We live in an apartment in a converted 1800s manor house - the stone walls are incredibly porous and unfortunately nearly all of the apartments have had problems with damp. The outside of the building has now been treated, but we are left with a large damp patch on one wall (with areas of plaster that have fallen away) and I don't know where to start when it comes to drying this out and repairing the plaster so I can redecorate.
Hi sounds like you need to get a dehumidifier mate. They dry out the moisuture in the air, drying out the walls and getting rid of damp spores.
You probably need damp proof courses too, they're materials inserted in walls and foundations to prevent moisture traveling from one place to the other. If your house lacks damp proof courses it could be advisable to look into having them installed - check with a building expert on the best way of tackling this.
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Buildings of that age were put together using a lime based mortar, they were made of stone, brick, slate all things that are damp to an extent, having said that, not damp to the extent your describe.
Key issues when trying to sort damp, is it caused by rain soaking into a porous wall, wind blown rain hitting with the force of a bullet can find its way through most poorly constructed walls, a leaking gutter or flashing, a pipe or tank in the loft or rising damp? Or the flat next door or upstairs with a leaking pipe, basin or shower tray?
To an extent your flats location, basement, ground floor, top floor will give a clue as to the most likely cause.
You don't mention how the outside wall was treated? Nor say if the problem has improved or got worse since the treatment?
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