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Pizza Freak 04-24-2008 08:26 PM

Building a Casa 90 into a hillside
Hello All,
I'm a newbie here, and this is my first post. If I make a mistake here, please let me know!

My boss (the wife) has given me the go ahead to have a wood-fired oven constructed in our back yard. We live in the Russian River Valley in Northern California, and the most important component related to my question to you all is that we live on a 16% slope...meaning that we have no flat land. Our house is carved into the hillside and we have a retaining wall on the uphill side that is approx. 32" high. My thought is...rather than building a foundation for the oven to take advantage of the retaining wall and essentially make a slab for the hearth that sits at that 32" level upon which I can construct the oven. I'll have to dig into the hillside a bit to accomodate the whole unit, but that is no big deal. We have 2.5 acres of old growth forest here (a lot of oak), so I have a never ending supply of wood!

So...considerations? Being on a hillside, we have some water moving down that hillside during the rainy months, and I'm sure I'll have to deal with that, in terms of drainage around the slab/oven. Also, given that this isn't going on a traditional foundation (cinder block, etc.), what considerations should I give to the construction of the slab/hearth? Are there any construction/material considerations I should take into account?

I'm sure that there is at least someone out there who has done a similar construction, and it is my hope to not "reinvent the wheel" here, and get some wisdom from this forum.

Thanks in advance!
Pizza Freak

gjbingham 04-24-2008 09:34 PM

Re: Building a Casa 90 into a hillside
I hate computers. I just posted (I think) and yet it is not here. One more try. Please excuse stutters.

Russian River - not too cold in winter? Mid-20s at the lowest? Steelhead swimming and frolicking? :) oops - different forum.

I'm thinking it sounds pretty easy. First check with your local building dept. to make sure you don't need a permit. Find out the recommended depth for footings for your area and soils. A French drain or other drainage system that surrounds the perimeter at or below the depth of the footings should keep your oven safe from frost heave an erosion due to your drainage situation. Again, the building department can recommend the best solution for your site.

Otherwise, it sounds like a standard build with a bit more backhoe or shovel work involved to remove some of the hillside.

I'll shut up now and let the more-informed speak. Hope that helps. Do I get some free oak firewood? :)

cvdukes 04-25-2008 08:36 PM

Re: Building a Casa 90 into a hillside
Pizza Freak,
I built my oven complex as a part of retaining wall at the toe of a dam holding back a 1.5 acre pond, so if no one else throws in some advice I may have a few pointers. Anything is possible but some things just take more advance planning and thought.

But first a few questions about your site. You say a 16 % slope... just so I'm clear on your measurement, does that mean that for every 100 feet of horizontal distance, your elevation rises 16 feet? If so, that sounds like a pretty gentle slope

Also, your 32 inch retaining wall... I am assuming that you want to have that under the front of the oven and extend the bulk of oven backward into the hillside? If so, a few more details on the long has it been there? Construction materials and foundation?... Drainage on the wall? 32 inches is getting right to the height that water pressure can easily knock over a lot of retaining wells (I've seen 12 inch walls fail here...but we tend to be wetter than many places ) the soil upgradient of the wall undisturbed soil for the distance going back from the wall at least as far as the oven will extend or or was it cut-and-fill (usually done by filling in behind the wall with dirt dug out from in front of the wall.)? Generally, what type of soil do you have uphill from the wall...very shifty sands/gravels, plastic clays etc.?

Pizza Freak 04-26-2008 11:17 AM

Re: Building a Casa 90 into a hillside
Thank you gjbingham & cvdukes for your replies.

