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Oven support wall cracking - Forno Bravo Forum: The Wood-Fired Oven Community



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Oven support wall cracking

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  • Oven support wall cracking

    My concrete block oven support wall cracked on both sides all the way down to the bottom. The cracks are hairline cracks. Has anyone else experienced this with their support wall?

  • #2
    Re: Oven support wall cracking

    Is the oven on it already?
    Which wall sides or back?
    Was there a footing or just a slab?
    Is there a crack on the slab? (wet down the slab and watch the water it will find even the smallest crack, you will see bubbles.

    A crack through block indicates serious movement.

    If there are no other visible cracks clean a 1' square section of the crack with soap and water rinse well after dry paint the area with white paint or a light paint. Put register marks (black lines) on the paint on either side of the crack parallel to the crack and a couple running right across the crack horizontally. measure the parallel lines and note the measurement (exactly) the horizontal lines can me measured after they have moved. this will tell you what direction it is moving if you cannot figure it out visually.

    At first guess Id say the slab on grade is cracked also, look closely
    Check it and get back...don't panic it can be fixed.


    • #3
      Re: Oven support wall cracking

      Slab movement was my guess. Several possible causes - base not compacted before the slab pour, lack of reinforcement within the slab (rebar or mesh), or slab is too thin.

      I will concede judgement to Uno - he is probably one of the forums best authorities on masonry.

      I'm very curious to the cause and solution if we are only talking about the block on slab.....without the hearth slab and dome. Just the block by itself really is not that much weight when considering the overall project.


      • #4
        Re: Oven support wall cracking

        The oven is already built. These cracks accured after the oven was used. I believe the problem is that the mason did not stagger the concrete blocks. The cracks fun all the way down the mortared seem. I believe the oven floor support moved when the oven got hot and caused the hairline cracks in the mortared seem. The oven floor itself did not crack but I can tell it sure does exspand when hot. Does this make sense? I also found a hairline crack in the front of my oven floor support. This is the part above my wood storage opening.


        • #5
          Re: Oven support wall cracking

          Why in the world did he not stager the joints?
          Are the block fully grouted (all cells)?
          Is there steel? where?
          How thick is the hearth slab?
          How wide is the span?
          How big was the steel to carry the block over the opening?
          You say oven floor did not crack is this a FB oven?
          A crack in the hearth (support slab) is/could be a big concern.

          CAUTION: Comments form a full blooded Italian to follow
          If this guy laid the block with the joints lined up he was not a mason and if he did this there is probably more that was not done right. The hearth slab (support slab) for this oven design even with the hottest longest fire should not rise to a heat level that would create cracks such as these if built correctly so it may not be the heat. If he was your friend I'm sorry but I call it as I see it, don't take it personally.
          On a positive note hairline cracks are not necessiarily an indication of structural issues. There are a lot of variables here so get back to me.
          Pictures would help a lot if you have em..


          • #6
            Re: Oven support wall cracking

            Is there any rebar running up through the block? I've seen (and laughed hysterically at) something done that way - it was reinforced with rebar and concrete was used to fill the holes (not unlike some mortarless systems - although those usually stagger as well).

            "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

            "Success isn't permanent and failure isn't fatal." -Mike Ditka


            • #7
              Re: Oven support wall cracking

              The foundation is about 5' by 5' and 4 inches thick with 3/8" rebar. The blocks are mortared and 3/8" rebar was used from the foundation up through the block and bent to tie in the support floor. This was done on every corner and in the middle of each wall. He also added rebar all around and across. Two 3/8'' rebar was also used from the slab up the block on each side of my wood storage opening under the oven support floor this rebar was bent to help support the opening, its about two feet wide. The oven support floor runs across this opening. First he layed about a 3" slab and then a 3" vermiculite plastic cement mixture over the concret slab. This is what the oven sits on. The person who built this structure is recommended by FORNO BRAVO. His name is JIM HEMSTALK. He is on your web site as a recommended mason in Northern California. I got the oven from my cousin who had them brought over from Italy The company is Trinchini Caminetti. I'm not sure if I spelled that correctly. The cracks are very small and you cant even see them unless you look closely. I'm going to assume this is a small problem. It makes me feel better.


