortars are equal in this regard. Those stones look to be on the softer side so choosing the right strength lime may need careful consideration (too strong and the stones could erode faster, too weak and the pointing will erode faster than it should).

 

[Itchinfilth]

How bizarre. Looks to be lime mortar used from here with the grit showing in the pointing in the lower courses? Is the sand deffo the stones and not the pointing falling out on the higher courses as they look to not have much pointing visible?