Damp Ground Under Floorboards

Hi. I bought my Victorian end-of-terrace house 2 years ago and am still trying to solve a problem with dampness in the subfloor void. As you'd expect, this has caused a lot of damage to the timbers, mainly wet rot and wood beetle damage.

Photos are here: https://www.dropbox.com/sh/q38spc43qjdn8ht/AADmcEs7GLdQ9bTLiAPx6Gj4a?dl=0


The air bricks are clear (2 front and 2 rear), and there are several large holes in the two sleeper walls which flank the staircase, so I think ventilation is adequate.

The sleeper walls are damp and salty, and have partially rotted the timber wall plates.
The brick piers are also damp and salty.
The wall plates that were sitting on the constructional hearth have all rotted badly.

I had the chimneys and parapet walls flashed when I moved in (previously cement filets), and am about to have the cracked flaunching replaced, which the first roofers had stupidly not told me about. I've also just had cowls fitted to the 3 pots that were left open.

Due to the speed at which the timbers are rotting, I don't see how this level of moisture could be normal, and it's only along the side with the chimney stacks on (external end of terrace wall).

There was a house previously attached on that side, which was destroyed by a WW2 bomb, and the chimney breasts that belonged to that house are still there. The upper flues are ventilated, but the lower ones aren't, so I'm going to address that. I'm also planning to Stormdry the ledges where rain has been soaking through the cement render.

Do you think it's possible that this problem has been caused by water coming down the chimney, or is that unlikely given how damp everything is? I've had drains and water supplies checked to no avail. I'm also waiting for the council to come and clear the roadside gully which is completely blocked.

I've done my research on what needs to be done to replace the timbers etc, but I still don't know how to address the root cause of the moisture ingress.

Any advice would be hugely appreciated. This house is driving me mad!

Thanks,
Joe

Typical problem, slate dpc is not 100% effective and the floor timbers are staying damp.
It looks like in places someone has inserting some modern dpc, that is good however it still needs a while to dry out. Drying wood in cold damp conditions will never be quick.
In our case which was similar I've levered up the joists slightly and inserted modern 115mm wide dpc.
In places where the wall plate was rotting as well as damp I cut out and bed a new one with packers or folding wedges as appropriate.
Make sure none of the timber is touching the external walls.

Thanks, John. I'm planning to do the same as you with the DPC under the new timbers.

Even if there was a perfectly functioning DPC, wouldn't the moisture from the ground still evaporate into the floorboards and joists though?

In the UK you can leave some timber off the ground and sheltered and it will never rot. It's only if the ventilation is not up to scratch that you'll get a build up. Or the wood is consistently colder than the air, which can happen in autumn for a short time.
There are tables online for how much moisture timber will get from humidity at a stable temperature, I think it's something like 21% at 90% humidity which is about the limited for rot.
For internal timber it would be about 14% at 60% or so.

The cement render might be too strong and as it goes to the ground could be trapping mositure in the brickwork.

Have you looked at the Peter Ward videos on Youtube? Render is often the culprit on these breathable lime mortared houses such as yours.

The outside pathway has been cemented, so you need to check that it hasn't bridged the slate dpc. Now slate dpc is normally okay, but if they've been sloppy with the cement bedding it in, then the damp creeps past it, and some of those joints don't look that neat. Makes sure you clean away any mortar that shouldn't be there.

You've got evidence of woodworm in picture 25, so best to treat the whole room to play safe, even though it's more likely been done by now.

They may not have bothered with a DPC behind the fireplace, as it would have had the firebricks etc in front of it, but as the whole fireplace is colder than the wall to the right of it may only be one brick thick, and that would explain why it's cold.

Picture 11 may well be damaged where they've previously tried to lift the floorboards.

The subfloor does look pretty wet, so it may be worth digging a test pit down a foot or so, and seeing if it fills with water.

You have a narrow alleyway, and that may not be allowing the concrete to dry out, and water may be getting trapped on top of the ledge on the chimney, hence it's wetter than the rest. It could be worth sloping that ledge so that the water runs off of it. But you may also be getting problems with the house, in that the cement render isn't allowing it to breath, and that's trapping moisture inside it.

Thanks, guys. This is all really helpful.

Footsoldier888:

I've seen Peter Ward's videos and I agree that the cement render is unsuitable. I spoke to a builder who recently lime rendered a similar house opposite mine and he said he'd do it for £10k but personally he wouldn't bother at this stage. Several other builders have agreed that whilst the render is inhibiting the wall from breathing, there is a problem with excess moisture in the ground which is seeping under the foundations.

I keep reminding myself that the house has been in this state (cement rendered end terrace) since 1945, and judging by how quickly rot is setting in, this moisture problem must be recent else the suspended floor would be a pile of sawdust by now.

I've got a Thames Water Network Engineer coming to look at it tomorrow, so hopefully they'll take it seriously and find a water source.

Doggit:

As the house used to be mid-terrace until a WW2 bomb destroyed the adjoining house, the exterior wall used to be internal, so the slate DPC is about a metre below FFL - one course above the soil. I can see it's been injected at some stage (through the cement render <facepalm>) so I'm aware that could be causing dampness to rise up the wall. Again though, the real problem is that the ground itself is so wet. The walls don't seem too bad.

I think the wall behind the fireplace is double skinned. I tried poking a screwdriver through a motar joint and hit a brick behind it. I suppose it could be infill though...

I'm going to put a couple of airbricks in the external stacks that belonged to the house that was bombed and will steeply slope the ledges on the chimney breasts then coat them with Stormdry.

I'll also take your advice and dig a trial pit and treat all the timbers.

Thanks so much for your help!