Carpentry 2: Gable Roofs Parts A & B
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Gable roof framing calculations are based on which two mathematical concepts?
- 1. Pythagorean theorem
- 2. Ration and proportion
Which formula can be used to find the size of any side of a right angle triangle if you know the size of the other 2 sides?
Pythagorean (a squared + b squared = c squared)
Roof slopes are always given as a ration of ___ to ____
rise to run
If the proportions of two triangles are the same, what can you say about the rise:run ratio of the same two triangles?
It will be the same
A roof has a slope of 1:2 and a rafter run of 3600 mm. What is the rafter rise of this roof?
A roof has a slope of 1:3 and a rafter rise of 1100mm. What is the rafter run of this roof?
What does the slope gain factor represent?
The length gain on the slope for each unit of run
What is the slope gain factor formula for 90 degree roof members?
Square root of: rise over run squared +1
When setting up the metric framing square to lay out rafters with a slope of 2:3, what are some of the rise:run number combinations you could use on the square to get this slope?
When using the rafter angle square to lay out rafters, the common scale only shows the rise number for the rise:run ratio. What run number is this square based on?
Measurements for common rafter shortenings are always done at what angle to the plumb line?
What is the upper end of a common rafter shortened to allow for?
What is the lower end of a common rafter shortened to allow for?
Explain the difference in the rafter shortenings that occur for the portions of the rafter that are inside and outside the exterior walls.
- All rafter shortenings within the boundaries of the exterior wall will be HALF the thickness of material the rafter butts into.
- The shortenings required on the portion of the rafter that projects beyond the exterior wall will be equal to the full thickness of the material the rafter butts into.
Explain the shortening required at the top of a common rafter if no ridge board is used in the roof system.
Shortening is not required.
According to the National Building Code, what is the minimum length of bearing for a birdsmouth seat cut?
Explain teh 1/3 to 2/3 guideline for rafter birdsmouths.
Approximately 2/3 of the plumb line left with 1/3 cut out for the birdsmouth
How would you calculate the theoretical length of a ridge for a gable roof?
The building length plus projection on both sides
The calculated line length of the centre gablestud is equal to the ______
Why is it important to accurately lay out the centre gable stud?
So the ridge height will be correct
A roof has a slope of 2:3 and the gable studs are spaced 600 mm o.c. What is their common difference in length?
What are the 2 common types of gable rafters?
Describe 2 methods of horizontally straightening the rough fascia
- 1. Eyeing it from one end and moving it in or out, as required.
- 2. A more accurate method is the use of a string line and blocks, similar to straightening a wall.
Identify 3 ways that the span of a rafter can be reduced
- Using caller ties
- Using struts
- Using dwarf (knee) walls
In a roof frame with a slope of 1:3, a collar tie must be installed that has a drop of 800 mm. What is the calculated length of this collar tie?
What must be added to the calculated length of a collar tie to find its actual length?
The horizontal distance beyond the birdsmouth (seat cut extension) less 10 mm (3/8") at each end, must be added to the line length at each end of the collar tie.
Why are struts cut 10 mm (3/8") lower than the top of the rafter?
To allow for shrinkage
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