2025 Pompeii Oven Build Log

[yoboseyo]

I use a product called “flexible pointing”, ( it may be called something else in different countries), which was developed to cover the mortar on tiled roof ridge caps. Because a tiled roof, just like an oven is subjected to continual expansion and contraction, the flexible pointing uses an extremely flexible acrylic and to achieve high build thickness, fine sand is also contained in the mix. Because it will also receive a lot of sun, there is plenty of UV protection also added to the product. It is fully waterproof, UV resistant and highly flexible so ticks all the boxes as a suitable product for WFOs. An old roofer told me that prior to its development in the 70’s roofers often had to re-mortar ridge caps to prevent roof leaks, but since it’s become industry standard, they never have to do those repairs.
As it is designed to be troweled on, for easier application I water it down 20% so it can be painted on in about 3 layers to achieve a thickness of around 1.5mm.

would you use this as a primer layer on the dome? or over or under the primer - I'm tiling over it so it has to adhere?

what are your thoughts on waterproofing vs not water proofing? Are the water resistant layers enough and water proofing is a nice to have, or a must?

I'm in Australia (Canberra)

[david s]

I use a product called “flexible pointing”, ( it may be called something else in different countries), which was developed to cover the mortar on tiled roof ridge caps. Because a tiled roof, just like an oven is subjected to continual expansion and contraction, the flexible pointing uses an extremely flexible acrylic and to achieve high build thickness, fine sand is also contained in the mix. Because it will also receive a lot of sun, there is plenty of UV protection also added to the product. It is fully waterproof, UV resistant and highly flexible so ticks all the boxes as a suitable product for WFOs. An old roofer told me that prior to its development in the 70’s roofers often had to re-mortar ridge caps to prevent roof leaks, but since it’s become industry standard, they never have to do those repairs.
As it is designed to be troweled on, for easier application I water it down 20% so it can be painted on in about 3 layers to achieve a thickness of around 1.5mm.

[Gulf]

Some have used products that are normally used for underground basements. One, over here, goes by the brand name "Redgard". Similar products may be available to you. However, I still like some type of open roof over an oven. That seems to be a great way to save the looks of an igloo and still protect it from the elements.

[yoboseyo]

The "tiling primer" , if it is acrylic based, is a "waterproofing membrane" imo. Yes, use something like the Pro Flex S2 for the tile adhesive. Also, seal those joints with an outdoor acrylic sealer. The stone sealer you mentioned sounds right for the final sealer. However, for any climate that receives precipitation with freeze/thaw, I recommend a roof over the dome. That allows for the oven's use in almost any kind of weather. A roof not being possible, I advise to cover the oven with some type of tarp during the coldest months. That is when true damage to the oven can occur.

The primer is water resistant, not water proof. Primer in question:

https://aus.sika.com/en/construction/tile-stone-installationsystems/primers-additives/davco-ultraprime.html

Practically speaking, I'm not sure. This is really a question of what is waterproof in practice

[Gulf]

The "tiling primer" , if it is acrylic based, is a "waterproofing membrane" imo. Yes, use something like the Pro Flex S2 for the tile adhesive. Also, seal those joints with an outdoor acrylic sealer. The stone sealer you mentioned sounds right for the final sealer. However, for any climate that receives precipitation with freeze/thaw, I recommend a roof over the dome. That allows for the oven's use in almost any kind of weather. A roof not being possible, I advise to cover the oven with some type of tarp during the coldest months. That is when true damage to the oven can occur.

[yoboseyo]

Hi guys,

Time has come to do the cosmetic tiling - I have chosen round natural marble penny rounds.

Need a tip on waterproofing - how much do I need and how?

The outer render layer is acrylic render premix.

I'm thinking - these are the layers
- render
- waterproofing membrane?
- tiling primer
- tile adhesive
- tiles
- grout with polymer additive
- penetrating stone sealer?

Do I need these waterproofing steps?
- firstly the waterproofing membrane - e.g., Davco k10 Plus or Dunlop Express Wet Area
- secondly stone sealer

I have a vent on the back of the dome so moisture won't build up inside it

Also, what tile adhesive should I use? Does it need to have S2 rated elasticity?

[tmoskos]

That looks like an awesome build project, especially with how detailed you’re keeping track of everything. I had a similar experience with moisture sneaking into my outdoor cooking area, especially after heavy storms, even with a good render. The weep holes with mesh really did help with drying things out quicker, though I underestimated how much water the insulation could suck up just from humidity and small leaks. I ended up having to torch the oven several times to get trapped moisture out, so adding those drainage channels early saved me a ton of headaches later. Anyway, if you get into adding any sort of glass enclosure or want privacy for the outdoor kitchen area, I came across some neat solutions like switchable privacy glass on http://smartglasstech.us/. They cover a lot about privacy and controlling sunlight, so could spark a few ideas!

[yoboseyo]

It will be the water added in excess to that converted during the hydration process.during the build. Eg water taken up when soaking bricks, water in the mortar, water in the vermicrete layer etc.

thanks for the illustration. helps a lot!

