Damp course found a foot under ground. What should I do?

matty_hunt

Hi. Need some advice please re dampcourse found 3.5 courses below ground. This is the back of my Victorian terrace. Previous owners had taken out back window and inserted a door and laid a concrete path to the door. The floor joists inside this door were rotting which led me to investigate an air brick, which I found buried under the door threshold. Having taken up some of the concrete and dug a trench, I have what you see below. I will have to replace some blown bricks and completely repoint the unpainted area. My question is, what would be your course of action here? Should I dig out all the soil and put a retaining wall in along the red dotted line to hold back the fence, dig another course down and fill with 20mm gravel to one course below slate. Paint wall above slate. Or retro fit a new plastic DPC somewhere around the green dotted line, back fill current trench with 20mm gravel, building trough for airbrick? Or something else? Any advice appreciated. Many thanks 

FreeBear

What I would do... Dig out all of the offending material to expose the slate DPC. Lift the fence panels out and insert an extra gravel board (quicker & cheaper than building a retaining wall).  Then pave the area leaving a 100-150mm gap by the wall & fill with gravel. Construct a step for the back door leaving a gap between it and the wall and ensure the air brick has plenty of air getting to it. This may necessitate using a reinforced concrete slab set on to a 100-150mm pier either side.

What I wouldn't do is go injecting chemicals or try inserting a DPM on the dotted line - This will do nothing to stop damp penetrating the wall and causing any new joists to rot.

matty_hunt

Thanks. I hadnt thought of putting in an extra concrete gravel board as I assume the posts are set in a block of concrete up to ground level. I'll dig out a little more and check it. I dont think I would have entertained the chemical injection route. My thinking of raising the DPM was to reduce the dig out on the assumption that as long as I have ensured the cavity is clear, it wouldnt affect the inner joists. 

FreeBear

Victorian properties rarely had cavity walls. You should be able to make an educated guess as to whether your house does. If the exterior walls are around 220mm thick, it will be solid - The pattern of bricks would also provide a clue. At 250mm or more, there may well be a cavity, but it might be filled with crud. Drill a hole or two in a mortar joint and insert a small endoscope (bore camera).

Even with a cavity, I'd still dig down below the existing DPC.

matty_hunt

Thanks. Yes, it has a cavity, built 1888 apparently.  I'd previously cleared it out, a lot of rubble and sand, though now that Ive found the slate Im not sure I went down far enough in the cavity. I need to replace some blown bricks so will make sure its 100% clear. On another forum, the chemical injection was suggested. A few videos later, it looks like it might be an option as it it could get the interior wall as well. Not as high as the green dots but by the slate.

stuart45

I wouldn't go for a Chem DPC myself. On a timber floor the DPC should be under the  plates on the sleepers and on that level for the inner skin.
Cavity should be clear at least one course below DPC if possible.

Mickey666

FreeBear said:

What I would do... Dig out all of the offending material to expose the slate DPC. Lift the fence panels out and insert an extra gravel board (quicker & cheaper than building a retaining wall).  Then pave the area leaving a 100-150mm gap by the wall & fill with gravel. Construct a step for the back door leaving a gap between it and the wall and ensure the air brick has plenty of air getting to it. This may necessitate using a reinforced concrete slab set on to a 100-150mm pier either side.

What I wouldn't do is go injecting chemicals or try inserting a DPM on the dotted line - This will do nothing to stop damp penetrating the wall and causing any new joists to rot.

I'd do basically the same, with the single exception of filling the gap between the new paving and the wall with gravel.  

Gravel against the wall is certainly better than solid earth/concrete but it can hold a surprising amount of moisture by keeping the sun off the lower layers and preventing evaporation.  

I would leave the gap empty but cover it with some form of grating . . . a bit like a linear drain really.  That will allow air to freely circulate against the house wall and keep it nice and dry.  Good ventilation is the best cure for damp.

matty_hunt

Thank you everyone for the replies. Ive a clearer idea of what Im doing now.