Cast dome build in North Wales

[RobinW73]

Hi folks, newbie here.

First off many thanks to everyone who has created this wonderful resource over the years. It is very much appreciated by those of us who are Jonny-come-latelys to the world of DIY wood fired ovens.

Secondly, apologies for the longer post. I'll be more consise in future.

A little about me - I'm Robin. I have just created (2023) a small off-grid glamping site on my farm in North Wales - solar power, reedbeds, compost toilets, etc etc. I am a competent DIYer/bodger. Ty Uchaf Farm Glamping if you want to search for us.

I have been thinking about building a pizza oven for the house/garden for a while, and now plan to start my build in the next few days but will start with one for the glamping site, next to the field kitchen (pic attached).

The Brief: I imagine the glamping pizza oven may get use 2-3 times a week in the summer for pizzas or the occasional steak; and not for extended/multi-use cooking purposes. So I'm planning a relatively simple (and speedy) build to learn some of the lessons ready for building the oven by the house, and I want to see how low a budget I can manage with and still get a usable outcome that will work and last for a minimum of 2-3 years before I need to do it properly. By low budget I'll be cheating as I have plenty of sand, cement, lime and concrete blocks from various other projects. I'm not creating the most beautiful oven - but will want a good looking but rustic finish. The oven will be subject to weather and so will have a waterproof render - but outside of the season I will be covering it with tarp. I may decide (on advice?) to extend the field kitchen roof over the oven.

So, to the plan...

I know, they are not the most popular or interesting builds - but the glamping oven will be a castable exercise-ball approach. This is preferable for me as I can cast in the workshop close to materials and tools and then transport to the glamping field, rather than build with a sand mould on site. Making formwork is no problem and transport is not an issue with a 95hp 40 year old tractor on hand...!

Here is a summary of the plan and the order of works - I would be grateful for advice on any mantraps, bearing in mind I'm not intending to create the world's "best" oven as per above. Indeed - if you think this overspecced for my brief please say so. I was initially considering just a "single skin cement/perlite dome and be damned" approach for my brief.

Base - 100mm of standard concrete on a rammed rubble substrate, straight to earth.

Frame - medium density solid concrete blocks (440x215x100mm) in a square M configuration. Standard brick mortar. Painted with masonry paint.

So far so good - this is how I build my BBQ frames.

Plinth - Shuttered and poured slabs, wooden frames removed after setting. 3 layers: 100mm standard concrete, with gravel aggregate and reinforcing bars. 100mm v-crete (5:1 with a 50:50 mix of vermicultie and perlite). 25mm Firebricks under the dome/gallery in a herringbone pattern, sitting on a thin layer of sand/fireclay to level and set within 25mm of vcrete (5:1) to the edge of the plinth. I may put a 10mm thermal break between the oven and the gallery bricks and fill with 10:1 vcrete. I will leave a week between each slab I guess.

Dome/gallery - 85cm exercise ball and wooden frame for gallery as per the standard approach. Door rebate in the frame, and a funnel at the base of the flue. The internal height of the gallery will be around 27cm (63%) based upon internal height of dome of 42.5cm. Gallery internal width will be 40cm and length 30cm. Flue diameter will be 100mm single skin flue - height of flue tbc.

I was going to cast in 4 layers: Firstly a dense refractory layer of 50mm of homebrew 3:1:1:1, with stainless steel needles and poly fibres, then 25mm ceramic blanket and then 50mm insultaing layer of 10:1 v-crete. I'll put a pressure relief valve at the apex of the dome in the v-crete layer to let out steam. Then a thin cement render.


A few questions:

1) Do concrete plinths really need to be so thick, given the weight on top is about 4-500kg spread over three legs? There's massive strength in 100mm standard concrete plus 100m v-crete. Would 50mm of concrete plus 100mm 5:1 v-crete suffice?

2) Rather than a cement render which is waterproof and may trap in moisture and need a valve, I ahve barrels of mature lime putty lying around. Has anyone made a breathable lime/sand only render? Does it need hairing? I'm a bit of a DIYer at lime plaster/mortar/breathable buildings (farmhouse is 1500s) and it seems to me that this could be a solution for a pizza oven dome.. or not?

Thanks for reading this far. I will post pics and progress over the coming weeks.

Robin

[wotavidone]

Good day. I haven't visited the forum for well over a year, but I'll see if I can still add some useful insights.

1) 50mm concrete is a bit thin. You're putting reinforcing bars in the plinth, and if nothing else that makes 50mm too thin. (50mm would certainly be too thin without reinforcing, too)
The bars need to be covered by a reasonable amount of concrete or they'll corrode and split the concrete. A compromise would be to use at least 75mm. Bear in mind, the load carrying ability is pretty much based on tensile strength, of which vermicrete has bugger all.
Of course, if you put enough reinforcing bars, then the concrete simply becomes the medium for filling in the gaps and keeping the spacing, at which point 50mm is probably plenty,

2) I envy you the barrels of lime putty, I'd be looking to calculate that into the homebrew mix instead of plain old lime. I also can't see why you would not be able to render with it. Sand/lime renders and mortars pre-date modern cements by hundreds of years. But, this is North Wales. Regardless of what I used for the render layer, I would be extending the roof of the outdoor kitchen (lovely work BTW) and building the oven under cover. Google says average rainfall in North Wales is nearly 3 feet per year. Just humidity in the air could be a challenge without direct rainfall on the oven.

