Building The Perfect Cabin Roof Part 1 of 5

Here’s how to build the perfect ridge beam every time!

Simplicity is the ultimate sophistication. ~ Leonardo da Vinci

Whether your building an off grid cabin, tiny house, or a shed in the back yard there’s a 100% chance you’ll be putting a roof on it and its got to last for decades.

Wouldn’t it be nice if the roof was just a simple quick design, build and poof.

Let’s face it, not everyone enjoys being one or two storeys off the ground, holding a heavy wooden beam and swinging a hammer

I know I don’t!

So.. I’m here to make it really, REALLY easy for you (and for me).

That’s why we started with the ridge beam and we built it in small, easy to handle segments.

Most Roof Designs

Unfortunately, most common rafter and truss designs used today are not easy to build and even more cumbersome to install. They can have complex angles to cut, involving math and measurements that some of us would much rather do without.

They can be heavy to hoist if you’re working solo, or with just one other person, in a remote location. They also have some real drawbacks that can adversely put undo stress on your structure even when built properly.

All that aside, we feel we’ve cracked the code on building rafters and found the perfect design that exceeds the classic rafter design and will provide a long lasting, sturdy and secure structure for your roof. If you haven’t yet gone through our Ultimate Roof & Rafter Guide have a breeze through it to become familiar with roofing terminology and designs.

Our Rafter Design

Right off the bat you might not spot the difference between our rafter (below)…

The-Off-Grid-Cabin-Roof-Rafter-Finished-3

… and that of the common rafter (below), but you soon will.

Common rafter butted up against ridge beam

We personally haven’t seen the rafter design we use on any other cabins. However, that’s something we hope to remedy after we’ve explained all of the benefits of its design.

The Typical Common Rafter

In typical frame construction a birdsmouth joint (or bird’s beak cut) is a wood-working joint generally used to connect a rafter to the top plate of its supporting wall. This cut is used at the bottom while the top of the rafter is butted up again the side of the ridge beam/board.

common-rafter-cuts

Rafter Thrust

In typical rafter construction, outward “rafter thrust” is compensated for by using rafter ties and collar ties but is not eliminated.

TimberToolbox has a great calculator for rafter thrust and you can check it out here.

RidgeBoardDiagram

CollarTieDiagram

CeilingRafterDiagram

TensionRodDiagram

Our cabin roof has three sloped surfaces (seen below) which I’ll be referring to in this and future posts.

  1. The main rafters on the rear of the cabin.
  2. The main rafters on the front of the  cabin
  3. The front deck rafters.

The Off grid Cabin Framing the Roof Complete

Our Rafter Design

In our rear rafter design, the birdsmouth cut is at both the top and bottom so that the entire weight is carried by the supporting wall as well as the ridge beam (see below).

The-Off-Grid-Cabin-Rear-Rafter-Design

The front main rafter also has a birdsmouth cut at the top only. The bottom of the rafter sits directly on top of the wall top plate (see below). There is no birdsmouth made here as there is no overhang required. the bottoms of thee rafter will be hidden inside the front deck roof.

The-Off-Grid-Cabin-Front-Rafter-Design

Lastly, the deck rafters have a birdsmouth at the bottom only which sits on top of a ridge beam (see below).

The-Off-Grid-Cabin-Deck-Rafter-Ridge-Beam

The top of these sit on top of a purlin and are nailed into the main rafters (see below).

The-Off-Grid-Cabin-Deck-Rafter-Purlin

The Benefits

Less Time: This rafter design is simple. It takes less time to measure, cut and install. Especially if working alone or with one person on the ground doing the cutting and the other up top installing them.

Less Money: On a simple gable roof like ours, the 2x10s (along with the ridge beam) are less expensive than ordering or building trusses. The cost of a crane (used to hoist up pre-build trusses) can range from $100/hr to $350/hr depending on the lifting capabilities required. Also, there was no way we were getting a crane to our remote build site.

