Calculating the angle either side of a hip rafter.

[epsilonGreedy]

I plan to make a wooden former to shape roll-top lead flashing that will then be fitted along the top of a hip rafter. The idea is to do most of the lead bashing & shaping on the ground before final shaping up on the roof. Given that each section of lead should not be longer than 1.5m, this wooden former will be quite small.

 

Given a 30 degree pitch hipped roof I have calculated that the pitch of each hip rafter is 21 degrees. But what is the angle of the slope either side of the hip that the lead should be shaped to?

 

Thinking extremes for a moment, if the pitch of the hip rafter was 90 degrees, i.e. vertical, then the slope either side would be 45 degrees and if the hip rafter was lowered all the way to the horizontal then the slope either side would be 0 degrees. Between those extremes I would like to know the slope either side of the hip rafter when the hip rafter is lowered to its main pitch of 21 degrees.

[Oz07]

This has blown my mind 

[Russell griffiths]

Don’t bother shaping the wings, form your roll on the ground and lift it up, the weight of the lead will lay itself onto the slate, then finish forming and trim wings when all in place, trying to make it tooooo accurate on the ground won’t work. 

 

Rough form then fit and tidy in place. 

[epsilonGreedy]

44 minutes ago, Russell griffiths said:

Don’t bother shaping the wings, form your roll on the ground and lift it up, the weight of the lead will lay itself onto the slate, then finish forming and trim wings when all in place, trying to make it tooooo accurate on the ground won’t work. 

 

Rough form then fit and tidy in place.

 

 

Ok I went back to the Restoration Couple lead work video on YouTube and this sort of confirms what you are saying.

 

He creates a rough right angled box section on the ground and then carries this onto the roof and bends in-situ around the mop stick. As you say the wings just flop into plate with a little extra bashing to shape around the tile to wood inside turn.

 

However for the gutter end pieces he does the reverse and shapes a tight fit around a mop stick former on the ground. See second video link.

 

 

Second link showing gutter end piece former.

 

https://youtu.be/d2mlnGuIWU8?t=22

 

It is interesting to see him using copper for retaining straps with a lead outer layer for a nice finish. @peterw suggested using stainless steel straps finished in lead and my local guy just uses lead with no second material to eliminate worries about an electrolytic reaction between dissimilar metals.