Calculating Firebox Clearances in a Corner Masonry Fireplace

Figuring out firebox clearances where a masonry fireplace sits at 45 degree angle in the corner can be difficult and access is rarely available. The big question is whether or not there is even space for clearances to have been met in the first place. Knowing this can be the difference between guessing or recommending a Level III inspection with some indication of what you should find.

We built this calculator to help our techs address and find these issues and have now built it into our app for you to benefit from. In the industry today this is the first and currently only calculator built to calculate the required dimensions.

Open up the v3.51 form inside the app, scroll down, and select “OPTIONAL CALCULATORS AND WORKFLOWS”. This will be located just below the sections for appliances and photo documentation.

Click “Select Items”.

Then select “Masonry Fireplace in 45-degree Corner – Clearance Calculator” and click “Done” at the top right of the page.

You will now be back on the prior screen. Find the section for the 45-degree clearance calculator and select “Add Item”.

Now scroll down and select what type of firebox is present:

  • a masonry firebox with a 2” firebrick lining
  • a steel form firebox
  • a masonry firebox without a firebrick lining or where the firebrick liner is thinner than 2”

Your selection will alter the information in the boxes below that specify minimum material thickness and air space required.

Now input the required variables:

  • (c) wall-to-wall structure width is a measurement of the distance from one corner of where the angled structure or veneer meets the adjacent wall of the structure, across the fireplace opening, and over to the other corner of the veneer or fireplace facing.
  • (rw) Firebox Width at Rear is the distance from one back corner of the fireplace to the other.
  • (d) Fireplace opening depth to outside facing materials is a measurement from the back of the firebox forward to the outside or front edge of the facing material. This edge will be the surface closest to the edge of the hearth extension. It generally extends beyond the firebox or firebrick and even with or past any lentils present supporting veneer materials.

Following your input of the required variables the form will calculate the potential maximum space available between the rear corner of the fireplace and the adjacent wall. If there is not enough space present to meet the minimum material thickness and air space requirements based on your prior selection it will show as a negative (-) number. If there is potentially more space than required you will see a positive (+) number.

If you have a positive (+) number this does not indicate that clearances are met, only that enough space is present they could have been met. If the masonry thickness varies or is thicker than the minimum required, or if additional materials were used to seal any air space that was present, clearances may still not be met. Again this only verifies whether it is possible for there be enough space. Only visual inspection can positively verify that ample space is present between the masonry structure and surrounding combustibles.

Using this information in h our reports at the moment is not automatic. You will need to screenshot the data used and add a photo section to your report to add this information. This will give you the freedom to insert it where you would like in your photo documentation section.

A diagram of what is being measured is shown below.

Please let us know if you have any questions as you use the calculator and if you don’t have the app get your device logins setup today so that you have access to it.

Last updated 5/2/2023

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