Unequal pitch roof trigonometry calculating unequal pitched roofs is easy if you know what trigonometry functions to use.
Tie in unequal roof heights.
The formulas below use the law of sines to calculate the unknown length or angle.
This first video shows how the finished roof planes in off angle roof construction will look when completed.
All of the principles necessary for framing such a roof have been developed.
They form an upside down v with the point at the top of the addition roof.
Drawing a superimposes the common rafters for the two roof pitches and illustrates the problem that occurs when roof layout starts at the outside edge of the exterior walls.
I used will holladay s a roof cutter s secrets pp.
Collar tie is a colloquial term for collar beam.
A collar tie is a tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge beam during periods of unbalanced loads such as that caused by wind uplift or unbalanced roof loads from snow.
Position the flashing between the two roof pitches.
Major pitch angle minor pitch angle span plate line difference calculate major minor runs theoretical plate line.
Where roofs of unequal pitch join it requires careful layout to create equalwidth soffits with equal height fascias.
Rafter tie a tension tie in the lower third of opposing gable rafters that is intended to resist the outward thrust of the rafter under load.
We wanted to make the porch roof as steep as possible while keeping the peak of the roof below the wall to roof intersection at the top of the shed roof.
A tension tie in the upper third of opposing gable rafters that is intended to resist rafter separation from the ridge because of wind or unbalanced roof loads.
It remains for the student to make the applications to uneven pitches.
After a taking some measurements and doing a bit of ciphering we established that the steepest pitch we could use on the porch roof was 4 in 12.
Not infrequently a roof must be framed in which several pitches are involved.
Slide the right edge of the flashing under the highest roof pitch.
I then did the calculations again using a different span same pitch and also came up with a difference of 23 3 4.
Place the 2 by 4 inch piece of lumber along the center of the flashing.
It is advisable to prepare a framing plan as shown in fig.
I came up with 291 11 16 for the back and 267 15 16 for the front giving me a difference of 23 3 4.
240 241 to calculate my front and back common lengths.
To maintain a consistant ridge height in these situations the roof pitch must be adjusted.
Framing a roof of uneven pitch.