To answer your questions cv, yes, the 16% grade is a rise of 16 feet over a 100 foot run. It doesn't seem like much, but we literally don't have a flat bit of land on the property. The retaining wall is made of pressure treated wood (which concerns me, of course) and was constructed in 1998 along with the house. It was, fortunately, over-engineered and is quite solid. I don't see it collapsing. There is a french drain at the base of the wall. Our cement patio meets up with the retaining wall. There is no backfill behind is all native, undisturbed soil. The soil itself is fairly sandy with clay & gravel farther down. If you go deep enough, you'll hit sandstone. The soil is not shifty, but we do have some erosion, mainly thanks to the gophers.

I should note that we only get rain in the winter here. The soil never freezes, so heave is not an issue. We can get anywhere from 25" to 35" of rain in the winter. The soil will be soggy all winter and bone dry all summer.

So, yes, I'll need to cut into the hillside a bit to accomodate the hearth, and the front of the oven will be directed towards the retaining wall (downhill). I'm essentially using the hillside as my "stand". I suppose my main concern is having the hearth sitting on the ground and being subjected to moisture during the winter (I'll want to use the oven in the winter, of course). Perhaps a good gravel base would help? Obviously I'll have to put in footings, a small retaining wall behind the oven, and some sort of french drain to pull water away from the hearth/oven. I'm toying with the idea of pouring a slab and making a short stand on which to pour the hearth. This would get the hearth off the ground and raise the height of the oven a bit, but it would add cost to the project. I'd prefer just to have the hearth on the ground.

Thanks for any wisdom you can supply. I'll only have one shot at this, so I want to do it right!
Pizza Freak

gjbingham 04-26-2008 09:05 PM

Re: Building a Casa 90 into a hillside
I still like the idea of a french drain around the perimeter. That should remove most of the water in the nearby soils. Gravel under the slab is probably almost a must as well. 4 - 6 inches of compacted gravel - tamped, will allow any excess water to escape from underneath the oven without developing soil erosion that could cause voids under the oven.

CJim - I'm wingin' it here. You know the answer? (yes! :) )

cvdukes 04-27-2008 02:04 PM

Re: Building a Casa 90 into a hillside
First, I'll have to say that I am little leery about using the wood wall to take the primary weight of your oven. No matter how well its pressure-treated, it all eventually rots ... I can't say much for the lifetime guarantee most pressure-treated wood comes with, I think the manufacturers count on you not being to prove it actually came from them.

Also, I think most preference would be for the hearth to be a little higher than 32 inches (mines up at 59 inches... but I'm tall and I also had to accommodate a smoker oven under the pizza oven). A couple of weeks ago, someone on this forum conducted a poll on the height... seemed that the preferred height works out to be somewhere around 60% of the height of the person that will be the primary cooker. Since you're the one building it, assume that your wife won't be using it nearly as much, so build it for your height.. There was also some discussion on building it up as least as high as your elbow (search on “hearth height” on this forum)

Given all that, I would step back from the wooden wall a few inches and cut a shallow trench parallel to the wall to pour a concrete footer. Then figure the distance up the slope where your oven and extra insulation around it would extend to…probably about five feet up the slope for a Casa 90c...then dig another shallow trench across the slope for an upper footer. With your slope, the upper footer would be about 9 inches higher than the bottom footer. I would then build a short 4" wall on the upper footer and a 13" inch tall wall on the lower footer (i.e, up to the height of the upper footer.) Easier yet would be to pour a concrete-formed wall that would tie upper footer and lower footer together with side walls about 4 inches thick…the concrete pour would be the front wall (just behind the wood wall) and side walls extending back up to upper footer, All the concrete walls should have a little bit of reinforcing wire (or even 2x4 fence wire in a pinch) If you do a poured wall, you could line the form with plastic sheeting that would stay in place on the upper side and underneath everything once you knocked off the forms..

FIll the space with an insulation (I prefer vermiculite concrete but do what works for you) between the front wall, side walls and back footer. The ground will suck too much heat out of your oven otherwise. The hearth would be placed on top of the insulation just above the level of the upper footer and the bulk of the oven would be back over the slope.... actually I would counter-lever a hearth landing (shelf) over the wooden wall…. You don’t want it sticking out into the patio space because that’s probably at a premium given your slope.

