Determine the size of the tin roofing material that you intend to use on your roof. Building supply houses have brochures that show tin roof panel size and corrugation centerline widths.
Use the ladder to access to the roof of the building where the tin roof will be installed. Measure the total length of the roof from front to back and peak to eave.
Allow a lap allowance for the width of each panel. For example: If the panel's corrugation centerline is 4 inches, multiply A by 4 inches and divide by 12. This gives you the additional amount of feet needed after the lap allowance is added. Divide the length of the roof plus lap allowance by the width of the tin roof panel (A).
Use the same method to allow a lap allowance for a vertical overlap. Substitute 12 inches in place of the 4 inches used for the width lap allowance. Divide the width of the roof plus the vertical lap allowance by the length of the tin roof panel (B).
Multiply A times B. This will be the total number of tin roof sheets required to complete your tin roofing project.