How To Make A Common Rafter For Any Width Structure

Picture of How To Make A Common Rafter For Any Width Structure

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 safe use of the tools needed. If you are unfamiliar with tools, and the dangers they possess these instructions are not for you.
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.

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MarkM3616 days ago
Mikeswife1 month ago
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!!
Joe Smania (author)  Mikeswife1 month ago

The top plumb cut is the starting point for marking. It is as close to the factory cut as possible, to the left.

How do you figure that angle?
JoeS3 Mikeswife1 month ago

Sorry for the late reply Mikeswife.

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.

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.

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.

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.

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.

The weight referred to above would be the weight of the roof it's self including everything rafters, collar ties, decking, felt paper, ice& water barrier, shingles, and nails which easily passes into the thousands of pounds on houses and garage size roof systems, also add any snow & ice to the equation and anyone can imagine the tremendous forces.

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.

Hope that helps ya.

Mikeswife1 month ago
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" side into the peak board, but surely more than 1 nail would be required. My framing nailer holds nails up to 3" 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!!
Joe Smania (author)  Mikeswife1 month ago

3" 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.....

Depending on your rafter stock, this is what I use for a nailing pattern....

2x4 or 2x6 gets 3 nails evenly spaced.

2x8 or 2x10 gets 4-5 nails evenly spaced.

2x12 gets 6 nails evenly spaced.

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.

Hope that's a help for ya, and post a pic or two of your beautiful roof.

Jlgulley31 month ago

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!

Joe Smania (author)  Jlgulley31 month ago

Thanks for your comment Jlgulley3.

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 "the cutting it off after" method.

Unfortunately I have no blog yet, but the thought has crossed my mind, and I just may start one someday... Thanks.

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....

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&1/4" to the first one and then every 24" from that first one. 16" O.C. is 15&1/4 and then every 16" 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.

Hope that has helped, any other questions.. don't be a stranger! Stop in and ask!

Post some pics of the play house when your done?

Why do all this math when all you really need is a simple contractor calculator?

Thanks for your question,

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 "wills1" and my reply to him for a more comprehensive answer.

P.S. Why memorize a booklet full of directions for a calculator when all you have to do is some basic math??

hangdawg015 months ago

could you do it without being a even number like your run is 10 feet 3 and 7/16

showing how to convert it

Joe Smania (author)  hangdawg015 months ago

I have added this scenario to Step One of the Instructable. Thank you for your feedback and this excellent suggestion.

Happy framing!

wills15 months ago

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!!

Joe Smania (author)  wills15 months ago

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!

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.

Joe Smania (author)  Steve.Lewis.Doppler7 months ago

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.


fjm51437 months ago

Hi, thanks for this article. You made the process so simple. Do you have instructions on how to make valley rafters?

Joe Smania (author)  fjm51437 months ago

I signed in with Google+, and it gave me a different user name... JoeS3. I didn't realize it until I hit the "Make Comment" button. Needless to say, the reply back to you from JoeS3 was me.

Thanks again.


JoeS3 fjm51437 months ago

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 & 3/4". In hind sight, I only give the mathematical process for foot increments.

It is on my list, and your interest may just be the motivator I needed.

Thanks again, J.

BIG - X2 years ago
Joe Smania (author)  BIG - X2 years ago
Thanks for the feedback BIG - X
zazenergy3 years ago
wow! very cool!
Joe Smania (author)  zazenergy3 years ago
Why thank you zaz! :)