Starting new 36" build

[deejayoh]

Hello -
After lurking here for over a year, I am finally ready to get started on the process of building my WFO. I have wanted one of these for about 10 years, so I am super excited.

My goal is to get the stand and the platform in this fall, and then begin on the construction of the dome in the spring.

My backyard is sloped, so I spent the summer installing a retaining wall and pavers to create a patio area that will be the space for the oven.

My plan is to place the bulk of the platform (~46") on top of the soil behind the retaining wall, and build a 2 foot deep stand in front of the wall to hold the rest.

This sketchup shows what I mean (although the dimensions may be a bit off).




My question is this: what do I need to do with the soil behind the wall to keep the platform stable? Is it enough to compact the soil, add 4-5 inches of compacted gravel and then pour the slab on top of that? or do I need to put in some additional support underneath to keep it from settling?

Note that this is in Seattle, so freeze/thaw is not really an issue

Thanks!

Dennis

[deejayoh]

Re: Starting new 36" build

Here is a picture of the actual space. Concrete bricks are just set in there for sizing, but that is approx what the left hand side will look like. I want to have the hearth sit on top of where the grass is now.


Also - two more questions:

1) Do I need to worry about heat loss more than if the hearth was suspended? I am thinking about a 3 1/2" structural layer for the hearth, plus 2" of FB board for insulation, - but am getting a little worried about heat loss.
2) If I isolate the concrete from the soil with a plastic layer under the concrete form, do I need to worry about moisture? This is Seattle after all, but the soil seems to drain pretty well. I haven't had any standing water issues (yet...)

[SCChris]

Re: Starting new 36" build

Considering that you’re in a rain heavy area, I’d spend time on how to keep things bone dry. If the foundation slab gets moist, you’re going to really fight with your oven. I’d get some method of drain so what water gets to the compressed gravel is drained off and out ASAP. Maybe create a water tight perimeter foundation wall that contains the gravel and below the gravel, install an oversized drain line. With an igloo or an oven house, you’ll need to get and keep the water away and from the portion of the slab that sits where the grass is.

This is just my opinion.

Chris

[deejayoh]

Re: Starting new 36" build

Chris -
thanks for the input. Better safe than sorry. I think I'll dig a french drain to pull water away from the gravel under the hearth. The hearth will be above grade, so I think the main concern with water absorption is from underneath. Hopefully a french drain + waterproof barrier will do the trick.

[GianniFocaccia]

Re: Starting new 36" build

DJO,
Welcome aboard. Glad to see you're already on the SketchUp plan - it will save you hours of design work. I placed my oven behind a retaining wall much like your plan, although I dug the entire oven base space out to provide for wood storage. If I can recommend one thing - it's to overinsulate I would consider going with more than just 2" of board. A number of builders have reported heat bleeding into their support slab).

Regarding effects of rain on your oven (top and bottom), consult with Keith (kebwi) who built a beautiful 36" oven in Seattle.

Congrats on taking the WFO plunge. Great-looking oven site.

John

[Amac]

Re: Starting new 36" build

Hi deejayoh
Like you I have just taken the plunge and started to bulid a 36" WFO. Coincidentally I also have a similar sloped terrain, and like you I also spent the summer building a retaining wall. I decide to dispense with the wood storage. I had already started before looking in at this forum and spent the last few days admiring the various designs already completed - especially Dinos, Les and Frances in Switzerland none which I expect to emulate but I am looking forward to it. I live in Ireland and here as you may have heard we have a more than well deserved reputation for rain!. However since I had alreadt started with the Stand which is roughly 6 ft by 8 ft and right now consists of 4" rammed rubble topped by 4" in concrete. On top of that I have put a 4 1/2" slab of Vermiculite cement mix (about 5' x 5') for the undefloor insulation. My next move is to bring the concrete base surrounding the vermiculate base up to the same level and carry on from there.
I had not given any thought to the moisture absorption mentioned by SCChris. What exactly type of problems can I expect and I hope it's not too late!

Anyway I look forward to following your building as the weatger here is almost impossible right now and I expect to get little done until spring,

By the way what is really bugging me is what type of cutting tools you all used and did you rent or buy them. I really dont expect to shape the bricks for the lower layers - just cut them in halves and so on and use shims or spacers and mortar to get the dome shape.
Sorry for the rambling post!
Good Luck
Aidan

[deejayoh]

Re: Starting new 36" build

Hey Aidan -
thanks for the note. Yes, we are probably facing similar rain challenges. But you are ahead of me in terms of getting your hearth poured. I still havent got my half-stand finished. Oh, and my back yard is 50 stairs up from the street, so when it comes time to fill those cores with concrete - and to pour the hearth for that matter - I have to carry the bags up! Think I will be hiring some day laborers to help with that one.

