Roof Framing: Part 3
It’s no secret that roof framing can be complex. In order to efficiently built an arguably “simple” roof demands that the construction up to that point includes level floors; and plumb, square and straight walls. Accurately calculating rafter lengths requires some basic Grade 10 math skills, but accounting for all the proper additions and deductions to those rafters demands an understanding of several building members, including ridge beam thicknesses/height, ceiling heights/ extra plating, differing wall heights with “irregular roofs” and even considering finishing details to adjust soffit depths in different scenarios.
In this blog post I will go over some of the complicating factors from an addition framing job near Georgetown, Ontario that demanded some extra consideration from us in order to nail important details.
Georgetown and the surrounding areas are old communities, and many of these older homes are beautiful, but can be especially challenging when being married to a new addition. These old homes are often not square, the walls are not plumb, the roof lines are not beautifully straight. All that to say, many measurements a framer in new construction could figure out on the ground with a calculator, need to be confirmed and considered before work commences with additions.
The roof of this addition is a “mono-slope”. Over 30 feet long it will slope upward off an 8’ wall and meet the peek of the existing roof on the other end. The 16” TJI roof rafters will clear span this distance at a “low slope” of 2/12 and the new roof will be a metal to match the metal on the existing roof to which it will “over frame”.
The complicating factor at our Georgetown addition project was that the existing house had what is called a “flying gable” where the gable rafters taper outward from the eave up to the ridge, in this case 49”, and since our roof was sloping up at 2/12 we needed to figure out what our wall height needed to be to accommodate that extra 49” of roof slope so that our new roof would meet with the tip of the existing flying gable perfectly.
What we did was we used a laser level and a story pole marked with top-of-rafter height near the new exterior wall location to figure the elevation of the existing ridge above top-of-wall (TOW). This number 74 15/16” was our total rise.
Now, with 2 known pieces of information, we could use our roof pitch (2/12) and the horizontal “run” of the flying gable (49”) to figure out how much this roof would rise over that distance. And the answer is 8 3/16”. So when snapping lines for my first raked wall, I was able to deduct the height of the rafter in plumb (pulled from our section drawing of the rafter), the roof sheathing, and the additional 8 3/16” to arrive at the exact height needed to mate the new roof perfectly to the old.
I will add an additional note which is when doing this type of figuring its especially important to review any details provided in the architectural drawings and even coordinate with other relevant trades. This case with a low-slope metal we confirmed with the roofer how they would prefer the roof assembly. They decided on a fully adhered underlayment an no strapping, which was not what I would prefer, but the homeowner wanted to stick with an assembly his roofer would warranty.
This is just one of many benefits of taking the time to develop a section drawing of your rafters to pull relevant information from. There are more difficult ways to achieve the same result that involve more risk, more high work, more hands and more guesswork. But a competent framer who takes the few minutes upfront can greatly reduce unnecessary labour down the line. If you have a house in the Georgetown area and are considering a renovation, or addition or even a new build please consider White Oak Homes. We love serving the homeowners in our community.
