Saturday, June 23, 2012

Small Building Construction, Part Four

Dear Preppers and Survivalists,

Folks, I didn't get some of the photos posted before we left on our vacation, so you'll need to check back. : - (

There is a scientific/mathematical method of figuring how to calculate the slope of the roof. I don't know what it is, so I used a SWAG (Scientific Wild *ss Guess) method by building a template.

Making the Template
On oriented strand board (OSB), the manufacture places different colored lines. These lines allow you to figure where to cut and where to drive the nails when used as sheathing.

Me? I used the lines as a reference for making my template.

Since the small building is 8 feet wide, I laid a 2X4 on a sheet of OSB at an angle. Marked the 2X4 where it went over the edges, top and bottom.

Next, I cut the board.

Picture of rafter.

Needless to say, I bought extra 2X4s so I could make some mistakes while I worked on the rafter. Plus, it's a little more involved then that.

So let me explain with pictures

I Hate Books
Not really because they gave me many good ideas.

My wife brought me this book on building chicken coops from the library. The book had an idea for notching the rafters for the roof purlins.


This idea added extra work, but it looks 'clean' when finished.

Be Warned:
If you live in big snow country, you might want to use 2X6s as your rafters and attach the purlins to the top of the rafters. No notching.

Building the Trusses
There are different ways of building a roof. You can decide to use a ridge board supported at the ends then attach each rafter to the ridge board or build trusses. There might be other methods, but I only have used these two.

We decided to build trusses.

First, I took a sheet of plywood, laid it long ways on some saw horses, and screwed some scrap 2X4s to give me the correct distance to simulate the top of the walls.

Next, I placed each rafter's bird's mouth (seat cut) on the 2X4.

This picture shows the bird's mouth very far away from the 2X4. You need to make sure the rafter is tight against the 2X4.

Less than Perfect
Next, I take the other end of the rafters and put them together.

Needless to say, my rafters didn't turn out perfect. This is one spot where I went for "less than perfect.'

Of course, as I moved the rafter one end would be tight against the other 2X4, but the other end would get a little loose.

So, I was constantly checking to make sure everything was in place.

Next, I screwed a small piece of plywood (1 foot by 2 foot long) to one side of the two rafters, at the very top.

Flipped it over and screwed another piece of plywood to the other side of the rafters.

I used four screws per side.

Next, I cut the over hanging bits off with a circular saw.

Notice, I staggered the screws on each side of the rafters, so the screws wouldn't be holding the same spot on the 2X4 rafter.

Once all of the trusses were finished, I placed them on the walls.

Next, I attached each truss to the ceiling joists with four screws, two on each side.

Then, I cut each end of the ceiling joists at an angle, attached the end trusses to the OSB, and cut the OSB to match the angle of the trusses.

Next, I added the purlins. Since we wanted a one foot overhang on all for sides of the roof, I made sure the purlins stuck out 12 inches from the trusses. Latter on, I will add the rake; after we put the solid decking and metal roof on.

This picture, to the right, kind'a shows a detail of the purlins attached to the rafter in the notch and the notch I had to cut in the OSB to allow the bottom purlin to fit.

As of this writing, my wife and I have added two sheets of OSB, for solid decking, and a 12 feet by 16 foot tarp to act as a temporary roof.

What I Would Do Differently
First, I would find someone to teach me the calculation for finding slope for a roof

Second, I would only place two trusses up on the walls at a time. It was a little windy and the trusses kept moving. I would get hit in the head every once in a while. Ouch : - (

Third, I would have cut the ceiling joists (the angle for the trusses) on the ground before putting the joists on top the walls.

Lastly, I would have bought the metal roofing before I started the roof. Doing this would have allowed me to plan the final dimensions of the roof instead of waiting.

Camp-Rigby - Anatomy of a Roof


A & B Construction - Anatomy of a Roof
Note: This link has the colored picture.


Structure Magazine - The Case for an Engineer of Record for a Metal Building System
Note: For the illustration.

YouTube - Framing Roofs, Part 1


Carpentry: Pro-Framer - Basic Roof Framing Instructions

1 comment:

Gareth Rounding said...

Its a lot easier than you might think to make your cabin yourself. The base is the most important part to get right. The sheds, cabins and houses all come with tantalized floor bearers, substantial floorboards and purlin boards.