Foundation for garden house

Hello Jackal,

a reinforced concrete slab is indeed a sturdy thing, but it requires a significantly higher effort. Under a reinforced concrete slab there should be at least a capillary-breaking layer (20cm thick gravel layer), otherwise the slab can frost heave and thus develop cracks. For a garden house measuring 3.00 x 4.00m and a slab thickness, as you think, of 10cm, 1.2m³ of concrete must be produced. With a normal-sized wheelbarrow (80l) that corresponds to 15 pieces. In addition, double the amount of the above-mentioned gravel is needed. That already adds up to quite a bit of work, even if you were to have ready-mix concrete delivered. A slab covering or paving is already considerably faster to lay.

Also, no weeds can grow under a hut with its own floor, since no light can reach there and it is quite dry underneath.

Hi Daton!! Thanks for your reply email and for your detailed answer. How you responded to me I like, it is also completely correct, you understand your subject, but for me too much from the textbook, no offense. I don't want to go into further detail either. I think with a concrete slab nothing can and will surely go wrong and whether it is more effort is debatable because we don't know the situation. I calculate with a wheelbarrow 18 pieces of gravel and let's say 10 bags of cement plus reinforcement. Unfortunately, I also cannot leave it at that with the weeds etc., nothing could happen but what about the outside area of the hut, ground etc., so water could also form there or... And I also believe that no tank drives over the concrete slab but a load of, say, 50-150kg will occur. I think the effort and of course the costs are lower But anyway everyone has their own opinion and that is good so no offense... Best regards.

Hello Danton,
for Schakal the thread is closed, I find your explanations very convincing and hope that I can keep the topic alive for a short time.
As a new garden owner, I found an extension on the existing stone arbor, about 3x3m, a fairly sturdy, covered but airy slatted construction. The four supports stand on point foundations, about 60-80cm deep, inside there are 30x30 concrete slabs laid directly on the soil and quite uneven.
Plan:
Use the existing foundations, rebuild the extension completely, and insulate it so well that it can be heated in the transitional period without large heat losses.
Even though the reinforced floor slab represents the simplest and cleanest solution, I would prefer to lay the small slabs again because
-it is annoying to lug a lot of building material
-such a foundation requires approval
-it is a punishment to ever dispose of the slab again (leased garden!)

In the attic of the stone arbor, I found a crumbly screed layer about 10cm thick (about 1 cubic meter) - looks a bit like blast furnace slag, from 1-2cm pieces to very fine dust. Probably post-war improvisation...
Furthermore, I have broken up an older foundation of a former shed and thus have another half cubic meter of rubble (pieces up to about 5cm) to deal with.
1st question:
Isn't it obvious to bring this stuff under the concrete slabs instead of ordering a container (1st layer rubble, then fine slag on top, then compact)? Or does it have to be gravel?
2nd question:
Since the room should have a wooden floor (kitchen): PE foil (pond liner?) on the concrete slabs and then beam sleepers and boards? Do I have to worry about condensation from below? And if I ensure sufficient ventilation underneath, how do I then insulate the floor against cold (baby crawls...)? Insulation boards?

When someone in other forums advocates loose slabs, one rarely finds out how it continues above them.

Thanks for every answer,

Sebastian

Hello Formfleisch,

if you want to use the construction waste, you must sieve it beforehand.
You should only use grain sizes of at least 1 to 2 cm. The finer the material, the greater the capillary action.
You can also use gravel or recycling material as a capillary-breaking layer, but it should be coarse-grained.
Above that, a layer of grit that can be leveled off to lay the concrete slabs on it.
As a moisture barrier, do not use a continuous foil, but only lay tar paper underneath the support beams, as otherwise any moisture that arises may not seep away through the joints of the concrete slabs.

I have attached a small cross-section drawing. From this, you can surely see more details of my suggestion.

Around the support beams, you should install perforated sheet metal or something similar so that no vermin can get under the hut.

How do you want to heat the room? Keep in mind that during the frost period, water-carrying pipes in unheated rooms should be drained.
For this reason, a small electric heater is suitable here.

Hello Danton,

thank you very much for the quick and detailed response. After having searched unsuccessfully on Google for several days, the drawing is especially a true blessing. I suspect that I can treat the stone porch similarly: foil on the stone floor, insulation, OSB boards? Otherwise, the level difference is very large, and back in '48, they obviously didn’t give much thought to thermal insulation...

Regarding the question about the heating: Actually, I wanted to get the chimney functioning again; the stones were removed up to the ridge in the 80s, then corrugated asbestos cement was installed over it. I wanted to install a small stove; firewood is already plentiful. However, today I thought that if I insulate the floor and roof as planned, probably an electric heater is not such a bad idea. It provides constant heat (especially at night), but I have no idea how (un)economical such a radiator would be...

Hello Formfleisch,

a wood-burning stove is a great thing, especially if there is enough wood available anyway. I would choose a branded stove from a specialist retailer because of the quality. A stove with 6 to 7 kW is more than enough for over 100m² of heated living space.
Very important, definitely talk to the man in black (chimney sweep master) beforehand to find out which regulations must be observed.

By electric heaters I meant those small ones on 4 wheels from the hardware store, not those heavy ones with fireclay bricks (storage heaters) inside.

You can also use both.

Attached is the alternative to the garden shed in [Mauerwerk].