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Old 07-28-2008, 09:52 PM   #1
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need some rafter help


i need some help with laying out a rafter, i know about spacing 16 O.C., but its the birdsmouth of the rafter im having trouble with, can someone here give me a formula for laying it out? or at least tell me how far the shoulder of the birds mouth should be cut? i dont know if its suppose to cover both the top and rafter plate or just the rafter plate. i plan to use a ridgebeam at 3 and a half inch wide, and rafters, im thinking 2x8s if that would be good, maybe on a pitch of either 6/12 or 5/12 or 4/12

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Old 07-29-2008, 12:15 AM   #2
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need some rafter help


You need the blue book of rafter cutting! It comes with the $5 framing squares you buy at Home Depot, at least in a condensed version. It has all shorts of cheat sheets for pitch, length, cut angle, etc.

Mine's at work, and I'm no help without it!

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Old 07-29-2008, 07:47 AM   #3
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The roof your framing is there a vaulted ceiling below or flat ceiling. you must take this into account. When cutting rafters with a vaulted ceiling I aways keep the beginning of the birds mouth even with the inside of the top plate. Which means when the rafter is installed , the ceiling line will die at the top edge of the plate. so the bottom of the seat cut does not stick into the room. cut your ridge cut first then pull your tape from there down to the length of your rafter, make a pencil mark at the top of your rafter and bring it down along the side of your rafter with the proper angle for your pitch , Not 90 degree's . take a framing square and hold the blade of the square along the line you just made keeping the tongue of the square facing the ridge cut. slide the square up or down until the outer measurement on the tongue of the square reads 3-1/2".check and see if your still square to the line on the side of the rafter. if so make this mark. this is your birds mouth cut. for your over hang flip the square over and lay it so the tongue is facing away from the ridge cut and still square with the line on the side of the rafter. if its a 12" over hang mark the side of the rafter at the tongue at the 12" mark. now hold the square at the top edge of the rafter like you did for the ridge cut and mark this angle intersecting the line you made for the overhang. now determine what you are going to use for fascia. 1x6 1x8 ??. 1x6 then measure from the top of your rafter down the width of your fascia board minus 1-1/2". which would be 4-1/2" now square this mark back to the ridge 90 degree's. your done. Good luck bob
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Old 07-29-2008, 09:23 PM   #4
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Originally Posted by buletbob View Post
The roof your framing is there a vaulted ceiling below or flat ceiling. you must take this into account. When cutting rafters with a vaulted ceiling I aways keep the beginning of the birds mouth even with the inside of the top plate. Which means when the rafter is installed , the ceiling line will die at the top edge of the plate. so the bottom of the seat cut does not stick into the room. cut your ridge cut first then pull your tape from there down to the length of your rafter, make a pencil mark at the top of your rafter and bring it down along the side of your rafter with the proper angle for your pitch , Not 90 degree's . take a framing square and hold the blade of the square along the line you just made keeping the tongue of the square facing the ridge cut. slide the square up or down until the outer measurement on the tongue of the square reads 3-1/2".check and see if your still square to the line on the side of the rafter. if so make this mark. this is your birds mouth cut. for your over hang flip the square over and lay it so the tongue is facing away from the ridge cut and still square with the line on the side of the rafter. if its a 12" over hang mark the side of the rafter at the tongue at the 12" mark. now hold the square at the top edge of the rafter like you did for the ridge cut and mark this angle intersecting the line you made for the overhang. now determine what you are going to use for fascia. 1x6 1x8 ??. 1x6 then measure from the top of your rafter down the width of your fascia board minus 1-1/2". which would be 4-1/2" now square this mark back to the ridge 90 degree's. your done. Good luck bob
thank you, i should be able to do the rafters now
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Old 07-29-2008, 10:01 PM   #5
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need some rafter help


Quote:
Originally Posted by buletbob View Post
The roof your framing is there a vaulted ceiling below or flat ceiling. you must take this into account. When cutting rafters with a vaulted ceiling I aways keep the beginning of the birds mouth even with the inside of the top plate. Which means when the rafter is installed , the ceiling line will die at the top edge of the plate. so the bottom of the seat cut does not stick into the room. cut your ridge cut first then pull your tape from there down to the length of your rafter, make a pencil mark at the top of your rafter and bring it down along the side of your rafter with the proper angle for your pitch , Not 90 degree's . take a framing square and hold the blade of the square along the line you just made keeping the tongue of the square facing the ridge cut. slide the square up or down until the outer measurement on the tongue of the square reads 3-1/2".check and see if your still square to the line on the side of the rafter. if so make this mark. this is your birds mouth cut. for your over hang flip the square over and lay it so the tongue is facing away from the ridge cut and still square with the line on the side of the rafter. if its a 12" over hang mark the side of the rafter at the tongue at the 12" mark. now hold the square at the top edge of the rafter like you did for the ridge cut and mark this angle intersecting the line you made for the overhang. now determine what you are going to use for fascia. 1x6 1x8 ??. 1x6 then measure from the top of your rafter down the width of your fascia board minus 1-1/2". which would be 4-1/2" now square this mark back to the ridge 90 degree's. your done. Good luck bob
i was thinking of a flat ceiling, i like the idea of less space to heat and cool, and i like the idea of attic storage and hiding space(these days a person almost needs that)
im planning on 2x6's for the outer walls, should i adjust the seat cut of the birds mouth to the width of a 2x6 or leave it at the width of a 2x4?

Last edited by feralfreak; 07-29-2008 at 10:09 PM.
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Old 07-30-2008, 06:48 AM   #6
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need some rafter help


Just keep in mind that you can not cut more then 2/3rds the width of the rafter. then you should be fine.

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