              • #8
                Re: Oven support wall cracking

                Originally posted by nyknicks View Post
                I'm going to assume this is a small problem. It makes me feel better.
                You should have said that sooner.... In that case I will not spend anymore time figuring out whats going on.

                Good luck..


                • #9
                  Re: Oven support wall cracking

                  I would still like your opinion on what you think. Does the constrution of the oven sound like it's going to hold up?


                  • #10
                    Re: Oven support wall cracking

                    Sounds like a small problem to me. Can you post some pictures?
                    My Oven Thread:


                    • #11
                      Re: Oven support wall cracking

                      Get comfy this is a long one...OK something is not right 5' square does not work out for standard block. With the slab on grade at 5' sq in order for full block to work out (for the sides) they would be 47 3/4" thats 4 full 16" block with head joints.
                      That means with the 2 side walls on the very edge of the slab the front and back walls would need to be the same dimensions 47 3/4 to fit inside of the 2 side walls without any cuts. So I don't know if you just gave me an approx measurement or what.

                      But here is the bottom line what ever the dimensions are. Having the blocks stacked in line with each other is like creating a series of individual walls the width and length of the blocks. So on the sides you have 4 or 5 16"X (6" or 8") walls butted up against each other, each independent of the other.

                      The fact that he put 3/8 rebar in the corners and the middle means that these "mini walls" have a piece of rebar in them nothing more, in this case of blocks being stacked the rebar may as well not be in there at all. 3/8 rebar is a small step above wire mesh and in my opinion if you gonna use it you might as well use the wire and save the money on materials, we never use less than 1/2" even for patios.
                      These walls are not tied into each other except where the slab is on top, this is like taking a playing card placing it face down and putting 4 other cards around it with their sides touching 2 on the long side 2 on the short side if you put tape where he put rebar and fold the cards under to make a base for the top card...its still very flimsy. Now take those cards that formed the base and cut them into 4 sections...not good.. this is what you have for a base. (based on your description)

                      If you were to bump this structure on the hearth slab say with a forklift (or an earthquake) the entire base would collapse like a deck of cards. So essentially what you have is a heath and oven structure resting a series of individual walls all the way around.

                      Note: If the blocks were bond beam and stacked this would be a different story because the concrete could flow through the entire wall.

                      If you were to stack them as mentioned it could still be made sound by putting vertical rebar (1/2") that was bent into the footing 18" and up through the block on either side of the butt joint so where each block meets you would have a piece of rebar on either side of the joint. Then if the top row of block was bond beam with horizontal steel all the way around the perimeter and all of this was grouted you could get away with it...but its much more work.

                      So here is my summary:
                      You have a 4" slab on grade that is minimal at best, with a perimeter footing, more than likely OK and should be just fine. without
                      You have a 3" hearth slab with 3/8 rebar which again might as well be wire making a 4' span with close to a ton of oven resting smack in the middle (coincidentally right where the crack is)
                      You have an EXTREMELY UNSTABLE base which has NO sheer strength.
                      My advice as a professional--keep pets and loved ones away form this oven in the event of an earthquake. In the mean time paint the cracks as I instructed above and keep an eye on them especially before and after each use. Id crawl under the oven asap and look at the under side of the support hearth to see if that hairline crack goes down the middle. I know this sounds alarming but this was build using minimal to inadequate materials and methods. My comments on this subject are based on years of masonry and concrete experience. But James graciously provides plans for free on this site that I believe are very good general recommendations for building the oven and base and just about everything you mention is contradictory to them.


                      • #12
                        Re: Oven support wall cracking

                        Uno, I'm curious....if all of the above turns out to be true; what can be done (if anything) to stabilize this oven? Or is the poor guy looking at a compete knockdown/rebuild?


                        • #13
                          Re: Oven support wall cracking

                          Thanks for the infomation. Is there anyway to make this structure more secure? I trusted the person who did the work because Fornobravo has him listed as a recommended installer. Can you tear down the structure and salvage the oven? How about building an inner wall? I left the 2x4 wood that was used to hold the backer board under the support floor. The cracks are no longer visible since the oven has cooled unless you knew where to look and got up real close.



