I have a small garden store which is built onto the side of the house. The walls are a mixture of whinstone and sandstone. The sandstone on the [u]interior[/u] wall surface is deteriorating at an alarming rate with piles of sand on the floor from the eroded stones. This store also houses the central heating boiler. Could the heat from the boiler be drawing moisture through the stonework from outside? Do you know what the solution might be?