External Concrete Plinth - Should I Remove?

[HandSlander]

Thanks @JohnD - should I be concerned about the lack of lateral support digging down 3 courses below DPC? Also are you suggesting a dirt-bottomed trench or should I be laying weed membrane and putting in a drainage pipe with holes on bottom?

 

[HandSlander]

Professionals now in (for landscaping work) and have removed the rest of the concrete plinth and broken up the solid concrete area around the manhole and downpipe drain/gulley...

I've been away during the works but upon my Sunday evening return, amongst the hottest of the year in London, some dampness in the soil even at surface level (dark area near the Tesco bag). Also the air brick on the bay has been opened up (just to the right, not pictured) and there is an undeniably musty smell coming from sub-floor.

 

[HandSlander]

reporting back to say job done, plinth removed, bricks re-pointed, area paved w new manhole cover, drainage upgraded included removing clay gulley and replacing with plastic, channel/trench against house filled with 20mm gravel. Unfortunately it wasn't possible to carry the gravel trench to the left due to the concrete from the manhole, but the builders put a raised brick paver (hidden in image) that prevents water running into the trench.

 

[DampHouseDIY]

Thanks for the great thread @HandSlander and advice @JohnD! A good read with the updates on progress.

I'm going through similar steps to reduce the condensation/humidity and general dampness in my ground floor flat in SW London. Having researched so much over the last couple of months since buying, I'm amazed at how little knowledge and care there is for Victorian/Edwardian properties and how best to repair/maintain them!

How are you getting on with the condensation/damp issues since your last update and the work done @HandSlander?

 

[HandSlander]

Sorry to say that although it undoubtedly improved the health of the building, it didn't fix the underlying issue. Also it turns out the 20mm gravel I used in the trench as pictured above (which was VERY cheap gravel from Selco), is far too tight and does in fact retain moisture against the bricks, so I will have to remove it all soon and replace with much larger pebbles to improve breathing and drainage... fortunately it doesn't get too much moisture particularly on the bay due to the sill and guttering overhang.

My latest theory about damp is through detritus build-up in the cavity as we did get some damp ingress on the front bay in a very uneven pattern... however realistically I may never truly solve this old building's chronic health issues.

I think you are right in that Victorian properties are not well understood, not just by Joe public, but also tradespeople. Things like suspended timber sub-floors, slate DPCs, cavity walls, bay windows, timber sash windows, ventilation, chimney breasts, clay drains, butterfly rooves and gulley drainage can all cause serious and potentially compounding problems... I recommend reading anything written by Ian Rock on the subject and I found the Haynes Manual on Victorian housing a good read, of which he is a co-author.

 

[HandSlander]

Well after a back-breaking couple of weekends removing all of the too-tight gravel from about 15 metres of "DPC Breathing Trench", I'm now done and it's drying out before being replaced by a much larger 50+mm aggregate to allow air circulation below the DPC and prevent any bridging. I probably found about 10 worms in the gravel, well above the soil level, so it was definitely too damp for this purpose, and I suspect the drying out will take all summer given the history. Just MAYBE this will fix my humidity issue.

However, one useful take-away from this process - this must be the best testament I've ever seen for the effectiveness of a 100 year old Slate DPC. You can see in the pic below an extremely clear line between sodden and dry bricks on this original bay window section, for which the original slate DPC is entirely responsible DESPITE being covered by ground level build-up for probably decades and the damage caused by the retrofit concrete plinth sealed into the brick, and completely unnecessary chemical DPC injections.

Lots left on my to do list including what you can see here where I need to (i) re-seal the paving to the channel drain using Sikaflex Jointing Compound, which has come away a few mm in the settlement of the patio construction (ii) repairing bay window sill cracks (NB: does not get much rain due to single-story bay window roof and guttering overhang).

 

[HandSlander]

Oh and I also upgraded the downpipes, accounting for the bulk of roof rainwater, which my landscapers had redirected as part of the new guttering / gulley system... it was showing some leaks in the joints which were counter-productively dripping into the trench.

This was my first time doing downpipes and it was super easy after watching the FloPlast instructional video, this was my second configuration (if you look carefully you can see the wall plugs to the left from my aborted first configuration), and in total I spent about £40 in FloPlast parts from Screwfix including leftovers that I'll refund. Quite pleased the 2.5m pipe lengths fit in my hatchback too!

Now it is a much cleaner fall for the water which is primarily coming from 3-storeys up, and I think it looks a lot neater too.

 

[admoyes]

@HandSlander thanks for the thread. Really interesting. How are things holding up in 2023? Humidity sorted?