A gambrel or barn style roof is similar to a gable roof, with the initial pitch from the peak being very slight, and a break about half way down the roof line with the lower section at a steeper pitch. Tin has been a roofing material for the better part of two centuries and is known for its rugged long lasting finish. Before installing the tin, strip old shingles or metal down to the plywood or OSB sheathing. Be sure to remove all old fasteners as well for a smooth surface.
Install a layer of roofing felt over the decking of the roof. Use roofing nails with nylon washers to fasten the felt to the roof. Roll the rows of felt horizontally across the roof, cutting it at the end of the roof with a utility knife. Lay the lower edge first, overlapping new rows from above by two or three inches. Use one nail every 10 to 12 inches.
Position the first panel of the tin roofing at the lower left corner of the roof. Align the panel edges with the edges of the roof. Screw the panel in place with 3/4-inch hex-head sheet-metal screws, with rubber washers installed. Use one screw in every third valley, four inches from the bottom and four inches from the top of each panel. Install the remaining panels in the row, overlapping each panel by two ridges.
Measure to the end of the roof and cut the last panel to fit with an abrasive cutting disk on a circular saw. Wear safety glasses and gloves for this operation.
Apply a layer of butyl sealer tape to the top edge of the row of panels. Overlap the next row two or three inches pressing it firmly down on the butyl tape. Install screws in every third valley to hold the panel in place. Add full length sheets as far as possible.
Measure the remaining distance from the last row to the break of the gambrel and cut enough pieces to fill a row. Install 8-inch flasing over the gambrel break joint, horizontally with half of the flashing above and half below the joint. Nail in place with rubber washer nails, with one nail every 10 to 12 inches.
Install the tin up to the gambrel break, laying the top edge over the flashing. Fit an end strip of butyl rubber at the end of the sheet. These rubber strips are formed to fit the peaks and valleys of the tin, providing a flat line at the end of the panel.
Install a 10-inch wide flashing across the gambrel break, folded along the centre so that the flashing lays flat on the roof above the break and overhangs the tin below. Resting flat on the end strip. Press it to the end strip and screw down through the flashing and end strip with 2 ½ inch hex head sheet metal screw with rubber washers. Nail the top edge as for the other flashing.
Install a tin roof ridge cap with space along the edges for the corrugated tin sheets. Attach it using the same screws as for the panels. Install the top section of the roof, above the gambrel break in the same fashion as the lower section. Install an end strip at the top edge of the panels for the last row. Fit the top edge of the last row, under the overhanging edge of the ridge cap to provide complete coverage.