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These are the strips that span perpendicular to the rafters to which you will fasten the tin or other roofing material.  For 29-gauge metal roofing, spacing can be up to 30 inches between centers. Nail the lathing securely, with a minimum of two nails at each rafter, being careful to keep them aligned. Typically, a lean-to roof will lay at a different pitch than your existing roof.  The lean-to's roofing will need to fit fairly snugly underneath the existing roof to prevent rain from blowing into your addition. You may need to install metal flashing to help ensure water proof joining of the two roof pitches. Some roof metal profiles have a "direction of run," so that the laps fit correctly to guarantee a good, weatherproof roof system. Wood screw threaded hex screws with neoprene gaskets are ideal. Use a piece of break metal configured in the right dimensions to provide a decent finished appearance to these areas. The metal should also close any gaps between the lathing strips and the roofing so that water will not blow in through them.

Summary:
Lay out the spacing of your lathing. Cut back the roofing on the existing building if needed so that the new roof can fit correctly beneath it. Lay your metal on the lathing, beginning at one end. Fasten your metal roofing with a suitable fastener. Install trim of your choosing to finish off your lean-to's roof.