Need Help - 5 Inch Chimney Form?


I am ready to make the brick cuts for the barrel bricks that are for the chimney (steps 21 and 22). The instructions call for a 4.5 inch cut along the length of the brick, but my chimney form is actually 5x5 inches so the L shaped brick slot will be a half inch short of the form’s edge. The diagram seems to show the L shaped brick actually going past the edge of form so I am confused. Any help would be much appreciated!

Thanks very much.

Hi Greg,

I know that looks weird and may even hurt your head a bit (it hurt mine until I actually did it!).

The foam form is larger than the actual chimney you will build later on. This is to account for mortar joints. The important dimension is in step 14, where you place the chimney form 4 ½ inches back from the form front. That will set it so it is 9 ½ inches in, measuring to the rear of the entire depth of the chimney form.

If your firebrick are not 9 inches long (and since there are no “standard” brick sizes in the U.S., that could be true) you need to go back to page 2 of the instructions and look at the diagram for how to lay out your top courses. That shows how to shorten the oven for shorter brick. But I’m guessing that you laid your brick out to the full length of the form, so in this case you need to bring the top 3 courses just a little longer to come to the chimney form.

I could be understanding your question incompletely, of course, but if I’ve got this right, this will bring the top courses to the correct position. You do want to maintain the same cuts (4 ½ inches) on the two brick that form the top front of the arch, because that will position your chimney correctly.

If any of this does not sound clear, please take a couple of photos and post them in a reply to this thread so I can see more clearly what your situation is. It would also help to know the dimensions (length, width, depth) of your firebrick.

Thanks Greg!

Hi Matt

I’ve attached a few pictures to help illustrate my question / concern. I am using standard 9" inch firebrick and yes the form will sit back 4.5 inches from the front edge.

As you can see in the pictures the L shaped brick is cut as directed but since the brick slot is 4.5" deep, it doesn’t reach the edge of the 5" form. Is this expected? Does this mean that the brick that follows will need to be notched to meet the top of L, or should I apply a half inch of mortar to get passed the form edge?

I hope this makes sense, thanks for responding so quickly I really appreciate your help.


You are absolutely on the nose here. The half inch is for a mortar joint between the L and the brick. The form itself goes into open air—it is the opening for the chimney so it does not meet anything at all.

You are right on the money here.

Hi Matt, I just wanted to confirm since the diagram seems to show the L brick’s long edge going passed the form’s edge. I was taking the diagram too literally. :slightly_smiling_face:

Many Thanks!

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