The formulas below use the law of sines to calculate the unknown length or angle.
Tie in unequal roof heights.
Place the 2 by 4 inch piece of lumber along the center of the flashing.
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.
Where roofs of unequal pitch join it requires careful layout to create equalwidth soffits with equal height fascias.
I then did the calculations again using a different span same pitch and also came up with a difference of 23 3 4.
Not infrequently a roof must be framed in which several pitches are involved.
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.
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.
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.
They form an upside down v with the point at the top of the addition roof.
To maintain a consistant ridge height in these situations the roof pitch must be adjusted.
Slide the right edge of the flashing under the highest roof pitch.
Another by product of this roof configuration is varying soffit heights and widths which require further design decisions.
Major pitch angle minor pitch angle span plate line difference calculate major minor runs theoretical plate line.
Framing a roof of uneven pitch.
Position the flashing between the two roof pitches.
I used will holladay s a roof cutter s secrets pp.
It is advisable to prepare a framing plan as shown in fig.
The 2015 international residential code does not require collar ties or collar beams.
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.
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.
It remains for the student to make the applications to uneven pitches.
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.
Cut two rafters with a circular saw to the pitch of the addition roof and nail them across the rafters on the old roof.
All of the principles necessary for framing such a roof have been developed.
Collar tie is a colloquial term for collar beam.
