How to pour a secure foundation for stove, uninsulated slab

[Nej Tack]

Hi BH,

I am going to install a new stove and need to reinforce the floor so that the tiles (spark protection) do not crack due to too much flex in my framed floor. The stove weighs about 350kg.

The foundation consists of 150mm gravel, plastic foil, and then the cast slab 150mm. The floor joists rest on plastic wedges on the slab, and right by the stove, the spacing between the joists appears to be 41cm.

My proposed solution so far is to remove all insulation between the joists, cover with non-aging plastic film all the way down to the slab. Tape the joints and cast EPS cement in the gaps I've created with plastic. After that, I need to create an even and flat surface with leveling compound. Here, I think the thickness will be about 25mm and then tiles on top. Alternatively, a bit of leveling compound and 2 glued floor gypsum boards. My thought here is that the plastic should work as a capillary break against the slab below so that moisture is not transported up into the joists. With EPS's insulating properties, I shouldn't create a massive thermal bridge even if the foundation stands on the slab.

Other ideas could be to cut the joists, set nogging and new wedges, and just pour a large slab. But I'm not too keen on this alternative.

Opinions are gratefully received! 😀

 

[klaskarlsson]

Hi BH,

I am planning to place a new stove and need to reinforce the floor so that the tiles (spark protection) don’t crack due to too much flex in my framed floor. The stove weighs about 350kg.

The foundation consists of 150mm of gravel, plastic sheeting, and then the cast slab of 150mm.
The floor joists are on plastic shims on the slab, and right by the stove it looks like there's CC 41 between the joists.

My proposed solution so far is to remove all the insulation between the joists, cover with non-age-resistant plastic sheeting all the way down to the slab. Tape the joints and cast EPS cement in the spaces created with the plastic. After that, I'll need to create a level and even surface with leveling compound. Here I think the thickness will be about 25mm and then tiles on top. Alternatively, a bit of leveling compound and 2 glued floor gypsum boards. My thought here is that the plastic should work as a capillary break against the slab underneath so that moisture is not transported up into the joists. Given EPS's insulating properties, I shouldn't create a massive thermal bridge even if the foundation stands on the slab.

Other ideas could be to cut the joists, set bridging and new shims, and just cast a large slab. But I'm not too keen on this alternative.

Opinions are welcome! 😀

Can’t you just add more joists/bridging which also, via plastic shims, have contact with the slab then? Wood can absorb an enormous amount of compressive force, it's the deflection that can be a problem with heavy weight.

Then if the framework has a rough dimension (say over 145x45) you should definitely ensure to have cross braces to prevent the wood from yielding with the grain direction, of course.

But with say cc30 + cc30, or if you want to go down to 20x20 (i.e., no more than 20-30 cm "void" anywhere), then you can probably easily park a car or two on top of a panel on it.