3) 100mm chimney is a bit too small for an 85cm ID oven, try for 1500mm if can get it.

Hope this helps.

[RobinW73]

Good day. I haven't visited the forum for well over a year, but I'll see if I can still add some useful insights.

...

Hope this helps.

Many thanks.

1) I'll probably just bite the bullet - and install 100mm concrete as everyone else seems to. I've got plenty of ballast and cement from previous projects so it's no bother. I hadn't appreciated there was "bugger all" contribution from the v-crete.

2) I've done a fair amount of lime plastering... so always have a couple of dustbins on the go. I've discovered that putty made with hydrated bag lime is just as good after a year for internal plastering as expensive slaked lime putty... so it's easy and cheap to do in you plan ahead. I may try and use it in the homebrew, but the putty is usually quite wet and I gather that I want a fairly dry and sticky homebrew? Thanks for comments on kitchen, a fun build I just made up as I went along. I will extend the roof - it seems the most sensible thing to do and then I could omit any cement from the render.

3) I think a 1500mm chimney might need planning permission! I'll go with 150mm. ;-) I was just trying to reuse some 4 inch flue I already have. I also have some twinwall but I think that's 5inch ID and 7inch OD which would look very bulky. I'll just have to buy a length of 6 inch diameter I guess.

Thanks - yes, helps greatly.

[david s]

1. The strength of a concrete slab is proportional to the square of its thickness. This means a 50mm slab is only 1/4 the strength of a 100mm one. Reducing the span between the piers by cantilevering the slab with the weight being placed more directly over the piers is a method of being able to reduce supporting slab weight and thickness.

2. A tall flue pipe1800mm high requires stays. If you are penetrating a roof, it becomes the support for the flue. Most building requirements are 600mm higher than the roof penetration with a double flue requirement if the flue passes through a ceiling.

3. A 100mm diam is too small for even the smallest oven. An 85 cm internal diam oven requires a 6”
(150mm) flue.

If using an exercise ball as a mould, how do you plan to move the casting on to the slab? Perhaps cast in 3 or 4 lifting hooks and cut them off once into position.

[david s]

Here is a solution you might consider. Because of the difficulty of building a stand on top of your existing floor and the weight the floor would be required to support, building the oven beside the floor leaves the oven mouth and operator under cover and the flue can pass outside the roof avoiding penetration, double flues and sealing issues. I've done several of these and prefer the method to going through the roof.



The same oven from the inside view. This also shows what I posted re cantilevering and supporting slab thickness.

[RobinW73]

Thanks DavidS - comments much appreciated.

Plinth/oven floor - I think, on reflection and for simplicity I will revert to a standard three-sided open frame of concrete blocks blocks and a 100mm slab.

Flue - I think I was talking at crossed purposes with wotavidone - I was talking about 100mm diameter flue and you and he were talking about 1500/1800mm height. I misunderstood and tried to make a joke about a 1500mm diameter flue needing planning permission!

I think, based on your advice, I'll opt for a 6 inch diameter flue but route it up through the roof.

If I install it through a corrugated roof high temp silicone flashing kit and storm collar, I shouldn't need to worry about stays or leaks. I will probably use a sheet of tin rather than bitumen sheet for the roof extension - and I've previously had good results with this method with the woodburner flues in the other glamping structures on site. Have you had issues doing it this way? I will also cost up the flue with an elbow option you suggest and comapremwith the flashing kit and collar and see which is cheapest. Git feel is it will be very similar to within a few quid.
I was planning to cast on plywood laid directly on the tractor rear forks in the barn/workshop. I can then drive into the field, level the plywood with the oven slab and slide it straight across on to the slab. That saves me the hassle of moving all the materials to site and the time/skill to make a nice round sandcastle mould. Can you see any flaws with this, other than the cast will be heavy to slide?

[david s]

That all sounds reasonable. Re the roof penetration, I don't know how tied you are to council regulations re a single walled flue. you might (or not) get advice from your local authority.

There could be a problem with corrosion around the flue pipe from bits falling from the cowl. My sister needed her roof repaired from this problem after 12 years with a slow combustion stove that had a flue pipe through the roof and there was considerable corrosion around the seal. Still 12 years is a fair time away.

[RobinW73]

Thanks - if it lasts 12 years I'll be very happy. Everything on the site is built for (and depreciated over) a ten year lifespan. I expect to be renewing and upgrading (or retiring!) well within in this timespan.

[RobinW73]

Finally I have started the build. Waited for the poor winter weather then the very hot spring weather to pass.

Concrete base (100mm) has been laid and now some 2 inch thick heather browns I had left over from renovating the house have been laid.

My plan remains to use concrete blocks for the pillars... but I almost think they'll look a bit too industrial against that nice base.