ᐅ Installing New Underground Drainage Pipes, Raised Floor, and Moisture Issues

Hello,

The topic title might be a bit confusing, so I’ll try to clarify the situation:

We have an extension on the main building with moisture problems and a large outbuilding with mostly exposed roof drainage. The moisture issues are caused by the outdoor ground level between the two buildings being raised so much that rainwater is pressing directly between both damp-proof courses. There is no proper waterproofing in the form of a bitumen thick coating – I can provide more details on this upon request.

Since the rainwater pipes currently discharge centrally under the terrace into an old well, I want to reroute them into a soakaway pit located far from the house.

This obviously requires new PVC drain pipes to be installed – and this is where the actual problem begins:

The courtyard is paved with grass pavers – about 200 sqm (2150 sq ft). Underneath there is roughly 30 cm (12 inches) of topsoil. Below that is the original pathway, paved with fieldstones (10-15 cm / 4-6 inches). Underneath again is more topsoil, followed by heavily clayey soil.

My question is: If I dig up the area, the pipe should be laid in gravel. However, my wife would like me to reuse the excavated soil – but I’m concerned this could cause settlement issues. What is your opinion?

Second question:
Would it make sense to excavate down to the secondary level, meaning down to the fieldstone base? In that case, it would be reasonable to first remove the soil and then lay the pipe underneath the paving, estimated depth about 60-80 cm (24-32 inches). What do you think?

Unfortunately, the previous owner of our house was very careless. He simply cladded the bedroom’s exterior base (plinth) with fieldstone slabs and then raised the ground level outside by about 30 cm (12 inches). Although the edges of the fieldstones are grouted, we still have moisture and mold in the room exactly up to the second damp-proof course.

As a layperson, it seems logical to me to restore the original condition after construction so that rainwater doesn’t run onto the base at all. Additionally, a drainage pipe (coconut fiber) will also be installed.

Your opinions are highly valued. We’re not very wealthy. All the prior repairs were necessary due to the former owner’s negligence, were not apparent before, and are now straining our budget.

Best regards – Maik

You place the KG pipes in a bedding layer of fill material (about 5-10cm (2-4 inches) below and 5-10cm (2-4 inches) above). The rest can be backfilled with the excavated soil. Compact it in layers (every 15-20cm (6-8 inches)) using a trench compactor; not much will happen that way. You probably won’t have 40-ton (40,000 kg) trucks driving over it anyway.

Regarding waterproofing, you can’t avoid proper sealing. Either a bituminous thick coating down to the foundation base, grouting, or other methods... Whether the fieldstone slabs need to be removed for this would have to be checked on site by someone.

So, the underground drain pipe must be laid with a slope. Therefore, you have already set fixed height points. Generally, the pipes are first laid in sand, then covered with crushed stone, as it can be compacted well. The top few centimeters should be covered with topsoil to allow grass to grow again. And yes, you are absolutely right. The water must be directed away from the base!

Hello and thank you very much for your reply.

Unfortunately, I don’t have a plate compactor, but I will get a 100 kg (220 lbs) vibrating plate.

The fieldstone cladding is going to be removed anyway. My plan was to encapsulate the masonry between the two horizontal damp-proof courses with a thick bitumen coating.

What exactly do you mean by "down to the footing"? Down to the start of the foundation? As far as I have seen, it was poured into a trench – so the walls are very uneven and would need extensive plastering.

I forgot to mention:
The walls above the plinth are built with insulating blocks, the plinth itself with hollow pipe bricks. I wanted to compensate for this thermal bridge with 80 mm (3 inches) insulation – using the thick bitumen coating directly as the adhesive. According to the manufacturer’s datasheet, this should work – but I’m open to being corrected.

Sand is in short supply here – I would need to order it.
What exactly do you mean by "Schotte"?

Best regards – Maik

Ha! It’s the R that makes the difference! Gravel! I meant gravel. Yes, and come on, a ton of sand should be obtainable for around 10 EUR!

Sure – you’re right.
Lol – just a single letter mix-up...

Question:
How far should the waterproofing membrane be applied?

The problem is that the base (plinth) below the first horizontal damp-proof course has an outward projection of about 5cm (2 inches) – at a right angle and approximately 50mm (2 inches) high.
Below that is the foundation, but it is set back about 3–5cm (1–2 inches) inward from the projection. In cross-section, this projection resembles a window sill between the base and the foundation.

I strongly suspect that the moisture inside the room was mainly caused by rainwater, since the outside ground level is about 10cm (4 inches) higher than the interior floor. The lower damp-proof course is roughly 30cm (12 inches) below the outside ground surface. So far, waterproofing has been minimal or absent.

Additionally, there is a thermal bridge due to the thinner wall thickness of the base and the use of a different building material with a poorer U-value.

My plan would be to apply slurry between the first and second damp-proof courses, then apply bitumen up to the underside of the projection, followed by placing a fillet (cove) and installing perimeter insulation on the base.

What do you think?
Good idea or not?
Any suggestions for improvements or better methods?
As I said – the foundation is not exactly level...

Best regards – Maik