Damp walls on extension

[DM7]

The inside floor seems to be at the same level as the ground outside. Often this is because the outside ground level has been raised and the original damp proof course has been buried; the fix in that case is to restore the original outside ground level. But in this case apparently the structure is only 15 years old and it doesn't look as if the outside has been changed in that period - so what has happened? Was the extension built without a damp proof course? Perhaps the client insisted that there be no internal step into the extension. This all needs to be understood.

If there is no damp proof course, or if it is below ground level and so does nothing, then their proposal has some merit. Inserting a damp proof course at a suitable level relative to the ground outside, and the doing ...something?... so that the dampness below that level doesn't get into the bottom of the wall and floor, could work in theory. But in practice these chemical treatments are ... controversial, maybe?

All the previous comments remain valid (read them all again!). Knocking it down and rebuilding it properly should not be ruled out.

The AI research agrees with what you guys in the forum originally said was the proper fix. This is what it said:

The building needs a French drain built around the perimeter to stop water penetrating sideways into the brickwork.

Then, if needed, install Dryrods internally as a second stage.

Here is the exact step-by-step order of work to fix the kitchen damp issue properly and permanently, using reliable modern materials.

Phase 1: The Outside Work — Stopping the Main Water Source

This is the most critical stage. We must stop the external concrete path and the pebble dash from feeding water into the brickwork.

1. Cut the Concrete Path

We will use a petrol con saw with a diamond blade to cut a straight line around 150mm out from the kitchen wall.

2. Dig the French Trench

We will break out that narrow strip of concrete and dig a trench around 200mm to 250mm deep.

3. Install the Drainage

We will line the trench with geotextile fabric and fill it with 20mm pea shingle. This completely breaks the path of rainwater moving sideways into the house.

4. Cut the Pebble Dash

We will cut a neat horizontal line through the pebble dash exactly 150mm above ground level.

5. Install a Bellcast Bead

We will remove the old rough render below that line and fit a bellcast drip bead.

Rainwater running down the wall will now drop off into the gravel, rather than soaking into the bricks.

Phase 2: The Inside Preparation — Letting the Wall Breathe

Now that the outside is protected, we prepare the internal kitchen wall so it can dry out properly.

6. Strip the Wall

We will hack off the damp, salt-contaminated plaster to expose the bare brickwork.

7. Drying and Monitoring

We will leave the wall bare for a few weeks. Because the outside French drain will now be working, the vast majority of the dampness should start evaporating out of the bricks.

Phase 2: Vertical Protection — Stopping Under-Floor Wicking

If the bottom bricks remain damp from the ground beneath the slab, we can install a modern vertical barrier.

8. Drill the Mortar Line

We will drill 12mm holes at 100mm intervals along a continuous horizontal mortar joint just above floor level.

9. Insert Dryrods

We will slide high-concentration Dryrods into the holes.

These rods dissolve into the damp masonry, creating a precise chemical barrier that stops ground moisture from climbing up the wall.

Phase 3: Re-Plastering

10. Apply Renovating Plaster

Finally, we will re-plaster the wall using a specialist salt-resistant, breathable renovating plaster, ready for paint.

By following this exact sequence, we fix the root causes from both directions and give the kitchen the best chance of staying dry long term.


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Not really somthing I would want to take on, as I just like doing plumbing and electrics now.. But at least I can let him know the proper fix, so that he doesn't get ripped off with a partial fix that might later cause him structural issues / come back again later..

 

[DM7]

Why do they need to install a water repellent to the skirting boards if their cure-it-all chemical dpc will solve the damp problem?

The Ai also spotted this same issue you mentioned:

This diagram [that they gave with their proposal] proves exactly why your outside-first plan is the right way to go. Look closely at how the damp company has designed their treatment:

They have marked your kitchen extension wall (the top room) with a blue line. According to their key, they are not just injecting a chemical DPC there—instead, they are hacking off the plaster full height and installing a slimline plastic membrane with dot-and-dab plasterboard.

What This Diagram Really Means​

They Know a DPC Won’t Work Alone: The damp company knows a simple chemical injection cannot stop the water crossing over from that high outside concrete path. That is why they are forced to use a full-height plastic tanking membrane on the inside to completely hide the moisture.

They Are Charging to Hide It, Not Fix It: Their plan is to lock the water inside the brickwork with plastic, letting the wall stay permanently waterlogged behind the kitchen units while the inside looks dry.

The Guarantee Trap: Look back at the fine print in their text: “You should ensure that the DPC is not bridged...”. Because the outside concrete path and pebble dash are physically bridging the wall, if that trapped water eventually forces its way into the kitchen floor or adjacent walls, they can void their 20-year guarantee because external faults were never fixed.