Less Work: Lifting a full length 20 foot 2×8 (54lbs) or 2×10 (66lbs) is much easier than trying to lift and maneuver a 75lb-85lb truss into place. You can check out www.engineeringtoolbox.com for a chart showing the different weights of typical lumber.

Increased Safety: I’ve seen people try and build all sorts of rigs and slides with ropes, pulleys and ratchets to raise a truss into place. A remote cabin location is no place for injuries! Lifting one half of the rafter at a time is so much safer than lifting anything pre-built.

Zero rafter thrust: Our design removes the horizontal “outward” thrust that is generated in traditional rafter/truss designs and instead creates a strictly vertical load which translates down through the ridge beam, through the walls/vertical support posts and into the ground.

Roof Loads

The roof is always under static (non-changing) and dynamic (changing) loads.

  1. Dead load: The ridge beam, rafters/trusses, roofing material.
  2. Live load: Loads imposed by use and occupancy such as snow, wind load, solar panels and even you standing on the roof.

Weather (wind, snow, ice) plays a significant role in the high load experiences by a roof. Here in Canada it’s not impossible to see two feet of snow on a roof. That translates to about 60lbs/sqft.

You can visit www.TimberToolBox.com and find several construction calculators including a horizontal rafter thrust calculator. This shows the outward stress experienced by the rafter collar tie/ceiling rafter/tension rod.

In the photo below I’ve used their rafter thrust calculator to determine the outward thrust.

The_Off_Grid_Cabin_Roof_Rafter_Thrust_Calculator

How To Build The Perfect Rafter

It’s time to build the perfect and you’re probably thinking…

This is going to be a designing, engineering and building nightmare!

Well, it certainly doesn’t have be.

We’re going to show you a quick and easy method of building roof rafters that is in many way superior to the traditional rafter design and yes, that includes pre-fab trusses.

If you haven’t yet read our Ultimate Roof and Rafter Guide you’ll probably want to go back and quickly browse through that now to learn exactly why we choose this particular rafter design.

The Birdsmouth

The birdsmouth is comprised of two cuts.

  1. Seat cut
  2. Heel cut

Birdsmouth Cut

TIP: Never make your heel cut deeper than 1/3 the width of your rafter.

Here’s a great YouTube video by Shannon from www.house-improvements.com showing the same measuring and cutting techniques we use.

REMEMBER: Our rear main rafter design has the birdsmouth cut at the top of each rafter and the bottom. The front main rafter have a birdsmouth at the top and the bottom sits directly on the wall top plate. The bottoms of the main rafters are also attached using galvanized 18-gauge hurricane ties and nails.

MEASURE
1. Determine the pitch of your roof. Ours is a 12/12 pitch meaning it’s a 45 degree pitch roof
2. Calculate the length of the rafter
3. Calculate the length of the overhang (ours has an overhang on one side only)

CUT
1. Cut the fascia end of your rafter (optional). We installed all our rafters and then made a chalk line across the top of all the rafters at the same time and used a level to mark the cut line)
2. Cut the overall length of your rafter (optional). We used the full length lumber over lap at the top and then cut the excess off after wards.
3. Mark the heel cut line(s)
4. Mark the seat cut line(s)
5. Cut the birdsmouth
6. Install the rafter

Measure Twice Cut Once

TIP: One thing Shannon mentions in his video is to measure and cut one rafter and use that as a template to cut the others. While you may get away with that on a small shed we DO NOT recommend doing that on a larger build. There will be plenty of OTHER times during your build when this may again seem like a time saving idea but DON’T! Measure every piece of lumber (twice) individually and cut it once (some minor exceptions are cutting the rafter length as described below).|

There’s nothing worse than taking a short cut which ultimately costs you time and money.

Mike Holmes

Do it right the first time. ~ Mike Holmes

Main Rafter Support Posts

The first thing we need to do is build the three main support posts that will carry the main ridge beam and rafters/roof.

If you recall from Day 3 of Framing The Cabin we tripled up 16 foot 2x6s to form the main center post.