As to the moisture issue - does your yard slope down toward your wall - or in other words, will the water be flowing toward your hearth? If so, I'd put a weeping pipe behind it to catch the water and carry it away from the hearth. Just like you would on a foundation. If it is basically flat, and your hearth is on grade - then I think having gravel and a water barrier underneath should be sufficient. In my case, there will never be standing water around the hearth to worry about - so as long as there is a way to carry away what does fall - I don't think there shoudl be an absorption problem

I'm more curious about your decision to use vermiculite. I am not sure how the insulating properties of that compare to the FB board. I personally need to go with the FB board because I can't add too much height to my oven w/o it getting hard to use. My wall is 37", and then adding ~4" hearth, 2 (or maybe 3") of FB, and ~2" of brick puts me at 45-46" oven opening - which is about as high as I want it to be. That's the main reason I am thinking 2" of insulation.

Oh - and for cutting tools, I am going with what seems to be the norm and buying the tile saw at Harbor Freight Tools. Maybe this weekend. I have been watching CL looking for a used one but none seem to come up. And I tried cutting some of the concrete block with an angle grinder but that just made a big mess, so I figure I might as well buck up now. Not sure if you have a similar franchise in Ireland.

[Amac]

Re: Starting new 36" build

Hi djao

..
As to the moisture issue - does your yard slope down toward your wall - or in other words, will the water be flowing toward your hearth? If so, I'd put a weeping pipe behind it to catch the water and carry it away from the hearth.

That sounds like good advice - yes the slope is towards the oven - normally it is very well draining - glacial deposit type stuff under the topsoil - but my retaing wall will just allo it to be trapped.

I'm more curious about your decision to use vermiculite. I am not sure how the insulating properties of that compare to the FB board. I personally need to go with the FB board because I can't add too much height to my oven w/o it getting hard to use. My wall is 37", and then adding ~4" hearth, 2 (or maybe 3") of FB, and ~2" of brick puts me at 45-46" oven opening - which is about as high as I want it to be. That's the main reason I am thinking 2" of insulation.

I am somewhat opposite - I need more height to make it comfortable - at the moment the vermiculite layer is at 30" and I will just get 3" or so extra on the floor bricks. I was considering partly for that reason and partly because of my fear of water wicking, adding an insulating board on top of a plastic sheet. But I also don't want to "trap" water underneath. Right now I amd leaning towards leaving it as it is. That vermiculite is strange stuff to mix - and drinks water during mixing at an astonishing rate.
Most pople on this forum seem to be opting for board now so you''re in good company there, and I possibly would have if I had taken a little more time before tearing in - but someone reported problems keeping it dry during the build, and that is virtually impossible here right now anyway.

Oh - and for cutting tools, I am going with what seems to be the norm and buying the tile saw at Harbor Freight Tools. Maybe this weekend. I have been watching CL looking for a used one but none seem to come up. And I tried cutting some of the concrete block with an angle grinder but that just made a big mess, so I figure I might as well buck up now. Not sure if you have a similar franchise in Ireland.

I have checked virtually all of Europe and I could not find anything resembling the HF this side of the pond. I already wasted money on a tile cutter which I found out too late wouldn't fit my bricks and was lucky to sell it quickly (at a loss). I'm a bit disappointed to hear that your angle grinder didnt work well, since that is my plan now I borrowed two from a neighbour, one 115mm and one 230 mm (9 in) and put a diamond blade in the smaller one. I used it to cut around the 4 sides of one brick and it broke reasonably cleanly. I've ordered 2 diamond (cont rim) blades for the bigger one and I'm hoping that one will cut the full brick. What kind/size of blade did you use with the angle grinder?

Right now the weather is not pleasant here, sharp frost last night, damp and cold today. That means that it will be March before I can start building. But I want to cut some bricks in the meantime.
I've covered the base with a plastic sheet which seems to collect a lot of condensation moisture underneath. I wonder if the vermicrete will still absorb rain moisture - but maybe someone else can help with that one?
Thanks for the response btw - keep posting and and best of luck
Aidan

[Neil2]

Re: Starting new 36" build

From your sketch it appears that your structural hearth slab will be partly supported on your new retaining wall / stand and partly supported on the native ground behind.

Uneven support like this raises the concern of differential settlement. I would take care to prepare the subgrade by removing at least of foot of native material or to firm ground and back filling with granular fill. Drainage of this subgrade layer, as outlined by Chris above, will be important.

To guard your hearth slab from the stress of differential settlement, you may want to consider making it more robust than normal. Perhaps a 6 inch well reinforced slab.

With respect to hearth height, a range of 42 to 48 inches seems to be typical of most of the builds on this site, but this is an individual thing.
Build a mock up at various heights and try a peel on it. (Many builders have commented that if they were to do it over, they would build their ovens higher.)

[brickie in oz]

Re: Starting new 36" build

Uneven support like this raises the concern of differential settlement.

A concrete pier/pile at the back would alleviate that problem and help carry the load, the slab could also be suspended off the soil via the pier/pile to help with the moisture issue that will arise.

