Tough One?

FramingPro

Is the total rise to the top or bottom of the rafter? Is the unit rise to the top or bottom of the rafter?

katoman

First off I'd recommend the book - Construction Geometry, by Brian Walmsley, you can buy it at Lee Valley. An excellent book for construction geometry. He will explain everything you need to know.

To give a short answer to your question - picture a triangle from the outside of your plate to the centre of the total span, and then to the height designated ( 4/12,6/12 )

This is the geometric triangle of the roof, or "line length". So your actual rafter length will be from the outside of the top at the birdsmouth drawn in a line along the rafter at that point on the rafter to the centre of the
ridge.

Some guys bring that line at the outside plate verticle to the top of the rafter. Now the length can be measured on the top of the rafter to the centre of the ridge.

It's hard to give a visual of what's happening with just words. Anyone here got a cad drawing you can show him?

FramingPro

Is that supposed to help with the total rise and stuff..it sounds like your talking about rafter length.

Chris@Sunrise

Dude youve been here before. Unit of rise is just that. Rise per given unit usually inches of rise per ft. Does not mater if your on the bottom or top of the rafter.

FramingPro

I know i know but wouldn't it matter?

katoman

Rafter length is determined by the rise and run. Better buy that book.

FramingPro

I know that..I layout my rafters with my square but im just looking for a way so i can calculate the ridge height if im making a rake wall,if im not i just nail a pair of rafters and lift the ridge until the plumb cuts fit perfectly

oldfrt

If you haven't had Geometry in school yet,pay attention as a lot will apply to this profession!

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steves

do your rise per run calcs then add the HAP, that will be the top of ridge above your plate height.

FramingPro

so if i have a 12:12 pitch and 48" run and a 3 inch HAP ill have 51" to the top of the ridge?

rselectric1

Framing "pro" are you serious? A tough one?

If you are having trouble with rafter math on a common rafter, what are you going to do about deducting half the thickness of the ridge, and figuring overhangs? What about hip rafters?

Buy the book!

I can only hope that framerman and loneframer dont notice your thread.

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FramingPro

I dont anymore..it was just the ridge height that was cloudy..i do overhangs and i made a small hip roof to try out the math