The Exhaust Fan Curb

I have purchased a fan that will mount at the top of the curve in the center of the roof.  The fan will blow in either direction, and is protected by a rain detecting circuit.

The penetration through the roof requires a flat mounting plane outside and inside.  To that end, we have left a hole in the roof structure that will accommodate a curb, or a vertical square box that extends above and below the planes of the roof at the extremes of the opening.  The curb itself is oak 16-1/2" x 16-1/2" outside dimensions, and is 2-1/2" high.  This provides enough height to extend approximately 1/4" below the inside ceiling low point and 1/4" above the outside roof high point.

Since the width of 16-1/2" is less than the space between the ribs, we had to create a pair of curved pieces that match the roof ribs in the space of the opening.  These "psuedo-ribs", running front to back, will carry the plywood ceiling inside and the roof outside where it meets the curb.

Today was spent creating the curb and the pseudo-ribs.  The pseudo-ribs were challenging in that they must match the inside and outside curves of the roof.  We have a combination disc / belt sander, which made it easy to get the outside curve correct, but the inside curve was another matter.

Eventually we created a 1/4" birch plywood template through a combination of the disc sander (outside curve) and hand-sanding the inside curve.  This template was then screwed to the backside of a piece of poplar and used as a guide template for a flush-trim router bit, thereby duplicating the piece in poplar.

The results were far superior to prior attempts.

No photos are available today, but when we mount the curb I will photographically document the relationships