Offset Chimney

[Mike Lamb]

I believe this was asked before but I couldn't find it.

Why are chimneys in old house (70+ years) offset in the attics. To help prevent backdrafting? I see it so often.

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[kurt]

So the chimney would exit at the peak, where it can be flashed so it doesn't leak.

They'd offset the chimney base so it wouldn't run up directly through the center of the house, and take it out the peak in the attic.

Or, so I've been told.

Made sense to me at the time. Back then, folks understood about the high sides of roof penetrations leaking.

[Bill Kibbel]

There are 3 different reasons a chimney may be offset:

• To penetrate the roof at the ridge so as to avoid having any roof slope above that would need to be diverted around the chimney (cricket)
• To avoid cutting and supporting a structural member
• Some architectural styles include chimney placement as part of the design.

[Neal Lewis]

Mike, what's your guess as to the age of the house? Without a ridge, I'm thinking at least 125 years old. At least that's what I see around these parts.

[Bill Kibbel]

I've posted this before in other threads:

It's pretty accurate for the mid-Atlantic region.

[SonOfSwamp]

Here in Middle Tennessee, I've seen plenty of 10s-20s houses with no ridge board.

WJ

[Mike Lamb]

Thanks. Sounds good to me.

I'm not sure how old. According to Bill's drawing 100 years+. The 2Xs in the forefront are 1 1/2" so that was an add on, and behind the chimney a real 2".

[SWagar]

In Portland this past summer, I inspected a house built in 1927 that had no ridge board.

[Rob Amaral]

Other reason is optimal draft... stubby little chimneys exiting at the ridge are not prone to the same problems with wind...

[msteger]

Yep, offset chimneys are pretty common around here in homes I've inspected from the 1700s and 1800s. I always figured it was to help flash it at the roof line to help prevent leaks.

[belvedere]

Originally posted by SWagar

In Portland this past summer, I inspected a house built in 1927 that had no ridge board.

Our house was also built in '27 and has no ridge board.