If any terms or wording I have used is confusing at all or anything seems unclear, please feel free to ask a question. I don't check everyday, but I do check in often, and would be happy to answer any questions or edit something that is unclear for you.....

These instructions should enable the moderately skilled handy-person to produce a set of common rafters for any building. These instructions are for the moderately skilled. I have omitted basic steps on the tools needed.
Rafter stock will be determined by geographic location, the width of your building, and local building codes. . . .
A pencil, tape-measure, circular-saw, speed-square, framing square, and saw-horses.
Risk of serious injury or death is possible!
If you are unfamiliar with the listed tools and the dangers they possess, these instructions are not for you.
These instructions are formatted in such a way that the peak of the rafter should be to your left, the tail should be to your right, roof side away from you, ceiling side close, laying on the saw-horses. Please read the entire instructions before beginning and then follow them step by step.

Step 1: Determine your run.

(1) Divide the width of your building in half, and remember that number. This is your run. If your building is an even foot-measurement, you can skip the edit below and go to the next step.

If your building is Feet and/or Inches, the edit below shows how to convert the inches and fractions of an inch to decimal values of a foot so the math in a later step will work.

Edit per hangdawg01's excellent suggestion...

If half of your building measurement is not an even foot measurement like 10' 3&7/16" this presents a problem because the Customary Foot Measurement System is a 12 based numbering system. We need to get it to Decimal Notation form which is a much more "math friendly" 10 based numbering system. A couple steps of division, and then add your answers together. It's pretty easy after you see how and do it once or twice..

Lets do this....

The formula is as follows.....

Even feet + [even inch divided by 12] + [ numerator of fraction divided by denominator of fraction, divided by 12]=R. Or using 10' 3&7/16" for our example, 10'+ [3/12] + [7/16/12]= 10.286 feet.

Now that we have this fraction in decimal form, we can multiply in a later step.


We solve for what is in the brackets first, so....

************ 3 / 12 = 0.250

7 / 16 = 0.437/12 = 0.036

********** Feet = + 10.000

************ Run = 10.286 Feet .


For full inches out of a foot, like 3" it is just a division. 3"/12" or 3" divided by 12" = N ( Answer N = 0.25 feet )

Now, for a fraction of an inch out of a foot, it is double division. Lets use 7/16" for example...
We have to divide 7 by 16 to get our decimal inch value, and then divide that by 12, to get our decimal foot value..
It reads like this in mathematical formula 7/16/12.
So 7/16= .437 inches. And then, .437/12=.036 feet. So 7/16 out of 1 foot is .036 feet.

And now we add the 3" decimal of a foot from above to it...
.25+.036= .286 feet. So 3 and 7/16 out of 1 foot is .286 feet, and now we would simply add the 10 feet to it.

I hope I've written this out so that it makes sense and is not overwhelming. It really is much easier and simpler than it looks in writing. Give it a try a couple times and you'll see.

