Correct usage of the rafter length calculator eludes me. Feet and inches for "Run" I understand, but feet and inches for "Pitch"?? Is there something wrong with this calculator, or does it just require some instructions (for me)?
Posts: 2 | Location: Pottstown, PA | Registered: 25 January 2005
This was an interesting calculation for us regarding the input fields. There are a couple of ways to express pitch or slope. This could be expressed in either degrees of the corresponding acute angle or as rise over run. We've chosen to use rise over run for the pitch. When expressing pitch, many contractors use a mearsurement of X inches(rise) over a constant 12 inches(run) to obtain the slope. That's what we've done here, only we've already accounted for the 12inches of run in the pitch since that is constant. All you need to do is enter is the rise of the pitch.
We should probably include an explanation of this given the specificity of this measurement.
Please let us know if this answers your question regarding our input fields. Let us know if you calculate pitch in another fashion and would like share it.
This is exactly the input we want to receive! Let us know of any other concerns. Thanks for helping us make HGTVPro.com a better site.
I think I understand, thank you. Let's check my understanding below however, just to be sure.
So then, a rafter run of say 16 feet entered as feet 16 and inches 0, and a roof pitch of feet 0 and inches 4 would yield a rafter length of 16'-10"? Did I arrive at the correct answer by the correct method?
Also, to clarify a few more aspects for me...Does this number estimate, for the sake of material ordering, the length of material needed? OR...is this an exact length across the top of the rafter? or the bottom?.. is the rafter angle cut to allow for fascia boards, does this account for the thickness of a ridge board...
I hope I have not worn out my question allotment. hehe
Thanks, Jeff
Posts: 2 | Location: Pottstown, PA | Registered: 25 January 2005
Rafter lengths are calculated by run and pitch (angle in x inches high per 12 inches long). So a 4 and 12 angle roof is a 4 and 12 pitch (4 pitch) roof. In the calculator just enter the run feet and inches, and the roof pitch in the inches field and it’ll give you the rafter length. Add the overhang to the run and that’ll give you the total length; add some length for the angle cut and that’s the length of the board.
So your example with a one foot overhang will go like this: Run: 17 ft., Roof Pitch (in the inches field) 4 will give you a 17 feet 11 inches rafter length in the calculator, and you’ll need a few inches for the 4 pitch angle cut.
The calculator should only have three input fields:
Run Feet: [ ] Inches: [ ] Roof Pitch: [ ] and 12
Posts: 1 | Location: portland, OR | Registered: 17 August 2006
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You need a code book. There is a geographical chart that has ground snow loads for your area. Chapter 8 has charts for rafters by snow load, if your area falls in between a chart go to the heavier load its more restrictive. You have a standard ceiling where the proposed attic floor joist rest on the exterior walls so now you can locate the chart you need (30psf, ceiling not attatched). There is 2 options for dead loads to select from before you go further. The 10psf will cover your basic roof products of moderate weight. The 20psf is heavier roof coverings like slate,tile,etc.. Now find the Hypotnuese or slope you want by building width and ridge hieght. Disreguard the overhang its only cosmetic. This gives you your span. So now you know all your criteria to follow the chart. Take your span length say 15 ft for example and you want 2 x 6 --16" o/c the max you can get for a 2 x 6 ---16 o/c is 14-4 for select doug fir you need to bump it up to a 2x8. Now you can get 18-10 for select doug fir or 15ft for #2. You can also reduce your spacing to 12"o/c and now a 2x6 gets you 15 again (under certain species). If I remember correctly the deflection is in inches say L/^=180 THEN you can deflect no more than 1" in a 180" span. For floors its double say 1" in 360" span. Thats it now you know how.
