The minimum base size is FAL (plus 1") X FAW (plus 1")The maximum base size is L (minus 3") X W (minus 3") Note: The lid size is 5" greater than the base sizeExample: FAL= 32", FAW= 16"L= 52", W= 24"The minimum base size would be 33" x 17"The maximum base size would be 49" X 21" Note: Based on the above example any cap that falls between the minimum and maximum size ranges will work. The minimum will provide protection of the flues at the lowest cost. The maximum will give the most protection and achieve the most balanced look. L Length of entire chimneyW Width of entire chimneyFAL Flue Area Length FAW Flue Area Width The screen height should be at least 5" higher than the tallest flue.