Help wanted with Brick Roof design and mortar recipe - pls

[colonelorbit]

Hi all, I am just getting ready to start my first oven build project and have been doing a bit of reading round this forum, as well as others and books on oven building. I have decided to go with a brick oven, with a barrel-vault design, as it seems the simplest and strongest to build and I make a fair amount of bread as well as pizza. I have sourced a load of reclaimed clay bricks (from a Victorian chimney) for the oven floor and roof, as well as most of the other materials. However I could do with a bit of advice on a couple of points:

1) Brick roof design. I would like to make the roof as strong as possible, so the structure will last and so I don't have to waste time, expense and additional heat sink on coating the roof in structural concrete. I have seen a few discussions and the odd photo of brick roofs made with offset bricks, so integrally tying into one another, but I can't find any infirmation/photos of how to tie the first row into the back wall, or whether this is necessary. I have thought that I could tie in the centre brick and the two end bricks into the wall, would this be enough? Also from my basic bricklaying experince (watching other people!) would a simple system of offsetting the bricks alternatively, so tying in every other one be enough to give additional strength?;

2) Fire brick mortar recipe. I would like to make my own fire brick mortar, to bind the bricks. Again I have seen a number of recipes, with or without Portland Cement and/or Lime, but have not come across a definitive answer. I have bought a large bag of Hydrated Lime, Portland, building sand, stone dust and can get some clay dust too. Has anyone got a decent recipe for make-your-own fire brick mortar, which is sticky enough to hold the bricks in laying, but low enough in Portland cement not to degrade with the heat of firing.

As you can probably tell from my posting I have little direct building experince, I do work in the building trade (as an environmental consultant) so have seen plenty of building done and am sure I can apply myself, but I have little actual experince of laying bricks - so would be grateful if the replies could be in nice simple laymans terms - nothing too technical - I do hope to get a bricklayer mate of mine to help me with the basics, but he has no brick oven experince!

Am looking forward to your replies and learned information, so I can put the last pieces of my project together and get started!
Thanks Col

[DavidApp]

Re: Help wanted with Brick Roof design and mortar recipe - pls

Hello Col

I am new to this oven building myself. I have done a lot of reading and built a barrell vault oven. Made some mistakes because I had not found this site when I started my build.

No insulation under the hearth was my worst mistake.

The high temperature mortar mix I started with was as follows.
3 Parts Silica Sand
1 part Fire Clay
1 part Lime (Type S)
1 Part Portland Cement.

I had problems finding the "S" type Lime. Got it at an oldtime hardware store.

Went with a product called HeatStop 50. It is a permix high temperature mortar.

David

[colonelorbit]

Re: Help wanted with Brick Roof design and mortar recipe - pls

David,
Thanks for the info on your mortar.

Watavidone,
Thanks for the advice on the 'running bond', that was my plan and it's good to get confirmation and thanks also for confirming the mortar recipe.

I was sure that I had picked up a thread once discussing a way of building the end wall so that it bonded with the arch, rather than just butted up to it (either internally or externally). I guess this would involve either some interesting cutting of bricks to make it fit or it only being bonded at the base of the wall and the key stone at the top; this latter method would still be effective especially if built inside the arch. However I can't find any more detailed references to this or other similar structures and wondered whether you had ever heard of or had any experience of this.

With the method of building the wall inside the arch, how would you fitr it together, would you:
1) build your end wall first, alongside the arch form and make up the gaps with brick slivers after the arch is built;

2) build the end all first, then draw on the line of the arch and cut back the bricks to an arch or;

3) build the arch first and build the wall within the arch.

All comments much appreciated. I do have a mate who is an out of work bricky and hope to tap him up for help on the day; I know he is good builder of walls and houses but has little or no experience of barrel vaults or similar structures so want to have as much design info as possible before I speak to him in detail.

Thanks

Col

[Tscarborough]

Re: Help wanted with Brick Roof design and mortar recipe - pls

Unless it is massive, it is not going to bond with the end wall, the dome will expand more than the back. It is guaranteed to crack at that joint, so you should make it a slip joint. A bit more trouble to cut, but it doesn't have to be perfect.

Cut it so that the back wall brick are half in and half out of the vault with a lip on the outside. Mortar it, but plan on it cracking (the mortar is more gap filling than structural).

[colonelorbit]

Re: Help wanted with Brick Roof design and mortar recipe - pls

Wotavidone, Tscarborough

Thanks again for your invaluable advice.

Have now got my base constructed (using recycled oak beams to make a square grid, 1.2m x 1.2m, set in the corner of walled patio) and have sourced most of the materials I need for the build. Just need to clean up my bricks (again recycled) a bit and get some powdered clay for the fire mortar.

Have got the design pretty much sorted out (using different resources) but could do with one more bit of help before I start setting out the oven floor:

1) What type of arch do you recommend? I have seen plenty of people using both semi-circular and long-radius arches with longer buttresses and both seem to have pros and cons. Semi-circular requiring less buttress, but having a wider gaps to fill with mortar. Long radius (flatter) arches requiring less mortar (and so more heat retention?) but requiring more buttress(?). I plan t make my oven floor approximately 75-80cm x 70-75cm, using standard clay (imperial) bricks.

2) How do you join the oven arches to the door arch? Most of the barrel vault ovens I have seen seem to follow the Alan Scott or Rado designs, with a row of leaning bricks joining the oven arches and the door arch (where the flue will be), so creating a sloping wall, internally to aid air flow. However, I'm no structural engineer or brick-layer, but i do have some civil engineering experience and I don't really like this design, it seems a weak link and makes the oven longer than I have room for. I'm sure I have seen photos of an oven where the door/flue arch was butted up to the outer face of the oven arch, resulting in a simple step down (internally). Would this work, I'm sure the Alan Scott/Rado design has a smoother flow of exhaust air, but would the simpler arch step-down design still be effective enough? - This design would maximise my oven space for the base footprint have have, but I don't want to compromise the air flow and build an oven that is hard to operate.

Any comments or information on these points is much appreciated. Will take some photos of the base so far and the rest of the work in progress as I go along and post on this or another thread. Thanks

[DavidApp]

Re: Help wanted with Brick Roof design and mortar recipe - pls

Hello Col

Some photos of my Oven.

As you can see from one of the photos I built the inner arch then the back wall. Finaly I did the outer arch. I did chamfer the bricks on the outer arch to reduce the morter joints.

I di have a crack that runs up about 11 o clock to the left hand side of the chimney.

I may have rushed the curing fires a bit or this design is prone to cracking.

David