What type of mortar for rendering dome and question about oven door construction

[ebbbe]

Hi,

Dome render:
I'm in the process of building an oven. At this point I have made the dome and insulated it with 3" of ceramic fibre blanket and added chicken wire. This weekend I'm planning on rendering the dome.

I have bought some cement based plaster mortar that is water resistant and open to diffusion. It is made to render the above-ground part of the foundation on houses.

What are your thoughts on this?

Thanks!

[mongota]

I'd venture that a "cement based plaster mortar" is portland cement, sand, and something to help with spreadability; either lime or a plasticizer of some sort. It'll work. Consider doing 2-3 thinner individual coats versus just one thick layer.

The 'water resistant' nature of the material can be dependent on how well you work it with the trowel.

Enjoy the process!

[ebbbe]

I'd venture that a "cement based plaster mortar" is portland cement, sand, and something to help with spreadability; either lime or a plasticizer of some sort. It'll work. Consider doing 2-3 thinner individual coats versus just one thick layer.

The 'water resistant' nature of the material can be dependent on how well you work it with the trowel.

Enjoy the process!

Alright thanks! I already bought it and applied the first layer. The thing with multiple thinner layers are part of the instructions, so I should be good.

With regards to it being water resistant, I think there is more to it than trowel work. It is literally has "waterproof" in the name and its considerably more expensive than normal plaster mortar.

thanks fpr the advice!

[david s]

The render of the outer shell for a WFO presents some problems not normally encountered in a conventional wall. Firstly the problem of steam build up under the rendered layer, secondly the variation of temperature experienced and thirdly the substrate to which it is applied. These problems require a stronger than normal finished result than a standard render will give. This can be achieved by applying over a chickenwire mesh or alternatively through the addition of random mixed fibres in the mix. Commercial renders are are designed to be applied in 2 or 3 layers 24hrs apart of around 5mm thick. If doing only one layer there is a greater chance of slump cracks developing Adding a higher proportion of cement to the mix in order to increase strength introduces the problem of making the resulting mix too brittle. The recommendation is to make each subsequent layer slightly leaner. The substrate should be firm enough to hold the layer without moving. Rendering on to a blanket layer is therefore difficult. Also, rendering against a very dry substrate introduces the problem of water being drawn away from the mix too fast to allow for adequate hydration. Application during hot and windy conditions should be avoided for this reason. To save time I do the rendering in one layer (with random glass fibres which eliminate the slump cracking problem as well as increasing strength) about 12-15mm thick and after wet sponging the surface to remove tooling marks, cover the whole oven in cling wrap to hold the moisture in for a week.
The degree of waterproofing in commercial cement renders is only partial as they are designed to be breathable in order to allow moisture to as well as reduce water in. When mixing the product, if it contains a waterproofing compound, when water is added it is difficult to mix in, most cement grouts have this same quality as well. The commercial cement renders are also usually mixed with 50% water and 50% acrylic fortifier.
The addition of lime in the mix imparts not only workability, but also some elasticity and crack healing properties. A waterproofing layer can also be added over the top of the finished render and there are many products available for this application, but choose one that has good UV protection. I use an acrylic product called Flexible Pointing which is extremely flexible and high build. I paint it on in 3 coats to create a thick (around 2mm) layer.
There are many ways to skin a cat, but I hope this info provides builders with some things to consider.

[ebbbe]

The render of the outer shell for a WFO presents some problems not normally encountered in a conventional wall. Firstly the problem of steam build up under the rendered layer, secondly the variation of temperature experienced and thirdly the substrate to which it is applied. These problems require a stronger than normal finished result than a standard render will give. This can be achieved by applying over a chickenwire mesh or alternatively through the addition of random mixed fibres in the mix. Commercial renders are are designed to be applied in 2 or 3 layers 24hrs apart of around 5mm thick. If doing only one layer there is a greater chance of slump cracks developing Adding a higher proportion of cement to the mix in order to increase strength introduces the problem of making the resulting mix too brittle. The recommendation is to make each subsequent layer slightly leaner. The substrate should be firm enough to hold the layer without moving. Rendering on to a blanket layer is therefore difficult. Also, rendering against a very dry substrate introduces the problem of water being drawn away from the mix too fast to allow for adequate hydration. Application during hot and windy conditions should be avoided for this reason. To save time I do the rendering in one layer (with random glass fibres which eliminate the slump cracking problem as well as increasing strength) about 12-15mm thick and after wet sponging the surface to remove tooling marks, cover the whole oven in cling wrap to hold the moisture in for a week.

The degree of waterproofing in commercial cement renders is only partial as they are designed to be breathable in order to allow moisture to as well as reduce water in. When mixing the product, if it contains a waterproofing compound, when water is added it is difficult to mix in, most cement grouts have this same quality as well. The commercial cement renders are also usually mixed with 50% water and 50% acrylic fortifier.
The addition of lime in the mix imparts not only workability, but also some elasticity and crack healing properties. A waterproofing layer can also be added over the top of the finished render and there are many products available for this application, but choose one that has good UV protection. I use an acrylic product called Flexible Pointing which is extremely flexible and high build. I paint it on in 3 coats to create a thick (around 2mm) layer.
There are many ways to skin a cat, but I hope this info provides builders with some things to consider.

Lots of info again, David. Thanks!

What I did was to apply one layer on chickenwire. The max layer thickness is listed as 10mm, which I tried to stay at. I then added a second layer of about 5mm with leftover fibres. I didn't have nearly enough cling wrap on hand, so I have covered the full rendered surface with wet towels as it is recommended when casting a dome. I didn't have problems with slumping or any difficulty of applying the render.

When the first week of damp curing is done, how long would you dry it out in the air before continuing with curing fires?

[david s]

It depends a bit on the weather, but as the outside never gets very hot you should be good to go. When renderers do house walls they never lock in the moisture for a week, but the strength is substantially enhanced if you do. Gerri. Cement boat builders talk about 28 damp days. While you’re not building a boat you still want a strong outer shell.