Clean Roof: Removing Rust Stains from a Metal Roof

What You'll Need
Oxalic acid
Hose with spray nozzle and spray bottle attachment
Wire brush
Drill with wire brush
Trisodium phosphate
Structural steel primer
Acrylic or solvent based paint

A clean roof looks wonderful on a house, but if you have metal roofs, rust stains can become a problem. Metal roofs have come a long way from the old days of using corrugated steel. These days, they’re often very elaborate and available in different colors to match the rest of your house. There are a number of advantages to having a metal roof on your house. It won’t need to be replaced anywhere near as often as a shingle roof and ultimately offsets the higher initial purchasing and installation costs. It will keep its looks much better than a shingle roof and hold up to all types of weather. However, as with all metals that come into frequent contact with moisture, it’s possible that your metal roof could develop rust, especially if it’s an older roof or hasn’t been installed properly. Sometimes it could be rust leaching onto the roof from an antenna or from a metal vent pipe. To get rid of these rust stains, follow these easy do-it-yourself process:

Step 1 - Removing the Rust

The simplest way to get rid of rust is to use oxalic acid. You’ll need a fairly concentrated solution of about 1 pound of acid per gallon of water. Apply it to the rusted areas as a spray using a hose with a spray bottle attachment. The rust should just drain away. Follow by rinsing with water until all the solution has gone. If there are problem areas, use a wire brush lightly and carefully to remove all signs of the rust.

Step 2 - Find the Cause of the Rust

Next, you need to investigate the reason for the rust. If it’s from a pipe, antennae or flashing, make suitable adjustments to avoid further problems. If the problem is with the roof itself, contact the company that made it. You might have to try and replace a section to avoid further problems.

Step 3 - Keeping Rust Off Galvanized Metal Roofs

If you have a house or outbuilding with an older galvanized metal roof, the best way to avoid rust problems is to paint the roof. Start by power washing the roof. This will remove any scaled rust as well as any flaking paint. If the roof has been painted some years before, use a drill with a wire brush to eliminate problem rust patches.

After that, you will need to clean the roof with trisodium phosphate, or TSP. This is an acid wash that will leave the roof completely clean. If your roof has algae or mold, you’ll need to add a little bleach to the TSP in order to remove it. Rinse the roof with water and allow it to dry fully.

Once the roof is clean, you’ll need to prime it. Use a standard structure steel primer. In most instances a single coat should be adequate. Once you’ve done that, you’re ready to apply the final topcoat in the color of your choice. You can use either acrylic or solvent based paint. Exactly what you use will depend on the weather where you live. Consult the staff at your local paint store, as they will be able to offer the best possible advice. A good paint job should last about 10 years before you need to apply fresh material.