The short answer
Water gets into thick rubble walls at the stack, the gable and the wall head, then stays there. Hard pointing, loose flashings and blocked gutters make it worse. Fixing the junction that lets water in is almost always better than treating the damp it causes.
A limestone rubble cottage is built of stones of different sizes bedded in mortar, and the wall is often a double skin with a rubble core between. It is sturdy, but it takes in water readily and gives it up slowly, and that one fact explains most of the trouble these houses have.
Q1.1How does water get into rubble walls?
Mostly from above. A chimney stack with worn pointing, a flashing that has lifted, a gutter that overflows down the face of the wall: each lets water into the top of the masonry, where it works downwards. Forest Marble supplied one of the two limestones that Cotswold stone slates were cut from, according to Cotswold District Council's guidance, but geology does not tell you what is on a roof, so we check each roof rather than assume.
Q1.2Why do stacks and gables leak first?
They rise above the ridge line and meet the weather from every direction. The flashing, a lead apron that seals the roof against the stack, has to be tucked into a joint in the stonework, and that joint is where old repairs give way. More on that work sits on our chimney repairs page. Gables bring a similar problem with verges and copings.
Q1.3Can pointing make things worse?
Yes. Hard cement mortar packed into soft old joints can stop the wall releasing moisture through them, so the water leaves through the stone itself and the face begins to break up. A softer, lime-based mortar suited to the stone lets the wall dry. We would sooner open a joint and repoint it correctly than seal it with a quick cement fillet that fails within a winter or two.
Where thatch sits beside a slate or tiled extension, the junction between the two deserves a careful look, because two materials that move differently meet there.