                          • #14
                            Re: Oven support wall cracking

                            There is a number of things you could do all of which aren't all that bad.

                            1. To beef up the slab perimeter (if you don't have the footing I mentioned) you can dig around the perimeter about 6" down. Clear out about 4-6" under the slab, put in some rebar, 2 sticks of 1/2" and pour a footing you can do this in stages or all at once, the oven isn't going to move even if you excavate all the way around at one time.

                            2. The walls can be deterred from kicking out by pouring a curb all the way around the base. Id go 4" high by 4" wide on the outside perimeter. If you have room on the existing slab for the 4" drill holes in the slab 2" out form the block in line with the middle of each block down about 2" insert a short length of 1/2" rebar so it comes to the middle of the 4" curb so 2" up from the slab. Then with a 10' length of rebar start at the front side of the base and bend it to go around the corner if the slab is close to 5' sq like you said you will need to over lap the 2 ends by at least 24", the entire length if you have it left over. This horizontal steel should be tied to the vertical stubs. If you do not have room on the slab you need to make it part of the new footing and it should be 6" wide by 4" high. If you have the footing and do not have the space on the slab you can do this on the inside of the walls with some modifications.

                            3. To add sheer strength to the base you could drill into the middle of each block on the left and right (inside) walls in the back about 2" from the back wall. Do this about half way up and along the floor like above the same distance. the bar should go about half way though the wall and only 2" into the floor (slab is only 4" thick right?) Get some bond beam blocks 6" is fine and dry stack them (stager the joints) on the floor up against the back wall in line with the steel each piece of steel should enter a row of block. Then grout (fill this with concrete) before grouting you should dampen the block and add a little extra cement to the mixture to make up for having to mix it soupy to pour in the block...its gonna be tight in there. An alternative which would be quicker but probably more expensive would be to buy 2 lengths of angle iron at least 4" wide the entire height of the inside wall (you could probably find these in the scrap section of your local iron yard) have them drill holes that would correspond to the centers of the block drill matching holes in the block epoxy in some 1/2 "- 5/8" all thread and this would join the 2 separate walls.>>>>strike that I forgot the back wall was stacked the angle iron will do no good....sorry

                            4. Adding support in the center of the oven is the easiest of them all. Measure the height get the bond beam blocks dry stack one course fill it repeat till you get to the last course since you cannot fill the last block it will have to be set in dry or (you can use liquid nails) then you will need to cut some wooden wedges and drive them in from both sides. Or you can make shoring out of wood. Id say a 2x6" ought to do it. Cut it to double it up against the hearth slab. Cut another piece for the floor, cut the remaining the distance from the top pieces down (actually an 1/8" longer) pound them into place. One on each side and evenly space across.

                            Now with the exception of the underbracing these suggestions are based on the assumption that the walls for the base are as I described before. If the blocks around the corners are all tied in there is no need for the sheer bracing of the back wall. and the side curb would not need to run all the way around the back just the sides.
                            Check this out and get back I'm getting tired its been pretty hot the last few days and it really takes a lot out of me working in it. When its not hot heck I can go all day..well almost but when its like this I'm outta there at 2:00 and wasted the rest of the night...
                            Last edited by Unofornaio; 07-24-2007, 11:39 PM. Reason: sleepy..


                            • #15
                              Re: Oven support wall cracking

                              Thanks for the information. I was wondering if you could use steel corner brackets and long steel plates to support the wall and use liquid nails to hold it on? Go ahead and laugh. Ok are you ready for some good news? The foundation has no cracks. The bad news is that half the oven foundation was placed over an existing patio and was tied in with rebar. So I can only put footings along the parts that are not on the existing patio. I already have a 4x4 across the center of the support floor and was wondering if that wood exspanded and is what cracked the mortar line? The cracks both start at the same spots the wood is at. The support floor is pretty warm the next day. Anyway thanks again for your help and let me know what you thing of the Steel to help with the wall.