Ottawa Oven - Onward Once More!

[Jed]

Sarah,

I wouldn't worry about the heat on the cement board so much. You have the flue tile in place (if I am 'seeing' your picture correctly) and you are proposing building a box with the box walls away from the flue, and fill in the space between the flue and the cement board (at least an inch?) with vermiculite that will protect the cement board from most of the heat.

When you fire the oven, how hot does your chimney flue get?

You are planning to build the structure with steel 'studs' in the corners to support the cement board, so the structure should be sound...

It will all be setting on a masonry structure that looks to be sound and able to support the weight....

And you are planning to 'cap' the assembly, keep most of the water off the structure...

And then finish with some 'cladding' to make it all look good, and keep the weather off the cement board....

I don't think of cement board as a 'structural' component, but the distances you are using (less than a foot and a half wide between the studs?), and the weight that is involved all look small enough that it should work.

If it is a worry, maybe double up the cement board; get a double layer that is screwed to the studs in the corner of the structure.

One opinion...

JED

[egalecki]

Could you wrap the flue in FB blanket first, put the wonderboard up, then fill in the voids with vermiculite?

[dmun]

I'm sorry, I'm completely off base here, I just looked at your picture again and you have a flue tile in place, and the sheathing can be made of anything non-combustible. For a moment I thought you were planning to build your flue transition out of wonder board.

Sorry about that. Carry on.

[sarah h]

Whew! David, thanks for taking a second look - I was worried for a while there. But I imagine you must have thought I was nuts if it sounded like I wanted to construct the flue from cement board!

Elizabeth and Jed, thanks for your input. I'd use ceramic blanket if I had some left but it's too expensive to order a whole box just for the little bit that I'd need. An inch clearance, maybe a tad more, is my plan. the reason I want to use cement board rather than brick or something cast is to keep the diameter to a minimum so that the chimney is not too overbearing in relation to the dome.

If all goes well, I'll be back at it tomorrow ...

[sarah h]

Well I'm still stalled out on the stonework and - not sure if others have had this problem - I cannot find any stucco around here! I thought it was something that was pretty common but apparently not ... unless stucco and parging are just two different words for the same thing?

I have moved ahead in one regard though - I finally achieved total white-dome status this past weekend and, wow, the heat from that is incredible! Now I know what the rest of you have been talking about.

At it's peak it was up somewhere in the high 700's and stayed that way for some time.
Twenty-four hours later, it was still over 300 degrees.
Forty-eight hours later, the inside top of the dome was still 205.
And that's without an insulated door!

My oven is amazing! Now, if I could just get it finished ...

[Jed]

Hi Sarah,

I've just finished the 'stucco' on my oven. I don't know the final answer, but will chat a bit about my experience... maybe the experience will help you find a better answer...

Stucco apparently can be mixed from different materials and still be stucco. The older mix is lime and sand. The current standard is apparently Portland cement and sand. Some formulas include some lime in addition to the Portland. And different combinations of sand and Portland cement depending on the application.

Mortar is Portland cement and sand. So with that in mind I purchased the Quikrete brand of premix Mortar (my question to the manufacture on the mix they used went unanswered ie. how much sand to how much Portland). To each 80# bag I added a shovel full of lime and a box of color. This is the mix that I used to coat my igloo shaped oven.

I am a bit dis-satisfied with the results. After being in place for only an hour or two, this 'stucco' mix started to spider web crack. Now, everything that I covered with this mix is spider web cracking like crazy.

So I got something wrong. Either the premix 'mortar' is not an appropriate substitute for 'stucco' (not enough Portland?), or I installed it incorrectly.

So, that is my story... One bit of experience... I can't make a suggestion on the correct way, but maybe this experience will help you to find a better way to get this job done...

Good luck with the project, what you have so far looks great, and white domes are a good thing!

This photo is the oven in it's current state...

Best,

JED

PS: and this second photo sort of shows the spider web cracking I have on the oven... Now looking for ways to 'repair' this....

[SpringJim]

Sarah

I have done lots of stucco work in Spain just using a sand mixture and portland....but that sand mixture is actually made out of crushed limestone.
So I thought stucco was just sand and cement but these couple of posts made me think about a bit more. Maybe the stucco in spain is better because of that local crushed limestone (possible dolomite?) sand.

