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My neighbour has recommended this Dryzone stuff.
Is he a snake oil salesman? Damp proofing creams like that do very little and the people who sell it for a living usually will recommend that you put specialist plasters up inside (normally closely related to cementicious render). Don't get me wrong there are times where these products can be useful but it is normally a lot less often (by factors of 10s if not 100s) than is suggested by the salesmen. As far as dpcs go @TW is right and a house the age of yours will almost certainly have a bitumen based dpc (it will show as a black line of mortar in-between 2 courses of bricks) which is easy to spot and in some ways superior to a plastic membrane as will tolerate more settlement of the building.
To my mind you should do the following next:
- Leave the trench as is but try to find the dpc if you have to scrape the parging away so be it looks like it needs repairing fairly soon.
- Take off the damp plaster - it will need replacing anyway you should be able to do this yourself.
- Leave it to see if it dries out after this, it should be noticeable if you can get a de-humidifier and run it in the room it should help
- While it is drying keep an eye out for cracks in the render as it could also be a / the problem.
- If this works leave the trench as is and try to find a way to divert the water collecting there away from the house there are a number of options such as pea shingle and guttering or ground drainage pipe. I have a strong suspicion that the clay soil is at least part of your problem.
Remember that eliminating damp is a process of elimination this will take time.
- Leave the trench as is but try to find the dpc if you have to scrape the parging away so be it looks like it needs repairing fairly soon.
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Bobbo
Update I’ve dug out the trench at the front of the house (where damp is showing internally) to half way down the first course of bricks.
The red circle shows a load of brick and concrete (an old garden wall possibly?). It’s about 120mm or so above the level to which I have dug, shown in green.
Lots of questions but I’m out of my depth so open to more suggestions/advice. Thanks again to @TW for his thoughts a couple of days ago!