How to build steel subfloor for pizza oven foundation?

[dandydiyer]

I'm building a pizza oven (pretty heavy) and I can't put down a concrete slab because there are tree roots that I need to work around.

So, am thinking of putting in some posts and building a steel subfloor. On-top of that I can build the concrete block base, etc. Is this approach workable? Attached is my original plan, so imagine that without the concrete base. 

Some random questions:

- What should I be looking for in terms of building the subfloor frame?

- Any tips for ensuring the top of the posts are all perfectly level?

- Can I put the concrete blocks directly on-top of it? 

- Do I still need to pour cement into the blocks?

Thanks.

[JacobZ]

Hi @dandydiyer,

While it would likely be possible, it wouldn't be particularly straightforward.

If you are looking to install a steel deck frame, we would need to ensure that it is capable of supporting the weight sitting on top of it. Our supplier Steel-Deck would likely be a good option that can assist in ensuring the deck is capable of supporting the weight of the pizza oven and its structure. Their contact information can be found on their Resources page if you'd like to reach out for a chat.

Unfortunately, you couldn't just install bricks over the top; you would have to add a flooring material such as James Hardie 2700 x 600 x 19mm Secura Exterior Flooring. You would, however, also need to confirm that this flooring material is capable of supporting the load on top of it. 

Generally speaking, when besser blocks are supporting vertical loads, they should be core-filled.

A rough estimate, assuming roughly 75 besser blocks are used, would put the weight of the core-filled structure at around 2.2 - 2.3 tonnes. You would need to ensure that this weight, distributed across this footprint, could be supported by the steel deck.

Building a steel subframe would certainly be an option, but it might be a fairly complicated solution, which may have an easier one.

Do you have any photos of the space you are planning to build the pizza oven that you could share?

Are the roots above the surface of the earth? Or are they buried beneath?

Is there any reason you couldn't build the surface up so that your concrete or block base sits on top of the roots?

Allow me to tag @Adam_W, @Nailbag and @DIY4Knuckleheds for their thoughts.

Let me know what you think.

Jacob

[dandydiyer]

Thanks for the thoughts Jacob.

Sounds like a steel frame might not be the best option for me (espec. after checking the prices). My concern with a slab is the compacted weight over the roots and the roots draining the moisture from the soil and cracking/de-stabilising the slab. This is why I thought maybe posts and a frame might be a better option.

Hard to tell in the pic, but the area is on a bit of a slope. I could certainly build that up and put a small retaining wall around it. 

2 tonnes of weight scares me a little I'm assuming the Versaloc blocks also need to be filled?

Thanks.

[JacobZ · Accepted solution]

Hi @dandydiyer,

I'm just trying to think of the most cost-effective, stable and easy way to do it without risking damage from the roots.

I think building up the land using retaining structures would be the way to go. You wouldn't even really need concrete if you didn't want to. You could just build your small retaining walls with sleepers, dump 100-150mm of road base to fill the retained area, compact and level it with a plate compactor and then build the pizza oven enclosure over the top. If you did it like this, if the roots grow up through it, there won't be any concrete to crack, and you could just recompact the road base if you needed to. You would just set the first course of blocks in place using mortar directly on your base and then build up from there. 

You could always add a layer of reinforced concrete over the top, but with a well-compacted road base, it wouldn't be necessary if you didn't want to.

Yes, typically, you would also fill Versaloc blocks if they are supporting any kind of weight from above, which, in the case of your pizza oven, they would be.

Let me know what you think.

Jacob

[dandydiyer]

Thanks again Jacob. Yeah I think that's probably the best way to go ahead with this. Thanks for the thoughts on this one.