Hardie Backer or Durarock

[Boogie-D]

Aloha!!! Been reading through all your amazing builds and picked up something that may make my life a bit easier...

in regards to forming the concrete table... I was going to build a complex ply wood frame that was removable from underneath the table... then I see that some of you have used Hardie Backer or Durarock under your table and have left it in place...

I can get both at Home Depot... I need 4’ x 6’ sizing...

Hardie Backer: comes in a 4’ x 6’ siding panel but is only 5/16 inch thickness (I have a lot of support and brace material)

Durarock: comes in a 4’ x 6’ sheet and is 1/2 inch thickness... from my previous experience is not as ridged as Hardie backers and will need a lot of bracing too...

thoughts or suggestions on using either these products to form my oven table?? Thank you very much.

[SableSprings]

Hey Boogie! Been watching & reading your posts. I appreciate that you are asking lots of questions AND doing a lot of reading & thinking about your plans. As to the bracing for the poured hearth (table top), I'd go with the Durarock and as you mentioned, some significant bracing. You probably only need to cut a couple of 2x4s or 2x6s underneath the cement sheet with 2 support posts underneath each board. Just be aware you need to put in a couple wedges in place so you can knock out the support posts or boards after the concrete has set. That weight pushing down on the under support is amazing and will really make the bracing difficult to remove without the wedges or knock-out blocks. Also, remember that you can lay the Durarock right on top of your stacked perimeter CMUs. Lots of folks pour alternate cores of the CMU with cement and put a 90° bent stick of rebar in each poured core. These bent pieces of rebar are then incorporated into the top slab pour with the rest of the rebar, effectively locking the top hearth slab to the base CMU structure. If you intend to do this, cut your Durarock so that it rests a couple of inches on the CMU blocks...not covering the holes. That gives you the option of filling the holes & locking rebar during the same pour as the hearth slab. I hope that made sense... Fill alternate holes with empty cement bags or plastic jugs/bottles to reduce the amount of cement you need...it ain't going anywhere with only half of the cement block holes filled...

If you have a seam in the Durarock that you're concerned about "leaking", simply cut & lay a piece of plastic over the seam. While you're planning this (and live in a rainy climate), plan on embedding some pieces of 1/2" or 3/4" plastic pipe under where your oven will rest. Drill 3-4 holes in the cement board and push up the plastic pipe so the top of the pipe is slightly below your intended slab surface/top. Push in a foam or Styrofoam plug in the top at your intended slab top level. Keeping the pipe just below the intended top allows you to screed the top of the "table" level without hitting the pipe. When the cement hearth sets and you remove the forms, simply take a piece of rebar and tap up each plastic pipe. This will clear the pipe and create a shallow divot so any water that gets under your oven has an escape avenue. If you really have some good eyeballs, putting a slight mound in your hearth will also encourage water to run off the slab instead of pooling on top. Once you've knocked out your pipes and removed your support pieces, take some screen mesh and silicone it over each pipe/drain hole to keep critters from making it a home .

Here's a couple good build treads that also incorporate tile sheets for drainage underneath an oven's base insulation.

Note picture in post #44
https://community.fornobravo.com/for...ge3#post428142

https://community.fornobravo.com/for...andpoint-idaho

[Boogie-D]

Aloha... Thank you very much Mike... Sometimes I feel really stupid for asking so many questions but really trying to make my first WFO build the best I can.... and I truly appreciate you all helping me to accomplish that... and everything you said perfect makes sense... my plan is to not have a foundation slab and 3-4 supporting walls... my plan is more like the flying saucer made of 4 sono tubes placed on 4 footers..