My deck is 66 foot long 16 foot wide how long should the rafters be
<p>It would depend on the pitch you want to put on it, Also, depends on if the deck's roof ties into the house, or if it's a double sided roof like a free standing structure.</p>
<p>Thank you for all the useful information you have provided in this forum! My question is in using the formula to figure out the length of the rafter, what would the overhang be for a 5/12 pitch? We are trying to get a take off list for the material required. Thank you for your time!</p>
A more precise answer to your question is... decide what overhang you want, what you are using for subfacia (if any). Subtract the thickness of the subfacia x2 from the overhang measurement. Then add 2x your overhang to the building with and continue with formula.<br>1 foot overhang with 2x6 subfacia deducted is 10.5 inches, 10.5x 2 = 21inches added to overall building width. Then plug that into formula and you'll get the length needed for the take off list.... :)
If you'd like a 1 foot overhang you'd need an additional 11.5 inches to your lumber length.<br>How I got that was the 13.0 length per foot run, subtracting 1.5 inches for subfacia.<br>Thanks for viewing and asking. A pleasure to help.....
There didn't seem to be much instruction on how to figure where to mark for the plumb cut meeting the peak of the roof. Everything else made perfect sense!!
<p>Thank you for pointing that out, I've edited that step giving it more detail.</p>
<p>The top plumb cut is the starting point for marking. It is as close to the factory cut as possible, to the left.</p>
How do you figure that angle?
<p>Sorry for the late reply Mikeswife. </p><p>Again, I'm not sure how big your roof needs to be, so I'll just try to cover all the bases incase you are building something on the larger end.</p><p>You find that angle by determining what pitch you want your roof to be, or by following your blueprints. Most residential roofs are between a 5/12 (which is a 22.5* angle or a 5 common on the speed square rafters gauge and on the shallow end as far as steepness) and a 12/12 (which is a 45* angle and at the sharp end for steepness.</p><p>There are lots of things to consider when choosing a pitch for your roof. Generally, the steeper the roof, the less the weight pushes straight downward. As in a 12/12 roof transfers most of the load to the walls, mostly downward towards the foundation.</p><p> The exact opposite happens with a more shallow roof. A 5/12 for example will need to hold more weight its self (like a floor would) and whatever percentage of stresses that are transferred to the walls are more pushing outward on the walls than downward on them.</p><p>We have all seen the roofs with a big droop in them right? Highest at the two ends and lowest in the center? This is most likely due to the walls being pushed outward over the years. As the bottoms move away from center, the top looses elevation. Collar ties, collar ties, collar ties. Proper collar tie installation, and roof design prevent this from happening.</p><p>The weight referred to above would be the weight of the roof it's self including everything rafters, collar ties, decking, felt paper, ice&amp; water barrier, shingles, and nails which easily passes into the thousands of pounds on houses and garage size roof systems, also add any snow &amp; ice to the equation and anyone can imagine the tremendous forces.</p><p>That's not to say one is better than the other, they both have pro's and con's. Just that there is lots to consider depending on what you are building, how wide it is, and what kind of weather you (or more so the roof) will be facing.</p><p>Hope that helps ya.</p>
<p> <br> <br>i really like this <br> article please keep it up.</p><p><a href="http://www.monacopropertylistings.com" rel="nofollow">immobili in monte carlo</a></p>
Also, when attaching the rafters, where on them do you nail? It seems obvious that you'd insert through the narrow part of the 1 1/2&quot; side into the peak board, but surely more than 1 nail would be required. My framing nailer holds nails up to 3&quot; long. Would I need to buy a gun with longer nails or do I toe-nail from the side or neither of those? ;) thank you for your help!!
<p>3&quot; framing nails are ok for most applications, but then again, I have no idea how big the roof is on the thing you are building, or what your local codes are, so.....</p><p>Depending on your rafter stock, this is what I use for a nailing pattern....</p><p>2x4 or 2x6 gets 3 nails evenly spaced.</p><p>2x8 or 2x10 gets 4-5 nails evenly spaced.</p><p>2x12 gets 6 nails evenly spaced.</p><p>I like to nail through the ridge at a slight angle into the rafter, and on the gable rafters I toe-nail, so I wont split the ridge, but there are many ways to skin this cat.</p><p>Hope that's a help for ya, and post a pic or two of your beautiful roof.</p>
<p>Joe, great instructible! Is it necessary to make two cuts on the ridge beam side of the rafter to remove 1/2 the width of the beam or could you factor that reduction into The original length? Also, do you have a blog on measuring and installing the ridge beam these rafters will tie into? I'm finishing a play house for the kids with a 5/12 roof to match my house!</p>
<p>Thanks for your comment Jlgulley3.</p><p>No you could subtract the full with of the ridge from your buildings overall width, and then continue with the formula, but for ease of explanation in this forum I just chose to show &quot;the cutting it off after&quot; method.</p><p>Unfortunately I have no blog yet, but the thought has crossed my mind, and I just may start one someday... Thanks.</p><p>The measurement of your ridge will be the same as your building length from gable end to gable end. Installation depends on the size, but this generally works for things like sheds and play-houses.... </p><p>Install 3-4 sets of your rafters and let them lean against each-other at the top with an angle brace or two on each side so they wont just flop over. Lay out all of the rafter marks on the ridge (2' O.C. is 23&amp;1/4&quot; to the first one and then every 24&quot; from that first one. 16&quot; O.C. is 15&amp;1/4 and then every 16&quot; from the first.) then push it up from below between the rafters, when you let off on the ridge, the rafters SHOULD bind on it and stop it from falling back down through. Best to have 3 people just to make it a very east task, fewer than that, and it can try your patience.</p><p>Hope that has helped, any other questions.. don't be a stranger! Stop in and ask! </p><p>Post some pics of the play house when your done?</p>
Why do all this math when all you really need is a simple contractor calculator?
<p>Thanks for your question,</p><p>For people who don't want to shell out $50.00 - $100.00 on the calculator, the brain works for free. Scroll down and view the comment from &quot;wills1&quot; and my reply to him for a more comprehensive answer.</p><p>P.S. Why memorize a booklet full of directions for a calculator when all you have to do is some basic math??</p>
<p>could you do it without being a even number like your run is 10 feet 3 and 7/16</p><p>showing how to convert it</p>
<p>I have added this scenario to Step One of the Instructable. Thank you for your feedback and this excellent suggestion. </p><p>Happy framing!</p>
<p>this just saved me a ton of money. i was worried about buying construction calculators and all the bells and whistles just to make a darn common rafter template...thanks, man!!</p>
<p>Oh wow! I couldn't have prompted you to leave a better comment! I liked to use the Construction Master Calculator back in the day, but then I thought... What would I do if I was on a job with a full crew of guys out in the boonies and the thing wasn't working on roof day? Shut the site down and drive 50 miles to Lowes? That's when I learned the old school way, it isn't much slower than the fancy calculator, and it always works... Thanks for the comment, and happy framing!</p>
<p>Awesome... very, very concise and helpful. Building a clerestory shed and the rafter template supplied with MIGHT have worked if the final dimensions where exactly as the plan suggests, but of course, the real thing varies by enough to make the template as useful as ashtrays on a motorcycle. Your explanation of the maths behind the calculations was perfect. Thanks a bunch.</p>
<p>Thank you so much for the feedback Steve. It's great to hear this is useful to folks out there in the web. Hope the shed turns out good for ya.</p><p>Joe.</p>
<p>Hi, thanks for this article. You made the process so simple. Do you have instructions on how to make valley rafters? </p>
<p>I signed in with Google+, and it gave me a different user name... JoeS3. I didn't realize it until I hit the &quot;Make Comment&quot; button. Needless to say, the reply back to you from JoeS3 was me.</p><p>Thanks again.</p><p>Joe.</p>
<p>Thanks for your kind words fjm. I have wanted to do a piece on valleys and hips but spare time eludes me. I'm thinking I'll do future instructables in video, even repeat this one including an easy way for buildings with odd widths like 22' 7 &amp; 3/4&quot;. In hind sight, I only give the mathematical process for foot increments. </p><p>It is on my list, and your interest may just be the motivator I needed.</p><p>Thanks again, J.</p>
Thanks for the feedback BIG - X
wow! very cool!
Why thank you zaz! :)

About This Instructable


65 favorites


More by Joe Smania: How To Make A Common Rafter For Any Width Structure
Add instructable to: