Floor joists, damp, and concrete!

[EmmyT]

Hi all,

My partner and I have bought a 1930s semi that we are doing up. This will be our forever home. There are two small patches of damp in the dining room. We've lifted the floorboards to investigate and found two joists (these line up with the damp patches on the wall) that are almost set, or at least touching concrete. The joists aren't soaked, just a tad damp.

We are running a dehumidifier, but I'm thinking the concrete needs to be removed next to the joists to create some ventilation. It's a tad rubbly as well. Looking under the floorboards it looks well ventilated with air flow. The concrete slab doesn't run the length of the room and stops where the circles are in the floorboards. Any idea what these are? The same is in the living room, but they aren't filled and the concrete is flush, level and neat with no damp.

So the plan is to remove concrete round the floor joists. Then should we add a membrane between the concrete and floorboards/skirting once the wall has dried out? I've attached pics.... Hopefully! Many thanks in advance. We are total novices but trying to use our common sense with this and Google to do as much as we can ourselves.

 

[JP_]

Could there have been an old fireplace hearth there? I had similar in my 1930s lounge, lots of woodworm all around the old hearth. So removed it to the subfloor and extended joists across.

I also put in additional air bricks while there, and cleaned out existing. So much dirt had piled up inside the airbricks blocking the air flow

 

[EmmyT]

I think it could be a hearth, but no working fireplace as such as there definitely no chimney breast in the dining room. So maybe a decorative thing. Will definitely check the airbricks for dust and dirt in any case. Thank you for the tip.

 

[JP_]

Was probably a chimney there years ago, maybe it was once the kitchen? My old dining room was once the main living/kitchen room, and small kitchen was added, apparently, in the 1950s.

This was a bit of the floorboard next to the hearth. Hearth obviously had no damp course built in to it.

Before / during removal

almost the end (last photo I took)

This picture still gives my wife nightmares - it was a dusty and horrible job!

Job done!

 

[JohnD]

It certainly is an old hearth. These are usually on a block of rubble with a stone or concrete slab on top.

Rubble is very absorbent of damp, and they do not have a DPM. Break it up and shovel it out. Clean any other rubbish under the floor, and unblock the airbricks so there is a good airflow to dry it. The old timber may have rot or worm from damp, treat it generously with a preserver after removing the rubble and cleaning the wood. If you can poke a flat-ended screwdriver into it, cut it away and replace with new.

You can also see old water pipes, perhaps from an old back boiler. Verify that these are not connected to anything as they may have leaks. In your first pic they are suspiciously close to the damp wall.


Edit
@JP_ excellent photos show exactly what I had in mind.

Including the worm.

 

[EmmyT]

Thanks both. Those photos are super helpful too @JP_. And @JohnD that explanation of what to do will do the trick I think. We will crack on with that. I'm hopeful the joists can be saved but if not we will definitely replace them. Thanks again.