Rafter Length Calculator

Typical Price Range To Install A New Roof
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Rafter Calculator estimates the length, board size, quantity, cost per board and total cost of lumber.

The estimates are based on your house foot print (length and width) and roof slope.

Rafter prices are based on quotes from multiple lumber yards and building material suppliers.

Gable width (Span): ft. in.
Roof Length: ft. in.
Roof Overhangs: ft. in.
Roof Pitch / 12" slope.
Rafter Spacing
Board Size (nominal)

Rafter Lenght Quantity Lumber Size Rafter Cost Total Cost
0 ft. 0 pcs. 0 ft. $0 $0

This calculator will give you the total number of rafters needed for the job (including ridge boards), length of each rafter, length of lumber (boards) for each rafter, as well as total cost for all materials, presented in easy to understand form.

It is excellent for framers, builder, contractors, GC’s and architects and will be helpful in planning new construction projects.

Cost of Roof Shingles
Low End
Mid Range
High End
Cost / S.F.
Total Cost

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How To Use Roof Rafter Calculator

Roof Framing CalculatorUse the diagram on the right to correctly enter your roof dimensions – this will help you calculate board length more accurately.

1) Enter house footprint: Length is the roof eave (gutter) side. Width or Span is the gable side of the roof, which is used in calculating rafter length and cost.

2) Enter overhang length: Most houses have overhangs (soffits) which are needed to protect the house exterior walls and foundation from water run-off from the roof. Enter desired overhang length – this will affect your rafter length and lumber needed for the job.

3) Enter roof pitch / slope: Roof pitch is the value of roof rise over roof run, using 12 as the base for roof run. Example: If your roof rises 5″ for every 12″ of run, enter 5 as roof pitch. If your roof pitch is in degrees, use our roof pitch calculator to convert degrees to “pitch” value.

4) Select rafter spacing and width:

Rafter spacing (and roof length) will be used to determine quantity of rafters needed for your project.

Lumber size/ width (2×8″, 2×10″ or 2×12″ boards) will be used to estimate cost of each rafter, as well as total cost (excluding local sales tax).

Rafters LONGER than 20 feet: Home improvement stores DO NOT sell pine boards longer than 20 feet. If you need rafters that are 20′ 1″ or longer, you will need an LVL (Laminated Veneer Lumber) engineered rafter board, which is this yellow color plywood-like board made for structural purposes.

Roof Rafter CalculatorLVL is VERY expensive, but solves the limitation of 20 feet, in a common pine board. LVLs come in 24′, 28′, 30′ and 32′ standard lengths. If you need longer LVL, they can be custom ordered.

The calculator will automatically switch your board from PINE to LVL when you exceed 20 feet long threshold on rafter length.

LVL prices are calculated PER foot PER size. There is no “board price”, meaning that a 24′ 2×10″ board will cost the same per foot as 32′ 2×10″ board. The only difference is length.

LVL Rafter prices (per linear foot):

Prices below were sourced from National Lumber in Newton, MA. These are “retail” prices and if you have an account and are working with an outside sales person, you can typically get 3-7% discount. Sizes are NOMINAL, which means that a 2×8 is actually 1.75″ x 7.25″ board.

2×8 – $4.37 / ln. ft.

2×10 – $5.23 / ln. ft.

2×12 – $6.48 / ln. ft.

2×14 – $8.35 / ln. ft.

2×16 – $9.24 / ln. ft.

Limitations Of The Rafter Length Calculator

This tool is designed for lumber length of up-to 20 feet, which can be commonly purchases at local home improvement stores and lumber yards. If your rafter length is calculated to be longer than 20 feet, then there are two options for you:

1) Most lumber yards and building materials suppliers sell an actual 2″ x 12″ by 24 feet boards for $45-55 each. Longer size boards are NOT available.

2) If you need a rafter longer than 24 feet, you will have to use engineered wood rafters.

For pricing we used Home Depot online prices, which are comparable to Lowe’s prices for the same size boards. Lumber yard prices would be slightly higher, but supposedly be better quality wood.

For special order pricing (24′ boards or engineered wood rafters) contact local building material suppliers or lumber yards.

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Author: Leo B
For over 20 years Leo has run a successful roofing business in New England, specializing in metal roofing, as well as cool flat roofing technologies. Having replaced and installed hundreds of roofs in New England, Leo has first hand experience with pretty much every residential roofing material and roofing manufacturer available in the US.
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11 comments on “Rafter Length Calculator

  1. This isn’t a question but it is a compliment to you and your informative site. I dont typically ask questions about things outside of the social box unless it’s an experienced family member or family friend of the same experience , ie 3rd or 4th generation contractors even still they all have their own Lengo and experienced definition of this and that. And most websites/blogs dont really explain things in retro form for my liking or to the question I’m searching on.
    So for me, stickly from my own experiences, readings and searches, to comment on your site here goes way beyond and out of my norm.
    You have done an amazing job with this page as for , design, info on each and explanation (break down) of the mentioned diagram(s). It is easily understood even for the everyday person that doesnt do this work for a living.
    Your knowledge exceeds anything that I have came across, expecially since the start of the search engines on the web, in all of my 30 years of doing construction.
    Finally someone with great understanding and someone with Heart in this industrial business that we call contractors, architects, designers etc.
    So kudos and glasses raised as i say Cheers to you sir for a wonderful informational site.
    Your page needs to be on top of the others when a person does a search cause then they wouldn’t have to search any futher than the readings and calculations of each dimensional questions they may have then on your page.
    So from one Lover of wood and dimension s to another, i do applaud you, Thank You so much for my findings on your page.
    Cause even I still have questions about things and I’ve been doing this for a while now. Sometimes even the old timers needs a refresher course.
    Roll Tide,
    BluEyez of Alabama

    1. Melissa,

      This calculator only estimates cost of materials. Labor costs are nearly impossible to estimate for roof rafters, because most such jobs are done during HOME BUILDING stage, and are part of overall framing cost. Framers do not breakdown cost of walls / cost of rafters, etc. They price work by the job or square foot, depending on house layout.

      In retrofit applications, there are 100s of external factors that affect labor cost.

      Hence, we did not even fathom to include cost of installation / demolition work – it is completely outside the scope of this tool.


    1. Tu, no, we do not calculate snow load. This is a dimension & cost calculator. We do not provide engineering advice. Your best approach is to look at local building codes to see which size framing required in your state/city.

    1. Tommy,

      What exactly are looking to purchase? Are you a contractor, and want calculator for your site? If so, we are finishing a product that you will be able to put on your website, with your own prices in it.

      I will be making announcement on this blog, but you can contact me via email for more info – info @ roofcalc.org

      Cheers, Leo

    2. Tommy,

      It’s completely free to use on our website – you don’t need to “purchase” it.

      If you are looking for commercial use options of this calculator – contact as at info @ roofcalc.org to discuss options.