Damp wall/floor around bay window

[MagneticField]

Hi all,
Wonder if I can call upon some of your wise heads (including @pigletjohn !) for an ongoing damp problem that I'm trying to solve (mid terrace late Victorian house).

There are some dark patches - including one that's very large in the centre of the bay - on the internal plaster. I've also, after investigating the uneven floor, realised that the subfloor joists and wall plates nearest the bay have rotted and need renovating (I'm assuming due to the lack of adequate ventilation). The previous owners appeared to have partially blocked one external air vent, with another covered up completely by the adjoining porch.

There is some kind of render on the centre section of the bay outside, which meets a plinth with no obvious separation of the two (this plinth runs along the neighbouring property and also into our porch). I've previously been advised by a surveyor to raise this render and install a bell mouth bead to the plinth - would this be the best course of action? I have already dug a French drain around the wall but not sure it's having the desired effect.

I've had various contractors/surveyors round but most recommend to have a chemical DPC installed, something that I'm very reluctant to do.

Finally, I wondered what would be the best course of action with the affected internal plaster. Would it be best to remove this and let the wall dry out for some time? Again, I'm wary of the sand/cement/waterproof approach that seems par for the course with the industry, but not sure of the best alternative when it comes to replastering.

Many thanks.

I think you're 100% right to be wary of the things you mention. We were advised against both tanking and chemical DPCs. Sorting out the vents and raising the render would be the first things I'd do, then I'd see what the results were and take it from there. No point replacing the joists/floorboards until you've solved the problem. Just clearing our vents banished actual visible white fungus on our joists- it's never come back and the joists are now dry.
We had a French drain suggested by our surveyor for the purchase. An 'old house' specialist surveyor told us later that they can be tricky to install correctly, you need the right fall on them. He suggested doing other things first (like cleaning out the blocked vents, sorting out the guttering, and checking for any cracked drains) and then see if they did the trick (they did). If one of your vents has been blocked by building work, it might be worth getting another put in? (He also said an absolute NO to injected DPCs and tanking, he said they would only make the damp worse.)

I can't see where the DPC is. You might be able to see it from inside.

Lower the external ground level to a couple of bricks below it.

Take a wider pic to include the roof of the bay and its gutter, downpipe and gulley. Quite likely the excess water is coming from there. Gullies are usually broken and leaking.

Take out that horrible mess of an airbrick and put in some new ones. One every metre is not too many and you need some on the other side of the house to enable airflow. Clean out builder's rubble under the floor and against the wall.

The gas pipe, if exposed to rain, is positioned so that water will run down it and through the wall.

You may find the joists are on a separate dwarf wall set back from the external wall but the cavity is full of wet rubble. This can be cleaned out (post again)

damp will result in woodworm
check airbricks are cleary and actually lead to a vent. Check window sills/frames for leaks and gutters then dpc as older house do not have dpv - best dpc in house buildt around the 1930's on usually semi detch, detached or rows of 4/6 terraced houses

DPC became compulsory in 1875 in England.

Thanks very much for the replies and advice everyone.

@PigletJohn - I've attached a wide pic of the bay, the downpipe has been fixed up and sealed recently after it had started to come apart in places (there is no gully underneath so it extends out on to street level).

It's been a headache trying to locate the DPC - could it be concealed behind by the plinth perhaps? I've included a couple of other pics where I've been able to take up sections of floor: one is directly under the front door, where there is a slightly cracking layer of slate below the damp wall plate. The other pic shows the same wall plate below the bay window, but with no apparent slate underneath.

The timber wall plate appears to be embedded in the bay dwarf wall, the top bricks of which are very wet. I've cleared out a bit of rubble from the cavity there.

We did have GCH installed earlier in the year, which is when the sub floor issues became apparent. The gas pipe placing is unfortunate, I wonder what would be the best remedy for this.

That may be true but in old house the cheaper membrane is eaten away and movements crack it
We had a 1930's rock-solid house and the previous lot allowed airbricks to close up when i replaced them i noted 10mm thich damp proof course was slate-like material.

Looks like the cement render was added later as the house next door appears to be bare brick?

This can stop walls from breathing as they’re designed to in older houses trapping in moisture. If inadequate ventilation then there is often the forming of condensation on the coldest surfaces of the room which in your case is that external wall low down (beneath your bay).

Have a good look at the gutter in case it is dripping.

Is the front patch paved? It is quite likely to be above the original ground level, and also might be sloping towards the house so that water runs to the wall.

The plinth is very likely bridging the DPC. It may have been added to hide (it does not cure) damp, and will have made it worse.

There is a good chance you can cure it by hacking off the old plinth and digging a trench against the house wall. This will enable you to see any water getting in, find the DPC, and will expose the lower brickwork, which can start to dry out when exposed to air. Damp does not easily rise more than two or three courses in good clean brickwork exposed on both sides, with no render, plaster, rubble or choked cavity against it. Also put in some more airbricks.

If, by taking up a board, you can see the rubbish filled cavity against the dwarf wall, it is possible to clear it. Some people use a fabricated tool like a long spoon. But if you can get a builders canister vac, with a steel (not plastic) tube, it will suck up loose sand and mortar. You can loosen it by poking with a piece of iron, or there might be a small gardeners tool to rake it. A large drill would also loosen it, though I have not done that.

When slate is used as a DPC, it is (almost) invariably run continuously, at the same height, all round the house. So once you have found it, you will know where it is. Slate lasts at least 300 million years and does not degrade. If the wall cracks, the slate will crack too, this is not a common problem. Much more common is a bridged DPC, choked airbricks, and blocked cavities.

It sounds like you have found some scraps of slate used as packing under joists where the dwarf wall was not flat and level.

I can't make out your later pictures. Move back to get a wider view, and include something to show scale.

It's also possible that water may be coming from a leaking water pipe. I haven't come across high groundwater in my area, it's generally drains, rain, or plumbing. I do however have a neigbour whose cellar floods in high tides.

Thanks for this - I'll keep an eye on the gutter for drips/leaks. New pics attached of sub floor at front door and bay window.

The front patio is paved but slopes away from the property. We attempted a French drain around the wall but I'm not sure it's draining effectively (to soil).

Could water be getting in between where the render and plinth meet as there's no obvious separation such as a bell mouth? Oddly, the plinth looks to be more angled where it goes inside the porch.

I'm afraid I can't really make out the pictures as the wall is dark. Is there a gap filled with rubble? The inside wall above the floor looks thicker than below, has it been insulated or clad in drywall?

Under the floor, is it bare earth or concrete?

The plinth in the porch probably looks different because the porch was added after the house was built, and the plinth altered later.

Leaving the floor open for a while will ventilate it and you may see it drying out a bit. Don't fall down the hole. Look out for any changes after rain.