I didn't introduce a lot of water that way, although I could be wrong. There's no vermicrete layer under the hearth. The bricks were soaked but the whole thing has been air drying for months - i only just did the rendering.

I have had 4 curing fires, so moisture would've been driven from the dome first, and is reaching the floor. I'm concerned about water ingress into the floor, although there might not be and I'll just continue firing.

I did put in a vent into the dome, which will help relieve moisture build-up. By your illustration, i could also drill some weep holes directly underneath the dome walls.

PS you're local fire dept. would not like the look of your (presumably single) flue pipe siting against the timber of your roof.

Ah yes. I decided that this option is better than a short chimney venting onto the timber joists. I can install an elbow if necessary.

[david s]

It will be the water added in excess to that converted during the hydration process.during the build. Eg water taken up when soaking bricks, water in the mortar, water in the vermicrete layer etc. do a rough calculation and you’ll realise there’s a lot. It may look and feel dry, but just as a pile of damp sand will dry on the surface an inch or so deeper will remain damp for many months because it's covered by the sand on top of it therefore the moisture is locked in.and as one litre of water creates over 1500 litres of steam, just as potters know, when pottery looks dry on the outside there’s still moisture in the middle of the walls of the pots and the bisque firing must proceed very slowly until the first 400C is reached.(steam spalling usually occurs at around 250C)
The visible water at the weep holes is proof of the presence of water exiting. The other indicator is how hot it feels to the hand because water or steam is far more conductive than air. Just take it slow. As you’ve documented there’s a huge difference in the temperature of the top of the dome and the floor (as well as the base of the dome, you’ll find), which means a huge difference in thermal expansion of those parts. This sets up huge stress. It is also far easier for the moisture to escape if the drying fires are done after insulating, but before final render, because that helps contain the moisture rather than allowing it to escape easily. Lucky you provided the weep holes.

PS you're local fire dept. would not like the look of your (presumably single) flue pipe siting against the timber of your roof.

[yoboseyo]

I'm up to the 4th set of curing fires, and I am noticing quite a bit of heat leakage through the bottom.

Remember that I have 3 layers of CaSi that is raised from the slab using tiles, and weep holes drilled into the slab.

On the 4th firing, I noticed that the underside of the slab was registering 30C, and it was a cold evening where the outside temperature was 10C. There's no heat leakage at all through the dome.

The top of the dome reached 400C. Yet the floor temperature was very uneven, even after a long period of firing, reaching only 250C at the hottest part.

I noticed that water was dripping out one of the weep holes.

I can't explain what's causing the water ingress. If water is leaking in from the outside somehow, that itself is not a big issue because the CaSi layer is suspended. I can't tell whether the CaSi layer itself is wet. I can't see how water could ingress and make its way up to the insulation layer, but the temperature leakage and hearth temperature suggests that moisture is a likely reason.

Also, it has been a few months where the oven has been air drying, so refractory materials/cement should be fairly dry.

Any ideas on how I can diagnose and rectify this issue?

Water dripping from weep hole in question:

[yoboseyo]

Cast the flue. Ended up running out of burnout fibres. I didn't know how to work with the stuff and I'm pretty sure I didn't distribute it evenly enough, buit whatever.

Attached the flue and the chimney. I was in two minds about whether to insulate the flue, and decided to do it anyway, as it would make it easier to finish. I wasn't sure whether render can withstand high temperatures

Also rendered the outside of the dome.

I think all but the cosmetic parts are done, and onto the curing fires!

[yoboseyo]

What steps did you use to cut the inner arch like that? I tried that design and it didn’t work so well for me. Going with something
more Conventional

I followed this youtube video as a guide:

https://youtu.be/6aMd3RykSYI?si=vYuYgCaLLxRefZtt

My dimensions were different so I drew it up in sketchup, essentially intersecting the dome and arch bricks, and ensuring what was my 7th layer sits exactly on the inner arch.

You can see some pictures of the inner arch brick layout dry run in my 3rd post. See how i used the IT pivot with a 450mm piece of timber

[MikeD55]

What steps did you use to cut the inner arch like that? I tried that design and it didn’t work so well for me. Going with something
more Conventional

[david s]

Don’t omit the clay in the recipe entirely, it adds refractory quality to the mix as well as making it more workable. The very fine polypropylene fibres I use are 19 mm long and added at the rate of 1% by volume. Be careful to disperse them well in the mix, they tend to clump. Mix for double the time you would normally expect.

[yoboseyo]

exposed cal sil at the sides of the entry

I will seal all the edges of the transition using rope

I’ve found the 3:1:1:1 recipe leads to excessive shrinkage (you don’t want shrinkage cracks), from the high clay portion so I halve it, with much better results. 3:1:1:0.5

would it be better to omit the fireclay entirely?

fine polypropylene burn out fibres

I just looked up this stuff. It comes in different fibre lengths from 6mm to 18mm. which should I choose? how much of it do I need to add?

Do you have any experience with tiling on hot surfaces? Would I be able to lay tile directly on the vent? I read in another post of yours that you have used Monoflex, which I can get. That wasn't for a vent, though. I'm not sure how hot my vent will get.