BTW....You can screen the sand to get different textures. Smaller sand will give you a smoother finish.

I've also seen the spider cracking on a stucco wall. You can keep it wet with a sponge or rag....after it begins to set up though...I find that this floats the surface cement into the cracks. I will also try and keep it wet after it is set up a bit.

After reading Jed's post, I'm thinking about adding some lime to my sand/protland stucco here.

Good Luck, Jim

When is that island oven going to be ready?

Originally Posted by Jed

Hi Sarah,

I've just finished the 'stucco' on my oven. I don't know the final answer, but will chat a bit about my experience... maybe the experience will help you find a better answer...

Stucco apparently can be mixed from different materials and still be stucco. The older mix is lime and sand. The current standard is apparently Portland cement and sand. Some formulas include some lime in addition to the Portland. And different combinations of sand and Portland cement depending on the application.

Mortar is Portland cement and sand. So with that in mind I purchased the Quikrete brand of premix Mortar (my question to the manufacture on the mix they used went unanswered ie. how much sand to how much Portland). To each 80# bag I added a shovel full of lime and a box of color. This is the mix that I used to coat my igloo shaped oven.

I am a bit dis-satisfied with the results. After being in place for only an hour or two, this 'stucco' mix started to spider web crack. Now, everything that I covered with this mix is spider web cracking like crazy.

So I got something wrong. Either the premix 'mortar' is not an appropriate substitute for 'stucco' (not enough Portland?), or I installed it incorrectly.

So, that is my story... One bit of experience... I can't make a suggestion on the correct way, but maybe this experience will help you to find a better way to get this job done...

Good luck with the project, what you have so far looks great, and white domes are a good thing!

This photo is the oven in it's current state...

Best,

JED

[SpringJim]

I snipped the following:

Stucco is a mixture of portland cement, hydrated lime, sand, and water. It is simply outdoor plaster. You can find it on houses, fences and anywhere that stucco is required for a fancy texture to a drab exterior. It can be easily repaired if needed, just make up a batch and re-apply the new mixture. It always blends right in with the old application. It's a durable covering for exterior walls. One of its advantages is that you can create different textures while masking imperfections in any wall. You can also add color to stucco. If you are building a block wall that you plan to stucco, strike the mortar joints flush. Clean an old block wall with soap or detergent before applying stucco.


For a scratch coat, you will need this mixture:



1 part portland cement

1 part hydrated lime

2 1/2 to 4 parts sand


For a brown coat, you will need:

1 part portland cement

1 part hydrated lime

3 1/2 to 5 parts sand


Finish coat ingredients:

1 part portland cement

1 part hydrated lime

1 1/2 to 3 parts sand


You will need a wheel barrow for mixing the stucco and a trowel or shovel. Safety goggles while mixing are a good idea, just in case the materials being mixed should splash in your eyes. Mix the ingredients in the shade, the direct sunlight will cause the stucco to dry faster while mixing.


It's easiest to make stucco from pre-mixed bags. but you also can prepare your own mixture, which is much cheaper. All you need to make stucco for scratch, brown, and finish coats are different combinations of portland cement, hydrated lime, and sand. Do not at any time add more water than the ratio calls for, unless you just need to retemper the mixture, and do this only once.


To mix stucco, add water to the combined dry ingredients until the mixture is a uniform color and the consistency of a thick paste. Make only as much stucco at one time as you can apply in an hour or so. If the stucco starts to stiffen, you can retemper it only once by adding water and mixing, this is very important! There are three coats required in using stucco. You have to apply three coats in order for your project to be completed.


For adding color to your stucco, you will need to add a mineral oxide pigment to the finish coat mix and you can have any color you wish. Keep track of the exact porportions so that you can match the color again later when mixing a new batch of stucco. Pigments work best with white portland cement. You can be a designer with colored stucco and give life to old stucco projects outside your home. You can also match a house that is colored in stucco, by making the same colored stucco to match a fence. You will be the talk of the neighborhood and you can tell everyone that you did it yourself!

[sarah h]

Started the stone covering on the dome today - might just get this thing finished some time in the foreseeable future!

Here's a photo. The mortar joints will blend in better when they dry.

[nissanneill]

Lookin' good Sarah.
Almost like a turtle with his periscope up.

Neill