In place of expensive sono tubes I will be using cinder block to form the 4 legs of my table (can see in my build thread)... since I am just using 1 cmu block stacked 4 block high in each corner, I will fill both holes of the cmu with concrete for strength.. I will prefill the interior holes first with concrete... and the corner hole will be left dry and will have rebar from the footing folded into the table and will be poured all at one time into the table ...


my table is 4foot x6foot and is going to cantilever the 4 cmu corner post 6 inches all the way around... this gives me the reveal I need to cover the cmu with lava rock.... so my plan is to set the 4foot x6foot Durarock directly on top and over over my 4 cmu table legs... and in the corner holes that are dry and hold the rebar I will use my jig saw to cut the squares out... thus allowing cement to flow into those 4 corner chambers... I will use a lot of 2”x4” to support this and my perimeter 2”x 6” dropped to the required 4 inches slab depth will attach to these 2”x4” supports... and then I will brace the shit out of it... i really appreciate the tip about the bracing blocks and wedges for removal...

also big thanks on answering my next question before it came... Lolol.. the drain holes under the hearth.. got it... you answered everything with that... how many, size, and technique... awesome!!! And I have been thinking about water run off.. a slight mound under the hearth?? A slight forward pitch to the whole table?? So much to consider... Sorry to ask so many questions... I truly respect everyone’s experience and knowledge... mahalo

[david s]

Aloha!!! Been reading through all your amazing builds and picked up something that may make my life a bit easier...

in regards to forming the concrete table... I was going to build a complex ply wood frame that was removable from underneath the table... then I see that some of you have used Hardie Backer or Durarock under your table and have left it in place...

I can get both at Home Depot... I need 4’ x 6’ sizing...

Hardie Backer: comes in a 4’ x 6’ siding panel but is only 5/16 inch thickness (I have a lot of support and brace material)

Durarock: comes in a 4’ x 6’ sheet and is 1/2 inch thickness... from my previous experience is not as ridged as Hardie backers and will need a lot of bracing too...

thoughts or suggestions on using either these products to form my oven table?? Thank you very much.

No cement sheet 1/2” thick is adequate to support an oven. The cement sheet is simply the base onto which is poured a reinforced concrete slab. The cement sheet simply stays there as it’s removal is extremely difficult. Not sure how big your oven is but the usual thickness is 4” a smaller oven can have a thinner slab especially if piers are placed inside the perimeter (cantilevered slab).
The oven will be heavier than you think and requires both a solid base and solid foundations under the stand. These can not be made too solid, but can easily be made too weak with results that can’t be rectified.

[Boogie-D]

david s aloha David... yes the durarock is just part of my form that will be left... yes I am planning to pour a 4 inch table slab with a rebar mat and rebar that goes from the table into the cmu connected to the footing.... I was not planning for a foundation under the sono tube as in the flying saucer picture below (Neil?) only a footer... if you look at my build site pictures, it’s located on a hill side...

if I am to put a concrete slab under the whole thing well bust out the wallet... and a lot more concrete $$.. and I better start digging my ass off.. (hillside)

I don’t own this land but long term lease it... the stand and table I build will stay here if I have to move... I can’t take that away... I do want to build it strong enough of course... and I realize the whole thing; table top, cast dome, cast insulation, floor bricks, and insulation blocks all add up to being very heavy... but the least amount of $$ I can put into the stand and table the better..

as far as my oven floor bricks... my CalSil insulation blocks... and my cast dome... I hope to be able to remove that from the table when I move.. I feel as if I can break down my stucco finish and break down the insulating layer and get back to my fire blanket... and be able to remove that.. at that point I would hope to separate the cast dome from the floor bricks but I am sure that will prove very difficult... I also have a fork lift and a trailer.. but do you catch my drift?? I hope to remove almost everything except the table.... so the table cost effective as possible, but strong enough to hold all that weight... am I way off to think I can do a flying saucer style table like this picture??

ya most people thing I am a bit nuts.. I sky dive, surf big waves, ride motorcycle, fish 5 miles out at sea in a kayak, heli board the steepest.. and want to build a wood fired oven?.. my wife and friends think my money and time would be better spent if I just went out and purchased a commercial no cord $2,500 gas oven with six burners and a griddle... this would be very easy... we could make bread again.. cook a pot pie... have a pizza... but then again I am a bit crazy... Lolol .. back to square 1...