How to Make a Valley on a Tin Roof

Without Valley Lap Joints

• 1). Place preformed valley trim in the valleys of the roof so the majority of the trim rests flush on the moisture barrier. The trim looks like a "W" with elongated wings at either end. The small central portion, called the water divider, will stick up. It should sit at the center of the valley.
  
  
• 2). Fasten the valley trim to the roof substrate with pancake screws. Place the screws near the top edge of the flashing on either side, spaced about two inches apart the length of the trim.
  
  
• 3). Install roofing sheets from the outer and lower edge of the roof toward the top center of the house. Use the 3-4-5 method of laying out a square triangle with sides 3, 4 and 5 feet long at the end of the run on the roof. Place the 3-foot side at the bottom of the run and the 4-foot side at the outside of the run. The 3-foot side must be square with the bottom of the roof. Square the edges of the roof panels to the 4-foot side.
  
  
• 4). Leave three inches from the center of the flashing to the edge of the roofing sheet on both sides when cutting the roofing material with the shear attachment at the valley joint so you leave a 6-inch space for water to run down.
  
  

With Valley Lap Joints

• 1). Install the lowest piece of valley flashing. Place the flashing flush in the roof valley with the water divider sticking up at the center of the valley. Install the flashing with pancake screws along the top edges of the flashing.
  
  
• 2). Place the second piece of valley flashing above the first and overlap it on top by four inches. If there is a valley hem, cut the first four inches off on either side of the second piece.
  
  
• 3). Put two lines of tube sealant spaced two inches apart on the first piece of flashing. The two lines should both be within four inches of the top of the flashing where the two pieces will overlap.
  
  
• 4). Place the top piece of flashing in place in the valley, overlapping the lower piece by four inches. Install the top piece the same as the lower piece, with pancake screws along the top edges.
  
  
• 5). Install two roofing screws on either side of the center of the valley flashing at the joint. Space the screws so they are between the two lines of sealant.
  
  
• 6). Install the roofing sheets. Use the 3-4-5 method to make a square triangle to place the sheets properly. Leave three inches from the water divider on either side free of roof sheeting when installing sheeting near the valleys so water can freely flow.