Id be putting lots of waterproofer in the hearth slab too as moisture will travel into the slab from the retaining wall, it doesnt take much moisture to make an oven under perform.

[deejayoh]

Re: Starting new 36" build

A concrete pier/pile at the back would alleviate that problem and help carry the load, the slab could also be suspended off the soil via the pier/pile to help with the moisture issue that will arise.

I really don't want to suspend the slab if I can avoid it. I don't see how moisture can travel up through a waterproof barrier into a slab. But settling is a concern. The dirt behind the wall was backfilled after the wall went in, and looking at it this weekend I can see it has already compacted about an inch in the past few months. I was thinking I could pack it down really well - but maybe I should drop a couple sonotube forms in the corners? There is a clay layer down about 2-3 feet that you need a jackhammer to break up.

Id be putting lots of waterproofer in the hearth slab too as moisture will travel into the slab from the retaining wall, it doesnt take much moisture to make an oven under perform.

Not sure what you mean by waterproofer. Are there additives for the concrete, or do you just mean sealants?

on the moisture issue, I was thinking that since the retaining wall is a separate structure, and below the hearth - that moisture shouldn't be able to wick into the hearth if I hadthe protection on the bottom. But I'll be the first to say I have minimal knowledge of concrete!

TIA for the guidance

Dennis

[brickie in oz]

Re: Starting new 36" build

maybe I should drop a couple sonotube forms in the corners?

That would stop any settlement issues for sure.

Are there additives for the concrete,

Correct, add them at the mixing stage and the concrete wont wick any moisture.

on the moisture issue, I was thinking that since the retaining wall is a separate structure, and below the hearth - that moisture shouldn't be able to wick into the hearth if I hadthe protection on the bottom. But I'll be the first to say I have minimal knowledge of concrete!

Ive seen moisture travel 6' up masonry over time, if you put a physical barrier in place it would help too.

Better to be safe than sorry, there have been a few tales of woe on the forum all due to moisture wicking, once it starts to wick after a build it would be almost impossible to stop without a pull down and restart.

Same goes for heat wicking, best to get it sorted out before the build.

[deejayoh]

Re: Starting new 36" build

Thanks Al - I also read your comments in the Insulation sticky thread

So, paying close attention here. I did some searching with the google and came up with Xypex, an additive that you incorporate in the mix that is supposed to prevent water wicking

Xypex Corporation

So I am thinking of using this (assuming I can find it!) along with a moisture barrier to have a hearth layer that is moisture impermeable.

On top of that, I'm thinking 3 inches of insulating board - local supplier doesn't say which this is but I will go with theirs or the FB board from our sponsor.

Seattle Pottery Supply E-Catalog: Insulating Board - BACKUP BOARD - 1900 Degree F. - 12 in x 36 in x 1 in - 16 Piece/Case

Think that will be sufficient?

[brickie in oz]

Re: Starting new 36" build

So, paying close attention here. I did some searching with the google and came up with Xypex, an additive that you incorporate in the mix that is supposed to prevent water wicking

Xypex Corporation

So I am thinking of using this (assuming I can find it!) along with a moisture barrier to have a hearth layer that is moisture impermeable.

The spec sheet reads ok apart from only being able to buy larger than needed parcels of it.
You should ask a hardware store about it or equivalent stuff.

On top of that, I'm thinking 3 inches of insulating board - local supplier doesn't say which this is but I will go with theirs or the FB board from our sponsor.

Seattle Pottery Supply E-Catalog: Insulating Board - BACKUP BOARD - 1900 Degree F. - 12 in x 36 in x 1 in - 16 Piece/Case

Think that will be sufficient?

3" of Calcium Silicate board may be overkill?
I have 40mm underneath my hearth and the oven stays hot for days and days.

[deejayoh]

Re: Starting new 36" build

Well, we are into February, but I am finally seeing some progress on my build.

Couple of changes from my initial post:

• I have decided to go with a 39" oven instead of a 36". almost went with a 42" but made a firewood vs. stairs tradeoff
• I decided to pour the lintel rather than use concrete block supported by angle irons. I figure the angle irons are going to rust over time and leave stains at best

So I got the stand built a couple of weeks ago, and today I hired a couple of guys from my friends construction crew to help me pour the lintel. Actually, I thought I would get the hearth poured too, but the &*!@ Salmon Bay Concrete store is closed on Saturday, so I could not pick up the Xypex AdMix C-1000 that I am putting in the hearth to make it waterproof.

Let me tell you, this stuff is super pricey. You mix it in at 2% of the weight of the concrete, and it's $6 per pound.... More than the concrete, but I figure safe is better than sorry.

I also have my ceramic blanket and duravent already on hand, so I am a bit ahead of the game there. Just need to get the bricks, mortar and FB board.

Finished Stand

Forms ready for hearth

Pouring the lintel