We recommend ½ cup of powder per gallon of water, we sell a 30 gallon tank so to fill that with product you would need 15 cups or 7.5lbs of powder. We find that a 30 gallon tank will spray about 3000 sq ft. so based on these numbers a 40lb bucket would cover 16,000 sq ft. However coverage and be improved greatly by applying the product correctly.
Apply product from a ladder with stabilizers resting on the roof above the gutter line. Position yourself at one corner of the roof to start and begin spraying at the peak of the roof directly up from your position, spray across the ridge allowing the product to run down over the roof covering the entire roof down to the gutter the move to the opposite corner of the roof and repeat.
Most inexperienced roof cleaning contractors start by positioning themselves in the center of the roof, figuring that they only need to set up one time in the middle of the roof and can reach the entire roof this way. The problem with this method is the “splash” when you are spraying the left or right edge of the roof from the center of the roof the product goes bouncing and splashing off the roof over the side, you are losing product by the gallons and creating more cleanup work for yourself as it shoots off the roof all over whatever is below. Another problem with that is that if the wind is blowing all that material splashing off the roof blows back against the house covering windows that you will need to clean later creating more work.
Final thoughts, keep the product on the roof, allowing the Roof Wash to flow down over the roof as it heads towards the gutter and this method will greatly extend the coverage per gallon of product.