"Let In" Purlin Planning, Sourcing, Fab and Install: ---Quote & Order
 
The following is a design for easy way to fabricate, small profile and water resistant purlins, that cast a small shadow. Maximum 48-14" rafter spacing for this design. Not recommended in extreme high wind areas.
 
Planning: Let in purlins must be positioned in line with blocking placement for 8, 10 & 16mm & 25mm polycarbonate. Purlins running the length of the structure, are fabricated on the ground and lifted in place. Splices must be positioned in center of rafters. A 1/4" gap at the splice will accommodate for Framing Layout. A 1" wide x 1 3/8" deep notch in rafters is required to accommodate for "let in" purlin and to allow for movement of condensation past these purlins. See purlin and spacer install drawings. Some rafter strength is lost when the rafter is notched, so the purlin fit should be snug. Load on rafter will tend to bow the rafter, tightening this joint. The aluminum will provide some of the lost strength.
 
Order 1 spacer per attachment point. 1 spacer is placed between each 48-1/4" on-center rafter. Spacers not required when 24-1/8" spacing used. See Polycarbonate Position & Attach for mid-sheet attachment of roof & wall sheets. At install predrill polycarbonate with 1/4" drill at these points and set snugly into polycarbonate, using the self -drill screws with 3/4" washers we sell. Slightly dimple polycarbonate and screw will penetrate aluminum enough.
 
Note: Always present your design to the building dept. for approval and provide proper Diagonal Bracing. The purlin design presented is meant to support the polycarbonate sheet, not strengthen the frame.

Purchasing: Source some 1" (O.D. - outside dimension) square aluminum tube, with an 1/16" wall thickness, for your purlins. Home Depot has this item in 8 ft. lengths.

 
To splice the aluminum into long lengths visit the electrical aisle and get some 1/2" schedule 40 PVC pipe. Figure one 12" length per splice. The fit will be loose, so get some duct or aluminum tape for wrapping splice to slightly increase the diameter of the splice. A snug but easy fit is best.
 
Fabrication: Assemble purlins into lengths required. 24 ft. sections can easily be lifted to the roof for assembly into longer lengths.
 
Installation: Cut 1" wide x 1 3/8" deep notch in rafters. This can be done by laying out area to be cut. With numerous passes of a circular saw, across rafter with blade set at depth of notch, create a comb like looking pattern to the inside of the lines drawn. Now simply tap on these small sections of wood and they will break apart from rafter. Clean up notch with a chisel. Aluminum should snugly fit in notch.
 
Tap in prefabricated aluminum purlins. Attach to rafters with 2-1/2" deck screws. At the splice two screws, 1/2" from outside edge of rafter, are set into rafters for strength.
 
There you go. Open design with minimal shadow!
 
Note: The sheet must be supported on all 4 edges with a minimum of 1/2" of sheet bearing on the frame. When using sheets that have been cut along the width, and no longer have a rib at the edge, position so first rib is supported by a minimum of 1/2" of frame.
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Quoting & Ordering----Samples & Design-Install Guide----Reload Sundance Supply Site