# Fire pit build over existing concrete deck - am I headed for trouble?



## mitchcos

Hi to all, looking forward to learning some great things! I am on the tail end of a patio/ swimming pool remodel in Nor Cal, and am starting to build a boulder firepit; followed by BBQ island/ pizza oven.

I am planning to build the fire pit on top of my new concrete deck and thought I should get some guidance as I'm concerned about heat damaging the deck and I prefer to avoid that. I was planning on using one course of fire bricks laid flat on top of refractory mortar on the bottom; surrounded by approx. 1' tall granite boulders. Am I headed for trouble or should this be adequate to insulate from the embers, which will be from wood. I did stub up a 3/4" gas riser for using a fireplace style log lighter, but could eventually use a 3/8 circular manifold set in lava rock if I tire of cleaning the embers out. In attacehd pic, see existing kettle, which is right over gas stub and drain I put in before concrete pour.

Thanx in advance,

Mitch


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## jomama45

First of all, do you get frost in your region, I assume so.

Second, you would definately want the firebrick at the base & the sides also.

Third, direct fire & granite DO NOT go well together. Granite is one of the worst materials to use here.


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## Wildie

In a similar situation I found that the heat causes the concrete to spawl when its damp!
The concrete absorbs moisture, that expands from the heat and damages the concrete!
If you must have a fire pit over concrete, I would suggest that it would made on a metal pan that has air separation from the deck! Firebrick on the pan would be a good idea!


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## mitchcos

Wow, I thought I was on the right track, I guess I'm glad I asked...

Yes, Sacramento gets frost, but I don't believe it is anything near the severity of other parts of the country. I believe the coldest I've experienced here in the past 18 yrs was 19deg. F once. and dont think we suffer frost that affects the ground very much, merely enough to affect the agriculture indusry which is a large part of our economy.

I had no idea that Granite is a poor choice to combine with open flame and will look into that. I was originally planning to finish the pit like I have done to the spa, using block; faced with flat cobbles dug up from the yard and capped with brick, but was afraid I would have too much since I've also done a large planter near the back of the house and plan to do the same on the BBQ island that comes next on my schedule. 

Thank you, 

Mitch


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## Gary in WA

Other than sooty embers all over the concrete, and certain rocks crack and explode, and fir trees burn even when they look green, I can't think of any. Be safe, GBAR


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## jomama45

Mitch, without any serious frost, I think you should be fine doing it. I thought you wanted to use only granite, but if your willing to lay block (with firebrick liner) it should last. BTW, what kind of pizza oven are you planning on putting in, a Renalto or something similar with precast?


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## mitchcos

Jo- Thank you for the input, I'm now re-thinking the thing after the replies received. I was planning to line only the bottom initially, and not the inner sides w/ firebrick but may do so now. I will lay out the boulders I picked to see how nicely they fit together before continuing. Are you suggesting using a course of block, then top with fire bricks for the bottom? 
I have the option of using a ring I was given a year ago by someone who built their own pit and were really disapointed by the lack of heat output. I may build this with fire brick lining on both the bottom and sides using that burner and switch back to wood if not satisfied. I ran 150' of 1-1/2 gas pipe, supplying the new spa heater and another stub for a new BBQ, leaving the pit supply (3/4" reducing riser/ sweep) at the end of the line. I was planning to burn wood when calcs were done to spec the gas line size, so I hope there is enuf cfm to make it work well. I have used the existing kettle seen in the pic's for the last year and do pay attention to winds as the two trees nearby (Sequoia's GBAR, not Fir - both actually 10+ ft. away) are a consideration. I'll likely place the bottom fire bricks on edge to give just that much more insulation between the deck and the fire and leave plenty of air inlets to help it draw.

RE: Pizza oven, I have been spending some time on the Forno Bravo site http://www.fornobravo.com/index.html#pizza_ovens and forum and was hoping to build one from scratch as I've been downsized by two companies here in the this robust (YUK!) California economy and need to be economical. I'll look into Renalto as well.
I have to keep the oven small enuf to avoid one that is too heavy because I wasn't sure how I was going to lay out the kitchen and didn't put down any footings under the deck, just standard 2x4 forms were used...

Hope this helps,and again, I appreciate everyones thoughts and welcome the input.
Mitch


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## Tscarborough

"heat causes the concrete to spawl when its damp!
The concrete absorbs moisture, that expands from the heat and damages the concrete!"

The actual issue is that the aggregate in concrete has a different coefficient of thermal expansion than the cementious matrix, and when direct heat is applied to it, the difference is enough to cause explosive spalling. 

