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DO NOT OVER PAY FOR CASTABLE REFRACTORY!! We have spoken with almost every castable dealer in the US and we can tell you prices range from $35 a bag to $80 a bag! $80! Castable Refractory should be around $35 - $45 a bag… anymore than that and you should call the manufacturer directly..
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Mix the castable refractory and the water to the manufacturers specifications! Castable isn’t like cement you use to pour a sidewalk - it’s much more dry and firm. The proper water ratio is VERY important when mixing the refractory and packing the oven mold.
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Pack the refractory SLOWLY yet FIRMLY! This is probably one of the most important parts of building the oven. When packing the refractory with a wood dowel, you will notice that the castable is very sticky. If you pack too quickly the refractory will stick to your dowel and create an airpocket in the refractory - and this is bad! Pack the refractory slowly and firmly… it’s not a race and it will save you a lot of headaches down the road!
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You may have some foam sticking to the finished parts of the oven when stripping the foam away. Try to get off as much as possible, but don’t put too much effort into getting every little bit off. The foam on the outside of the oven won’t be seen… and the foam on the inside will not withstand the test fires. Just remove as much as possible, but don’t waste too much time trying to get every little foam bead off the finished oven parts.
The three piece upper slab to the base is VERY heavy. Each piece weighs over 250 lbs. Plan ahead for assistance. I used 4 people. If you built your base five blocks high, it WILL be difficult to place these slabs. I recommend you measure and mark appropriately, and place the REAR piece first if you do not have room behind your base to maneuver.
I used a hoist I rented from my local Home Depot. Made it much easier
You know, I looked at that hoist yesterday at my local HD. It would probably work great on a hard level surface. I just don’t see it working at all on my soft, inclined grass. (I’m talking about a hoist that has forks and looks a bit like a pallet jack.)
I built the slabs away from the base so that I would have a level surface. Tuesday my son in law helped me move them over to the base, with the assistance of an appliance dolly. No way the two of us will be lifting them up on the base, even though I ended up building 4-high. Thanks for the advice.
No Jack’s needed… if you send me an email with your contact information, I’ll tell you how to get the slabs on the base with just two people and minimal effort.
Keywords: Vertically Positioned Slab and Pivot Point 
I purchased and used a 1-Ton shop crane from Harbor Freight. It was relatively inexpensive and I got some deep discounts on it. Even with the crane, I had to do some tricky maneuvering and managed get all 3 slabs in place over a leisurely 3 days. If I had to do it over, I would invest the time and effort to build bracing around the base to pour a single slab in-place. I wouldn’t mind the time and complexity of bracing in exchange for un-injured backs.
It took only a few minutes with 4 people lifting. But if it was another level higher, I would have needed bigger people!
Interesting to see the different responses on how easy/difficult this was.
For me, this was, by far, the most difficult aspect of the oven. There were four of us. We, per instructions, put the middle one on first. However, we had no space behind and lifting the back piece over the middle one was a struggle, to say the least. Especially as the mortar hadn’t hardened. We had to place it dry, them lift it in the air a bit and put the mortar on before lowering it again. Thankfully no one got hurt, but we definitely were concerned.
No of us were strong construction workers. I guess the physical shape of the lifters makes a difference.
I used 3 pieces of pvc pipe to roll the slabs to the base, ( think ancient Egyptians building pyramids) then lifted far end to stand up against base the n slid it up base to pivot point and pushed into place. Do back , middle then front slab. I’m 6’ 200 lbs and 57 yrs old.
Sorry I missed your note, Kevin!
I very cleverly put my two sons-in-law in charge of getting the slabs up on the base. One is brains, and the other is brawn. Good combo.
They decided independently that they wanted the materials hoist after all. So, I mixed and placed mortar, then the four of us (including my spouse) walked each slab over to the materials hoist, then let the hoist do the work. The toughest part was keeping them from the tendency to scrape the slab along the wall—and thus scrape off the mortar.
In retrospect, I think this is (mentally) the toughest part of the build. I’ve done a lot of construction and reno work over the past 15 years in our home, and learned how to jockey heavy materials and panels by myself. But typically, most of us don’t lift heavy slabs of concrete, ever. Now that I’m over that hurdle, I think I’d be able to do it again. 
Dear all,
We just put the slabs by hand. I was thinking to follow Kevin’s proposal to pivot around longer axis (raise one end, put two concrete bags under it, pivot around this area to raise the other end, put two more bags there etc. - you effectively half the weight of the slab this way); but at the end four adults could really easily raise the slab and put it on the table. Here is our first try to see how difficult it is:
As you can see, we managed to lift it without much strain. We also felt that sliding the slab horizontally would remove the mortar from the support walls. At the end we had only three folks raising the rest of the slabs, but it was on the boundary between difficult and very difficult. Four people can do it fairly quickly.
Hope this helps!
Congratulations. But do keep in mind that what may be easy for four fit and relatively young persons may not be as easy or safe for others. I’m glad that the four of you were able to place your slabs.