New Build Question - Existing foundation

[Engine357]

This is my first post, I apologize for the super sunny photo. I am starting a 42” build and have knocked back all my ivy where I have a river rock slab. It is a super beefy slap with no cracks. Besides the non smooth surface it has a 1/4” per foot slope towards the dirt you see on the left for drainage. Will either have to be corrected? On the Oregon coast we get 90” of rain per year and drainage is an advantage. What is the best way to approach this? Could I utilize this slab and just level the hearth? Thank you!

[MarkJerling]

Do I understand correctly that this is an existing concrete slab? If so, how thick is it? What's under it?

[Engine357]

It’s an existing slab with some kind of a river rock pebble blend. I didn’t put it in. It’s about 6” thick and from what I can gather it’s just dirt underneath. The concern is it not being level but it’s 1/4” slope per foot which I know is the standard for good rain water runoff.

[MarkJerling]

It’s an existing slab with some kind of a river rock pebble blend. I didn’t put it in. It’s about 6” thick and from what I can gather it’s just dirt underneath. The concern is it not being level but it’s 1/4” slope per foot which I know is the standard for good rain water runoff.

Assuming it's on firm material and assuming it won't therefore settle once you add an additional load to it, I can't see a problem in you using it. Logically, you'd want to drill & epoxy in some starter bars and pour a level base on top so you can work up from a level surface. Not 100% neccesary, but would make life easier. Otherwise you need to cut your first course of bricks or blocks to correct for the slope.

[Engine357]

Do you have a recommendation for what tool to cut and angle in the first course of cinder blocks?

[UtahBeehiver]

I am going to chime in. If you are going to go through the time and effort and cost to build one of these ovens why risk the uncertain structural strength of the existing soild subsurface. These ovens are "really really" heavy and in the scheme of things the slab cost is nominal, a proper concrete slab is paramount to a good solid oven structure. The existing stone fill is fine for a sublayer for a concrete slab as long as it was compacted but do yourself a favor and start out right.

[MarkJerling]

Do you have a recommendation for what tool to cut and angle in the first course of cinder blocks?

I'd set up a level string line and mark the cuts on the blocks using a chalk line and then cur the blocks using an angle-grinder or proper brick saw. But, remember, you need to be sure that the slab is not going to move when you add more mass to it. If it's a good 6" slab on compacted basecourse, then it will probably be fine, but you won't know until you add load to it. The last thing you want is for the slab to settle under the load and your base becoming the leaning tower of Piza!

I would reccomend that you dig a small (1ftx1ft) test hole about 2-3ft deep on one side of the slab so you can see (a) how deep it goes and (b) what's under it. If it's on loose soil, then rather break it up and start over but if it's on good basecourse then it's usable.