Normal firebrick, 2-1/4" thickness, provide adequate protection for a firepit when laid as a paver (4"x9-1/2" face exposed) over concrete. Use a good refactory cement with no mortar joints, and be sure to allow for water to drain from the pit in some way. Most natural stones do not play well with direct flame contact, and THAT involves the moisture content of the stone.

Looks nice, and if it was me I would just do the gas thing for the simple reason that there is no clean up and when it is on, it is on.


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## mitchcos

What is the reason you suggest no mortar joints, and would that be your suggestion for both side bricks as well as the bottom? So -mortar btwn the fire bricks and deck; none in the side jonts and since I really don't understand your reason, what about the sides? Fire brick on end around the circumference, inside the boulders, mortar between boulders and bricks...sides? 

I did put down a pvc drain pipe in the deck (center) thinking I could slope the mortar on the bottom towards the center and that I might be able to come up with something later on that could line the pipe in metal to keep it from melting, bo no idea what... Stay with me here, I'm trying to understand/ learn, but it's not intuitive yet!

Mitch


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## Tscarborough

Normally, a mortar joint is from 1/4" to 3/8" in width. When laying firebrick, the joint should be as small as possible and no more than 1/8", especially outdoors.

Any area that may be exposed to direct flame should be covered with firebrick.


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## jomama45

Mitch, I didn't mean you had to lay block under the firebrick, but you could if you want it high. Generally speaking, the bigger diameter & lower the walls you build for the firepit, the less chance to damage the structure with heat. I agree with TS that the firebrick can be laid in mort dirctly on concrete patio. Why dont you think about using the gas line for a log lighter for burning wood? Either way, it sounds like you should have plenty of gas to run a ring also.

I took a look at the pizza oven link, & I did something similar to the modulars a few years ago. You may want to look into Rumford, check out their website, as they are probably much simplier. Either way though, I don't think I would build a pizza oven on your patio, as they get heavy if you use all masonry for the structure.


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## Sandywjo

Tscarborough said:


> "heat causes the concrete to spawl when its damp!
> The concrete absorbs moisture, that expands from the heat and damages the concrete!"
> 
> The actual issue is that the aggregate in concrete has a different coefficient of thermal expansion than the cementious matrix, and when direct heat is applied to it, the difference is enough to cause explosive spalling.
> 
> Normal firebrick, 2-1/4" thickness, provide adequate protection for a firepit when laid as a paver (4"x9-1/2" face exposed) over concrete. Use a good refractory cement with no mortar joints, and be sure to allow for water to drain from the pit in some way. Most natural stones do not play well with direct flame contact, and THAT involves the moisture content of the stone.
> 
> Looks nice, and if it was me I would just do the gas thing for the simple reason that there is no clean up and when it is on, it is on.


I just spoke to Rutland company that sells both firebricks and castable refractory cement. They advised to just buy the Castable refractory cement and put about 1.5 to 2 inch layer on the bottom of firepit (on top of the cement.) That's it. But you said it needs to be able to drain water? Well then their idea would not work I guess. I would rather do the bricks that seem cheaper but don't really understand how this method works. You mention the bricks and using refractory cement but no morter. You can't just lay the bricks down flat? Sorry, I am not normally familiar with this and I am trying to help my son find a way to protect his concrete patio from the firepit that was built on it. (sides and structure itself all fireproof, but was built right on the cement). Could you explain the exact method for putting these bricks down as a base? Thank you.


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## John Smith_inFL

this thread is from 2009 - I think they have figured it out by now.


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## 1scottkohlberg

Sandywjo said:


> I just spoke to Rutland company that sells both firebricks and castable refractory cement. They advised to just buy the Castable refractory cement and put about 1.5 to 2 inch layer on the bottom of firepit (on top of the cement.) That's it. But you said it needs to be able to drain water? Well then their idea would not work I guess. I would rather do the bricks that seem cheaper but don't really understand how this method works. You mention the bricks and using refractory cement but no morter. You can't just lay the bricks down flat? Sorry, I am not normally familiar with this and I am trying to help my son find a way to protect his concrete patio from the firepit that was built on it. (sides and structure itself all fireproof, but was built right on the cement). Could you explain the exact method for putting these bricks down as a base? Thank you.


Did you go ahead and do the fire pit on the concrete?


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## ront02769

I spent a couple of years as a bake master for a clam bake place (maybe just an east coast thing), anywhere from a couple of dozen to 1,400 guests depending. Wverything was done on concrete slabs, bigger for big bakes, smaller for the little guys. So the the big ones our starting point would be a 14x14 area 4’ stack of logs interspaced with 30 lb steel ingots, light ‘‘em up and go from there. At the end the concrete was not pristine, but then it had been there for like 20 years so what do you expect.


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