Materials to do a concrete poured wall with proper footers should run less than $100…given everything you’re going to spend on a oven, that’s just a few percent of the overall cost. You really wont need French drains around the upper side of the oven… the four or five foot width of it presents such a small area for water pressure to hit on to cause a problem… leaving plastic sheeting in place on the upper side of the upper footer and under the insulation layer just gives extra peace of mind.

Pizza Freak 04-28-2008 06:30 PM

Re: Building a Casa 90 into a hillside
Hey gj & cv...

Thanks a lot for your input. If I ever get this done, you'll have to stop by for a pizza or two if you're ever in the neighborhood. I'm a winemaker, so we'll have something available to drink as well.

I like your idea particular the perimeter footer idea. I was never planning on having the retaining wall accept any of the weight of the oven, and, to the contrary, I'd like to set it back from the wall in case we need to replace it at some point (so the oven needs to be fairly well secured into the ground). I think a 40" height for the oven would be the lowest acceptable, so that isn't much above the retaining wall.

If I can bounce a few other ideas off of you I'd appreciate it. If I do this perimeter wall idea...connecting the upper and lower about establishing a good gravel bed in the interior and pouring a slab of rebar reinforced standard concrete on top to tie it all together for the foundation of the hearth? Then put some FB board, or something similar, on that, and finish the hearth off with firebricks.
This gets my oven up a bit, and with the setback from the wall I'll have some room for a landing, and I should be pretty well insulated from the soil.

Am I getting close to a solution?
Thanks again!

cvdukes 04-28-2008 08:11 PM

Re: Building a Casa 90 into a hillside

Originally Posted by Pizza Freak (Post 31039)
Hey gj & cv...

Thanks a lot for your input. If I ever get this done, you'll have to stop by for a pizza or two if you're ever in the neighborhood. I'm a winemaker, so we'll have something available to drink as well.


Thanks for the invite, but my trips to California are few and far between..but its interesting that the two responders to your posts thus far (GJB & me) are also winemakers...but then I would guess probably half the folks on this forum brew somthing (beer or wine)...I think we all see the WFOs as just a natural extension of a brewer's hospitality.

I think you are on the right track with a slab of reinforced concrete inside the perimeter wall, but I would caution that you don't want to make the bottom of the slab sloping down towards the retaining wall. If you have a sloping slab on sloping subsoil, you might get some sheer movement if you get shifting soils (don't yall get shakers out there?) or a lot of subsurface water. Better would be to cut your soil inside the perimeter wall into a series of steps so the slab would rest on a horizontal surface ...or even slightly reverse slope the soil steps into the hillside...nothing severe (5% or so) but it serves to help lock the foundation into the hillside, especially when you have to replace that retaining wall! The upper surface of the slab could be whatever is easier for forming.

I was following a forum thread earlier posted by NissanNeill (trying to figure out his rope seals on the oven doors) when I realized that his WFO is built into a hillside above a retaining wall. I'm sure he could give a lot more pratical cosniderations since it sounds just like your project.

gjbingham 04-28-2008 09:56 PM

Re: Building a Casa 90 into a hillside
Beer, wine, bread, pizza? What's the common denominator? I think we all have a yeast infection! Except Frances (I hope!). :eek:

Pizza Freak 04-29-2008 06:47 PM

Re: Building a Casa 90 into a hillside
Thanks cvdukes,
I'm feeling a bit more confident that I can manage to anchor the slab into the slope. Getting the base relatively flat via excavation won't be difficult and I need the exercise (digging). I should be able to keep the slab flat.

A couple of hours ago I had the unique experience of watching a 100+ yr old oak tree fall into a creek bed in our back yard. It tried to take out some redwoods and bays, but they put up a good fight. It sounded like a major explosion! Guess I have a good supply of wood for the future!!!!!

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