I’m a journeyman carpenter but have never had the priviledge of building a mansard roof like the one seen in the “this old house” special on the second empire remodel. It has a bit of a radius to it and I was looking for some ideas on how to build the rafters in a timely and cost effective way. One idea that occured to me was a slight radius cut in LVL’s but I thought that one of the many masters on here would have a better idea.
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Without a link, I can't see those particular examples, but first - to be clear - sometimes people get confused between mansard and gambrel which are similar but not the same. Since you mention the term 'second empire' I do believe you are indeed talking about a true mansard which runs all four sides and not two only.
In my experience, the wall is framed to support the upper part of the roof, and the mansard steep lower roof is flanked out from it over a soffit and sometimes corbels. For the curve, the rafters are set straight, and a kicker is set out near bottom, sistered onside of the straight rafter, or a curve is cut from lumber and stitched to the top of the rafter. There are probably other ways, but cutting all from a LVL seems the most expensive to me.
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also...with plywood sheathing, true curves are fairly easy... but bending anything more than 3/8 is problematicalin an area like that we'd probably use three layers of 1/4then again... if the roofing is cedar shingles , you can usually get your curves with the shingles aloneMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
With cedars, I've managed to get a pretty good curve just by ripping a bevel on a 2x4 shim out under the first course.
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We completely renovated a 1904 victorian here in town with a sprung mansard roof. The technique that the carps used back then is similar to what piffin described.
The third floor, floor joists ran past the outside wall became the framing for the soffit overhang. The straight roof rafters were laid in at a 72 degree angle, and then had another framing member where one edge had been cut into a curve sistered onto it at the bottom of the rafter to create the spring in the roof.
As far as I recall, the length of the sistered-on spring members was about 42".
We had to replace about 2 dozen of these rafter build ups so we replicated exactly what the original carps had done. We used plywood on the straight part of the roof but then used 1 X 4 material to create the sprung. There is a thickness difference between the two materials but the shingles absorbed that.
We also found buried in the walls many old newspapers and pay sheets for the workers who constructed the house. Back then the lead carps earned $1 a day, and the helpers earned $.55 cents. Things sure have changed.