Firebox Sizes

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To make our fireplace surrounds work with your existing conditions, you need to take a few measurements. Following is a guideline for measuring your firebox. First, you need to figure out which firebox diagram most closely matches your conditions. Next, step through each measurement using the table on the next page as a guide.

Masonry or Rumford boxes make it easy to anchor cast stone surrounds and have the same considerations as a zero clearance box except the glass doors and vents.

Zero Clearance boxes have the same considerations of a rumford masonry box but a few others to add to the list. These are some of the dimensions and limitations that should be filled out and addressed when looking for a surround to use with a zero clearance box.

 

Below are notes on the measurements that are called out in the drawings listed above.

  1. Opening Width: This measurement tells you the maximum width your surround must be. There are three basic methods for fitting a surround to your firebox opening: One, order a surround which is larger in size than your firebox opening and cover the exposed surfaces between the surround and opening with a slip; Two, order a surround larger than your opening and leave the existing material between the surround and the opening; Three, order a surround that is close in size to your opening, and cut it to fit the opening.
  2. Opening Height: This measurement tells you the maximum height your surround must be. The same issues apply to this as above.
  3. Overall Width: This measurement is primarily important when the firebox protrudes into the room (i.e., measurement L). In that case, the surround may need extra structural support for any parts of the surround that stick out past the face of your firebox. Consult your mason or designer for more details.
  4. Overall Height: As with the overall width, this measurement is primarily important when the firebox protrudes into the room (see Overall Width).
  5. Leg Width: The leg portion of the surround usually covers the existing masonry. If the surround leg is narrower than the masonry, a slip can be used to fill in the gap. Note that on the zero-clearance box, the glass doors usually require at least a half inch of the metal showing to allow for retraction of the glass panels.
  6. Header Width: On zero-clearance boxes, often most of the metal around the firebox opening is covered by slips.
  7. Opening Lip: On a masonry firebox, this measurement is usually less than the standard flat hearth thickness. However, the hearth mortar-setting bed may need to be increased so it sits above or flush with the opening.
  8. Clear Space: make sure to allow room for any gas valves or fixtures; if they fall in the range of a veneer or slip they may easily be field cut but a leg or molding would not be an easy adjustment. Take into consideration any bookshelves, windows or finish elements that may encroach on the surround.
  9. Hearth Thickness: A firebox with a full hearth will require a raised hearth (see page 2E.1).
  10. Hearth Depth: A raised hearth
  11. Hearth Width
  12. Firebox Thickness: If the masonry box protrudes from the wall it may require extra pieces to wrap back to the wall. See the custom-work section (page X.4) for more details.
  13. Top of Vent: Working vents must not be covered by a slip as that will impede the function of the fireplace and could result in a fire hazard.