Hearth Extensions for wood burning fireplaces

[msteger]

For fireplace hearth extensions, I have been using the rule of fireboxes level with the floor must have at least an 13" hearth extension to a non-combustible material and those above floor level must be at least 18". I saw that documented 5 or 6 years ago.

I was looking through the Code Check Complete book and found a totally different rule for hearth extensions. On page 47, it says 16" minimum hearth extension if firebox opening is less than 6 sq ft.

20" minimum hearth extension if firebox opening is greater than 6 sq ft.

The difference being the rule I have been using deals with height of the firebox to determine depth of hearth extension whereas the Code Check documentation has it determined by sq ft opening of the firebox.

I can't seem to easily find the documentation right now for the first method, but only what's in Code Check Complete. What do you guys use for this rule regarding hearth extensions for wood burning fireplaces?

Thanks.

[Brandon Chew]

I use 2006 IRC R1001.10:

§R1001.10 Hearth extension dimensions. Hearth extensions shall extend at least 16 inches (406 mm) in front of and at least 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m2) or larger, the hearth extension shall extend at least 20 inches (508 mm) in front of and at least 12 inches (305 mm) beyond each side of the fireplace opening.

(note: the same requirement is in the current New York residential code and prior versions of the IRC, but with different section numbering -- R1003.10)

[Scottpat]

Originally posted by Brandon Chew

I use 2006 IRC R1001.10:

§R1001.10 Hearth extension dimensions. Hearth extensions shall extend at least 16 inches (406 mm) in front of and at least 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m2) or larger, the hearth extension shall extend at least 20 inches (508 mm) in front of and at least 12 inches (305 mm) beyond each side of the fireplace opening.

(note: the same requirement is in the current New York residential

code and prior versions of the IRC, but with different section numbering -- R1003.10)

I use the same. Have not heard of the 13"/18" rule.

[Bill Kibbel]

I'd suggest tossing Code Check and get the NFPA 211 to review for chimney, stove and fireplace inspections.

[Jim Baird]

IRC'06 calls for 16"-18" for <6 SF->6SF.

[Neal Lewis]

Originally posted by Jim Baird

IRC'06 calls for 16"-18" for <6 SF->6SF.

Jim, Brandon quoted the exact code already; eighteen inches is not correct.

Quote: §R1001.10 Hearth extension dimensions. Hearth extensions shall extend at least 16 inches (406 mm) in front of and at least 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m2) or larger, the hearth extension shall extend at least 20 inches (508 mm) in front of and at least 12 inches (305 mm) beyond each side of the fireplace opening.

[Jim Baird]

Cool.

[Jim Katen]

Originally posted by msteger

For fireplace hearth extensions, I have been using the rule of fireboxes level with the floor must have at least an 13" hearth extension to a non-combustible material and those above floor level must be at least 18". I saw that documented 5 or 6 years ago.

I was looking through the Code Check Complete book and found a totally different rule for hearth extensions. On page 47, it says 16" minimum hearth extension if firebox opening is less than 6 sq ft.

20" minimum hearth extension if firebox opening is greater than 6 sq ft.

The difference being the rule I have been using deals with height of the firebox to determine depth of hearth extension whereas the Code Check documentation has it determined by sq ft opening of the firebox.

I can't seem to easily find the documentation right now for the first method, but only what's in Code Check Complete. What do you guys use for this rule regarding hearth extensions for wood burning fireplaces?

Thanks.

I've never heard the first rule you mention. Perhaps you got it from the Carson Dunlop materials. They have lots of strange stuff that seems to be based on Canadian codes.

- Jim Katen, Oregon

[Darren]

Everyone seems to understand section 1001.10 about the extensions; how many actually call out section 1001.9?

'No combustible materials shall remain against the underside of hearths and hearth extensions after construction.'

I find wood on the underside of about 90% of ALL hearth extensions.

[msteger]

Thanks Guys. Not sure either where I learned the 13" and 18" rule, but I'll stick to the one in the IRC. I also find plywood or OSB under the hearth extension most of the time.

[Neal Lewis]

Darren, recently I found a plywood form still in place directly under the FIREBOX FLOOR! Good luck getting it out of there.

[Darren]

Neal,

That's why I open all exterior clean-out doors and stick my mirror inside; I have found several that way too. As a matter of fact, I believe I posted a picture here several years ago showing that.