Fireplace Clearance Calculator

Mantel height, hearth extension and trim clearances, with the code behind each number

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Mantel clearance required12" above the opening (IRC R1001.11)
Your mantelPasses at 12"
Stricter chart rule (12" + 1" per inch)20"
Hearth extension20" in front, 12" past each side (60" wide), opening 7.5 sq ft
Mantel from floorBottom 42", top 45"
Typical mantel top for this ceiling54" to 60" from the floor
Shelf depth that suits the opening8" to 10"

Turn these numbers into a build plan.

  • 36" x 30" opening
  • 12.0" required clearance
  • 20" front, 12" sides hearth extension
Materials List
Recommended materials and quantities
Hearth extension (noncombustible)60" x 20"
Mantel shelf48" long, 8" deep
36" x 30"Mantel 12" above, needs 12"Hearth extension 20" deep, 60" wide
Red: nothing combustible within 6". Amber: 6" to 12", projection limited to 1/8" per inch.

Tiling the hearth and surround? Get the count from the tile calculator or, for brick and stone, the masonry estimator. Trim around the surround is in the trim calculator.

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Project plan · built from your numbers

Hearth and mantel for a 36" fireplace

A 8" deep mantel needs at least 12.0" above the 36" x 30" opening; yours is at 12", which passes. The plan adds a 60" x 20" noncombustible hearth extension.

Materials
$385
Time
1 weekend
Level
Intermediate
8 steps 8 tools 8 materials
  1. 1

    Identify the unit and who does what

    Find the fireplace or insert label: factory-built and insert units set their own clearances, and their manual overrides this calculator. If it is a gas fireplace, a licensed gas fitter must disconnect and reconnect the gas line and test for leaks. Do not touch gas piping yourself.

    • Read the manufacturer label and manual
    • Gas units: book a licensed gas fitter
    • Check whether your area needs a permit
  2. 2

    Confirm the clearances

    Code calls for 12.0" between the top of the opening and a 8" deep combustible mantel, and a hearth extension 20" in front and 12" past each side. Your current mantel position already passes.

    • Mark 12.0" above the opening
    • Mark the 60" x 20" hearth footprint
    • Side trim clearances pass
  3. 3

    Remove old trim or mantel

    Pry off the existing mantel and any combustible trim inside the clearance zone. Protect the floor and keep the firebox closed so debris stays out.

    • Old mantel off
    • Combustible trim cleared from the zone
    • Floor protected
01 · Your numbers
Opening
36" x 30"
Required clearance
12.0"
Hearth extension
20" front, 12" sides
Est. cost
$385
02 · The plan

8 steps · 8 tools · 8 materials

5 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

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Hearth and mantel for a 36" fireplace

Finished in 1 weekend · $385 DIY

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Fireplace clearance rules at a glance

WhatRuleCode
Hearth extension, opening under 6 sq ft16 in front, 8 in each sideIRC R1001.10
Hearth extension, opening 6 sq ft or more20 in front, 12 in each sideIRC R1001.10
Mantel and trim within 6 in of the openingNothing combustibleIRC R1001.11
Mantel and trim 6 to 12 in from the openingProjection up to 1/8 in per inch of distanceIRC R1001.11
Mantel and trim more than 12 in awayNo projection limitIRC R1001.11
Wood framing2 in from front and sides, 4 in from the backIRC R1001.12
Factory-built, gas, electricManufacturer's listingIRC R1004, R1005

Mantel clearance chart

Minimum distance from the top of the opening to a combustible mantel
Mantel projectionIRC R1001.11Chart rule (12 in + 1 in per inch)
1/2"6"12-1/2"
3/4"6"12-3/4"
1"8"13"
1-1/4"10"13-1/4"
1-1/2"12"13-1/2"
2"12"14"
3"12"15"
4"12"16"
5"12"17"
6"12"18"
8"12"20"
10"12"22"
12"12"24"

Worked examples

  1. 8 in oak mantel over a 36 x 30 opening. IRC: 8 in is more than 1-1/2 in, so it needs 12 in. Chart rule: 20 in. Hearth: 7.5 sq ft, so 20 in in front and 12 in each side.
  2. 2 in trim board 8 in from the side. At 8 in, the most allowed is 8 / 8 = 1 in. 2 in is more than 1 in, so it fails: move it to 12 in or use a 1 in board.
  3. Gas insert. Find the clearance table in the manual or on the label behind the lower grille, enter it in the calculator, and use the larger of that and any local rule.

Why other charts say 20 inches

Searches for a mantel clearance chart turn up a rule of 12 in plus 1 in per inch of projection. It is a conservative convention that many manufacturers and older charts use. The IRC for masonry fireplaces stops at 12 in: beyond that distance any projection is allowed. Both are shown above so you can see which one your inspector or manual is using. Planning the surround? Count tile with the tile calculator or stone with the masonry estimator, and see the TV mounting cheat sheet before you hang a screen above it.

Fireplace clearance FAQ

How high should a mantel be above a fireplace?

Under IRC R1001.11, nothing combustible may be within 6 in of the opening, and within 12 in a mantel may project only 1/8 in for every inch of distance. A 3/4 in trim can sit at 6 in, a 1-1/2 in projection needs 12 in, and any deeper wood mantel needs 12 in. Many people then set the top 54 to 60 in from the floor for looks.

Why do other mantel clearance charts say 20 inches?

Some charts and manufacturer manuals use a stricter rule of thumb: 12 in plus 1 in for every inch of projection, so an 8 in mantel needs 20 in. That is not the IRC rule for masonry fireplaces, but it is what some factory-built and gas units require. Follow the manual for those units and the IRC for masonry.

What is the hearth extension requirement?

IRC R1001.10: at least 16 in in front of the opening and 8 in past each side, or 20 in and 12 in when the opening is 6 sq ft or larger. A 36 x 30 in opening is 7.5 sq ft, so it needs 20 and 12. The extension must be noncombustible.

How far does a mantel need to be from a gas fireplace?

Whatever the unit's listing says. Direct-vent units commonly require 6 to 12 in for a shallow mantel and more for deep ones, and some require a hood. The clearance diagram is in the installation manual and often on a label under the lower grille.

Do side trim and pilasters need clearance too?

Yes. The same rule applies to the sides: no combustible trim within 6 in of the opening, and within 12 in it may project only 1/8 in per inch of distance. A 2 in trim board at 8 in fails, because only 1 in of projection is allowed there.

Can a noncombustible mantel be closer?

Yes. Stone, tile, brick and metal are not combustible, so the projection limits do not apply. Watch for wood blocking or brackets behind a stone mantel; those count.

How deep should a fireplace mantel shelf be?

6 to 8 in for openings under 36 in, 8 to 10 in for 36 to 48 in, and 10 to 12 in for wider fireplaces. Deeper shelves need more clearance under some manufacturer rules.

Is a mantel clearance diagram required for inspection?

For new masonry fireplaces and many insert installs, inspectors check clearances on site. A printed sheet with the opening size, mantel projection and distance, and the code section speeds it up. Print the calculator result with the diagram.

What if my existing mantel is too low?

Raise it, cut it shallower, replace it with a noncombustible one, or (for listed units only) add the manufacturer's approved hood or deflector. Older homes built to earlier codes are not automatically safe.

Can I mount a TV above my fireplace?

Yes, with care. Keep the screen above the mantel, which deflects heat, and check that the viewing angle from the couch stays under about 15 degrees or use a pull-down mount. The TV mounting cheat sheet covers anchors and wiring.

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