ᐅ Basement originally partially constructed with concrete, now possibly fully built with masonry.

Hi,

In previous discussions and offers, we agreed with our provider (architect/owner of the construction company, including their own site, excavator, employees, etc.) to build the basement roughly half in 24cm (10 inches) thick concrete and the other half with masonry.

The background is that we have a sloped site, and the basement is about half above ground level, containing two living rooms. The rooms at the rear, towards the slope, are a heating room, storage room, and bathroom.

We do not have a soil report. The soil is clay/loam-like (my father-in-law called it red marl). So, it is not very permeable. Above us runs the road, and beyond that, the slope continues for about 50-100m (55-110 yards), then you reach the top of the hill. Two more plots will be developed above us. The house and road are roughly at the same level (plus a step).

We now want to finalize the contract details. The architect recently called me and said he would prefer to build the entire basement with masonry (thickness unknown but thicker than 24cm (10 inches)), including drainage. The drainage would not clog because it would be in a gravel bed.

I have researched and, as expected, I am uncertain about which option is better and what questions I should ask the architect or what details should be agreed upon.

Basically, there are two methods: waterproof concrete (often called a white tank) and sealing with bitumen or plastic membranes (often called a black tank).

Regarding the cost of both options, I have read very different statements; for some, one was more expensive, for others, the other was. Or it was previously like that, but now they say the costs are about the same. The waterproof concrete method is said to be much more common nowadays.

I also found, but only from one source (other sites had exactly the same text), that bitumen variants last only about 30 years, while waterproof concrete easily lasts 80 years. Is there any truth to this? Has anyone had to remove their soil because water appeared after 30 years?

Back to the upcoming discussion with the architect. Should he provide a written guarantee of watertightness? Is the concrete method definitely waterproof and generally requires less long-term warranty? What else should I pay attention to (I won’t sign immediately, but a more confident approach wouldn’t hurt)? What might be a rough but reasonable wording for both options in the contract?

Thank you very much.