Steps to construct a foundation slab
To lay a foundation slab you need a 20 cm deep trench with a deeper stem wall around the perimeter at 40 cm depth. Around the perimeter we build formwork from OSB boards and stakes, so that the upper edge is about 10 cm above ground.
We backfill the trench with sand (10-15 cm) which must be properly shaped and compacted. This creates a “form” for a slab about 20 cm thick + a 40 cm rim around.
After filling the sand you can lay a building film over the trench but it is not mandatory.
The next step is to install sewer drains in the bathroom and kitchen. The main line going to the septic tank should have a minimum diameter of 150 mm, while the drains from the toilet and sinks/showers should be 100 mm and 50 mm respectively. The drains should protrude about 10 cm above the level of the slab and be capped during pouring concrete and building the cabin.
At this stage we also make water and electrical connections to the places where they will be located.
Then we install a single-layer reinforcement mesh made of ribbed wire. It is advisable that the reinforcement does not lie directly on the sand or film but is supported from below by pieces of concrete, blocks or stones.
For larger structures we suggest laying reinforcement around the perimeter of load-bearing walls with two layers. In this prepared form we pour the concrete. We can also skip traditional rebar and order concrete from a concrete plant with polypropylene fiber additives.
After it has cured we perform surface waterproofing in the form of a thermally welded roofing felt or dysperbit.
After completing the works it is necessary to protect the slab from rain or snow. The total cost of materials for a cabin sized 7m x 5m is about 5 000 zł