One must know the exact total surface area (in square feet) of a roof in order to be able to figure the correct amount of roof coating material needed to properly coat (paint or waterproof) a roof.  The following factors affect significantly the correct calculation of surface area, and thus the correct amount of coating materials needed.

(A) Roof Overhang & Exterior Wall Thickness (see Col. 1 on chart below)

The inside dimensions of the building do NOT include the overhang of the roof beyond the exterior walls or the thickness of the exterior walls.  For example, if the interior dimensions are 50 ft. x 60 ft. for an interior area of 3,000 sq. ft., and there is a 2 ft. overhang all around the building, and the exterior walls are 10 inches thick, then the flat surface area of the roof must be calculated not by the formula, 50 ft. x 60 ft., but on 55.67 ft. x 65.67 ft., which equals 3,657 sq. ft., which is actually 21.9% more than the interior area of the building!

(B) Roof  Slope (see Col. 2 on chart below)

The slope of the roof will increase the square foot surface area to be coated, compared with the flat square foot surface under the roof.  For example, if the slope of the roof is "4 in 1", i.e. a 4-inch rise for 1 foot (12 inches) of flat surface, the sloped roof surface area will be 5.21% greater than the flat surface area it covers.  Thus, in the example above, the 3,657 sq. ft. under roof equates to 3,848 sq. ft. of sloped roof surface.

(C) If Flat Tile - You MUST add for Tile Thickness (see Col. 3 on chart below)

The "butt" end of the tile, if the tile is one (1) inch thick, and the length of the top surface of the tile to be covered is twelve (12) inches, will increase the total surface to be coated by 8.33%.  Thus, in the example above, the 3,848 sq. ft. of sloped roof surface actually amounts to 4,169 sq. ft. of surface area that must be covered by the Roof coating.

(D) If Barrel Tile Roof - You MUST add for the Curvature of the Tile, plus the "butt" ends of the Tile (see Col. 4 on chart below)

To find the total surface area that must be coated, assuming the radius of the barrel tile is 3", (= 6" diameter) and the tiles are laid so that the top of one inverted tile to the top of the next inverted tile is 9", then one must add 33.33% to determine the actual surface area to be coated.  Note:  One must also add for the "butt" ends of the barrel tiles, just as explained in "C" above.  Thus, in the example shown, if the roof is covered with barrel tile, the actual square footage of surface area to be covered is not 4,169 sq. ft., but rather it is 5,559 sq. ft.

(E) If Corrugated Sheet  - steel, aluminum, fiber-glass (not shown on chart below)

To find the total surface area that must be coated, if the corrugation is 2 1/2 inches from the top of one corrugation to the top of the next corrugation, and the depth of each corrugation is 5/8 inch, then one must multiply the width of the sheet by a factor of 1.08 (before multiplying by the length) to determine the actual surface area to be coated.

(F) Wastage (not shown on chart) Never does 100% of the coating in a container end up on the surface being coated.  There is always a certain amount of wastage, such as the coating material one cannot easily get out of the container, or the coating material that is left in the applicator, either brush, roller, or sprayer.  Professional painters, based on years of experience, know that they must add an extra percentage for wastage, normally an extra 10% for brush or roller application, and an extra 25% for spray application.  Be sure you calculate this factor when determining how much Roof Coating you will need.

Interior
Area

ADD for Thickness of
Ext. Walls (10") &
Roof Overhang (2 ft.)

ADD for Slope
of Roof
(4" in 1 foot)

ADD for Tile
Thickness
(1" butt end)

ADD for Barrel
Tile Curvature
(3" radius)

Equals Flat Area
from Drip Edge to
Drip Edge of Roof

Equals Surface Area
for Fiberglass, Asphalt
Shingle, Roll Roofing,
Tar Paper Roofs, etc.

Equals Surface Area
for Flat Tile Roof

Equals Surface Area
for Barrel Tile Roof

Sq. Ft

1,000
1,500
2,000
2,500

3,000

3,500
4,000
4,500
5,000