chimney height / clearance above peak - more of a draft or safety issue?

[bag of hammers]

I believe the specs for chimney are 3' above peak or 2' above the point where 10' horizontal line from the roof meets chimney. Is this requirement more from a safety perspective (cap / gasses too close to roof) or draft perspective? And does the chimney height measurement include to the top of the cap? or top of the pipe?

 

[LLigetfa]

Tastes great or less filling? The debate rages on.

Actually it's 3 feet above the roof surface at the chimney. That could put it 2 feet above the peak.

Why are you splitting hairs trying to include the cap in the measurement? Better a foot higher than a foot shorter if the difference doesn't require additional bracing.

There is a pressure plane between the roof and the top of the flue that you want to stay out of. Since roofs vary in pitch and some are complex with hips and valleys, there is not a simple rule to follow, only the minimum code.

 

[billb3]

safety and draft

if the cap blows away - you suddenly would be non-compliant if you measured to the top of the cap.

I've seen it measured both ways, may be subject to inspector's interpretation and that's a minimum.

 

[bag of hammers]

Tastes great or less filling? The debate rages on.

Actually it's 3 feet above the roof surface at the chimney. That could put it 2 feet above the peak.

Why are you splitting hairs trying to include the cap in the measurement? Better a foot higher than a foot shorter if the difference doesn't require additional bracing.

There is a pressure plane between the roof and the top of the flue that you want to stay out of. Since roofs vary in pitch and some are complex with hips and valleys, there is not a simple rule to follow, only the minimum code.

Reading about how people are adding to their chimney to improve draft. So I was just kinda curious if the minimum height was just to help avoid draft isssues. I assume this pressure plane causes havoc with the draft...?