Rafter Help Please - Building & Construction - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum rafter help please

03-04-2010, 07:57 AM   #1

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I am by no means a carpenter, but I am building a shed and need some help with the rafters. I dont know how to do the math. My shed is 20x24 ft I need to know the ridge board ht and rafter length. specs are as follows

20x24 building size
20 roof width
6/12 pitch

ridge and rafters are 2x6 thanks very much

03-04-2010, 06:51 PM   #2
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Quote:
 Originally Posted by lenh I am by no means a carpenter, but I am building a shed and need some help with the rafters. I dont know how to do the math. My shed is 20x24 ft I need to know the ridge board ht and rafter length. specs are as follows 20x24 building size 20 roof width 6/12 pitch ridge and rafters are 2x6 thanks very much
There are online calculators available! When I Googled 'rafter calculator' I found this site ( http://windyhilllogworks.com/Calcs/raftercalc.htm )

You should be able to find several, by doing the same!

03-04-2010, 08:10 PM   #3
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Quote:
 Originally Posted by lenh I am by no means a carpenter, but I am building a shed and need some help with the rafters. I dont know how to do the math. My shed is 20x24 ft I need to know the ridge board ht and rafter length. specs are as follows 20x24 building size 20 roof width 6/12 pitch ridge and rafters are 2x6 thanks very much
The ridge will be approximately 5' 4" from the top of the plate to the top of the ridge.

For a gable roof like that you really don't need to set the ridge first. You nail the front and back rafters at each end of the building(or at the end of a two-piece ridge) and slide the ridge in between the four rafters. The rafters will always set the height of your ridge.

You should be using a 2x8 ridge when using 2x6 rafters. The length to the plate and deducting 1/2 the thickness of the ridge will be, 11' 1-5/16" without the overhang. I added a 1' overhang in the drawing.
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Joe Carola

 The Following User Says Thank You to Joe Carola For This Useful Post: lenh (03-05-2010)

 03-05-2010, 06:07 AM #4 Member   Join Date: Jan 2009 Location: Long Island Posts: 348 Rewards Points: 250 Hey Joe, what program is that and I have another question for you, does the rafter have to seat completely on the top plate? The full 3 1/2" inches?
 03-05-2010, 07:45 AM #5 Join Date: Mar 2010 Posts: 2 Rewards Points: 10 wow you guys are very helpful. Joe your reply gives me the info i needed. I want to thank you for taking the time to reply with drawing it much appreciated. However I do have a question, you say to set the front and back rafters and let them set the height i dont really understand what you mean. again thanks.
 03-05-2010, 09:35 AM #6 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 You should consider using Simpson rafter anchors rather than the traditional birdsmouth construction. I used them for my last shed project, much stronger connection between the rafter and the top plate, so provides better resistance to wind uplift. Faster to install also. Only adds a few dollars to the overall project cost. However,it does change the geometry a little, and you no longer need to cut a birdsmouth. Check out the Simpson Strongtie site on the web for full details. By the way, there are competing companies that make metal connectors, however I am not familiar with their products, hence I mention Simpson.
 03-05-2010, 11:28 AM #7 Member   Join Date: Sep 2008 Location: SoCal Posts: 1,802 Rewards Points: 1,016 Have you considered having trusses made up? I have stick built several garage and shed roofs - and have used trusses. Trusses are great - quick and easy to install, strong, and not expensive.
 03-05-2010, 11:57 AM #8 Member   Join Date: Jan 2009 Location: South of Boston, MA Posts: 17,248 Rewards Points: 2,000 Where are you located ? Any snow load ? I have always cut the birds mouth and used the Simpson strong ties on larger roofs My small 10' wide shed I just cut the birdsmouth Are you just putting in collar ties or rafter ties ? Any plans for storage area above the wall area - IE the "attic" of the shed ? Any plans to insulate & heat or cool the shed ?
 03-05-2010, 03:51 PM #9 Member     Join Date: Dec 2009 Location: Near Cleveland, Ohio Posts: 2,379 Rewards Points: 2,586 Lenh, First off, please update your profile with at least an aprox. location so we can better answer your questions. Secondly, if you are not sure about pretty basic rafter cuts and installs, it might behoove you to hire a carpenter to help you on your project. You will learn a lot, the project will go up a lot easier, and it will probably keep you from getting a few gray hairs. I have helped out friends on many occasions. They supply a handful of friends for labor, I do all the layout and instruct them what to do, and the wives/girlfriends cook up a mess of good food and provide the beer for when we are done. Mike Hawkins
03-05-2010, 11:34 PM   #10
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Quote:
 Originally Posted by II Weeks Hey Joe, what program is that and I have another question for you, does the rafter have to seat completely on the top plate? The full 3 1/2" inches?
"Usually" the rafter begins angling up at the 'arris' of the top plate which is the top inside corner of the plate. The Rafter is usually cut full width of the plate as well.
Does it have to??? No, but you can run into some difficulties calculating rafter lengths and angle by changing the normal layout.
Don't ask how I know this to be true.
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 Short of cutting off a body part, the worst that can happen in woodworking is manufacturing really nice looking kindling. --- Quoted from lenaitch

 03-06-2010, 06:24 AM #11 Member   Join Date: Jan 2009 Location: Long Island Posts: 348 Rewards Points: 250 Thanks jlhaslip
03-06-2010, 08:35 AM   #12
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Quote:
 Originally Posted by lenh wow you guys are very helpful. Joe your reply gives me the info i needed. I want to thank you for taking the time to reply with drawing it much appreciated. However I do have a question, you say to set the front and back rafters and let them set the height i dont really understand what you mean. again thanks.
Depending on how many guys you have and what experience you/they have,you can use a 24' ridge.You set and nail the front and back rafters at the birdsmouths into the top plates on the left and right side of the house. The plumbcut at the top ridge end will butt into eachother.

If you have four guys, two guys can hold each end of the rafters and the other two can lift the ridge up and slide in in between the plumbcuts of the rafters until the top of the ridge is flush with the top of the rafters. Nail the rafters to the ridge and the ridge height is set.

If you have to splice the ridge using 2-12', you set the two end rafters and then set two more near the end of the 12' foot ridge and do as I just said above.

If you're not comfortable with that you can pre-set the ridge at the correct height. If your not sure with the height I gave and any math, all you need to do is cut a 2x4 block (ridge width)and tack it onto the plumbcut of one rafter and take that rafter and tack it into the plate at the birdsmouth in the front of the house and then tack another one at the back of the house. Slide the back rafter up until it's flush with the top of the 2x4 block and tack it in place. Measure from the top plate to the top of the block and that's your ridge height.
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