Mortar Damp Adjacent to Damp Proof Course?

30 May 2015
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United Kingdom
Could you please advise me on a possible problem? I’ve noticed that even in dry weather but after a cool night, the mortar along two rows beneath the damp proof course (DPC) in my kitchen extension is damp in the early morning but this soon dries out during the morning as the temperature rises. There is no evidence of any damp on the walls or the floor inside the house and the bricks above the DPC are dry.

The rows of mortar in question are A-B and C-D in the enclosed photo. Row A-B is also the position of the damp proof course.

Until recently, the wall had a crazy paving path cemented right up to the wall, but the path slopes away from the wall to drain the water. I’ve now cut a channel between the bricks and path to a depth 2 bricks below the path to leave a clear gap (see photo Wall Bottom). The bricks below the path are quite damp due to the heavy clay soil.

It seems to me that water might be being sucked up from the damp clay soil through the bricks and mortar by capillary action forming damp patches along the DPC since it can travel no further and eventually evaporates from these areas.

Is this the most likely explanation or are there other problems I should be looking for?

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Could you post pics of the two full elevations in the pic - marked with the position of the FFL?

Besides your most likely explanation, it could be that: Moisture from below the solid floor might possibly be coming across a debris blocked cavity? Or there could be wet cavity insulation?

Thing is, the damp presenting on the outer skin is not showing on the interior surfaces so its not that important an issue.
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Thanks for your replies.

I've included two photos showing the side elevations. The extension was built in 1988. The DPC is the row A-B and you can see the floor level is immediately below the tall window in the second photo, just above the DPC.

All the exterior brickwork below the DPC is currently dry even though we had 2 hours heavy rain yesterday. I tend to see the damp patch on the mortar in the morning after a cool night then it soon dries out.

There is a radiator below the window in the first photo with the pipes buried in the concrete floor. The floor inside is bone dry and there is no loss of water from the CH header tank with the supply turned off for a day so this rules out a leak from the central heating pipework. There is also a drain from the kitchen sink under the floor to the manhole cover under the planter in the first photo. The drain runs freely and there is no evidence of any damp on the inside floor.

The crazy paving was cemented right up to the wall. I've now cut a channel 3" wide and 6" deep to help keep the lower bricks dry since the clay soil and the bricks under the patio are quite damp. It seems most likely that the damp patches I'm sometimes seeing is due to the water being sucked by capillary action from the lower damp bricks then saturating the mortar along some parts of the DPC from where it evaporates. Is there anything else I need to check?


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