How to Heat Roofing Tar
Generally, hot tar roofs are hard-wearing. However, they require maintenance from time to time just like any other kind of roofing system. Working with roofing tar can be hazardous, though, and especially where heating elements are involved, it's best to have a professional in the mix.
If you're going to work with hot tar yourself, always wear protective gear like boots and thick clothing, work slowly and methodically, and follow all instructions and warnings that come with your products to avoid serious injury and dangerous fires.
Step 1 - Prepare an Area for Heating
Clear a level, open area where you can safely place the cast iron high heat burner you'll use to heat the tar. Cast iron burners are regularly used by roofing contractors to heat tar and asphalt heater pots, more commonly called kettles.
Clean the area and ensure there will be no kids or pets running around underfoot while you're heating, and that there's nothing flammable near your kettle like fabric or wood.
Step 2 - Get Your Roof Ready
Identify the parts of the roof you'll be working on, so you can plan how to effect those repairs quickly and correctly. If you're tarring a whole roof, think about the direction in which you want to work, and the order in which you want to complete the various areas. It usually makes sense to start in a corner and work backward toward a safe exit point.
It's also important to determine how much material you need before you start the process. Once you start heating and applying the tar, it will be tricky to change course or go get more supplies. Plan for at least one gallon for each 100 square feet if you're covering a smooth surface—as much as twice that amount if the texture is more layered, like corrugated metal.
If you're applying tar to a roof for the first time, you'll probably want to tack down a protective layer of something like asphalt to insulate the roof from the heat of the tar you're applying.
Step 3 - Heat the Tar
Watch the weather to decide when to start—you'll need a few clear days of sun for the tar to set properly.
If your tar comes in a full tootsie-roll, you will need to cut it into chunks that will fit into your pot. This is best done early in the morning when it's cool and you can split it like a log with an ax. If the roll gets hot, the ax may stick and it will be very difficult to chop up.
The cast iron high heat burner should already be set up with the tar kettle, so you can start by putting enough tar inside the heating pot to nearly fill it, and lighting the burner. Adjust the flames of the burner as necessary to melt the tar while staying a safe 25 degrees below the flashpoint of the material.
It's good practice to heat the roofing tar batch by batch to ensure its quality. Once heating is completed, carry the tar carefully to the application or repair areas.
Warnings and Precautions
Do Not Attempt Without a Thermometer
You'll need one that can go over 500 degrees Fahrenheit. You don't want your tar to reach that number, though, you want it to remain at around 450-475 degrees. Tar can flash into sudden flame if it's too hot—it's basically chunky oil, which does burn.
If it does flash, cut the heat way down or off immediately and put a solid metal cover over the pot. Watch it closely with the dry chemical fire extinguisher at hand as it can flash again. If it gets out of control, the entire kettle can burn.
If it goes out, wait a minute and then carefully ease a chunk or two of cold tar into the pot and replace the lid. This will help bring the temperature down, which should return stability.
Never Throw Water On a Tar Fire
It cannot be overemphasized how serious a mistake that would be—it will result in immediate and severe burns from superheated water droplets and burning tar flashing back in your face. Plus the distinct possibility of spreading the fire to yourself, your roof, or your yard.
This is the same reason we do not drop a frozen turkey into a boiling hot deep fat frier unless we have a five alarm fire response and marshmallows handy to entertain the neighbors with. It's just that serious, and one moment of inattention or rush can cause disaster.
Do not stir the tar in the pan or it can flash. Allow it to melt slowly to the right consistency while monitoring the temperature constantly. When it is ready it will be a little thick and soupy, but flow well enough to be spread easily with a new clean mop from a metal bucket or with a large hand-held brush.
Extreme care must also be taken when taking the tar from the kettle to the work area, and when mopping it in. Tar can, if hot enough, still flash in the bucket, and spills can be extremely dangerous.
Wear Protective Gear
For this reason, you should wear heavy work boots, long denim or canvas work pants, and a long sleeve shirt when undertaking this project.
You should also have some heavy duty and/or heat resistant gloves, and a protective mask, ideally one with a high density, HEPA rated respirator.
Keep First Aid Handy
If a chunk of tar does land on any exposed skin, don't try to peel it off immediately—that can cause more damage. Instead, run cool water over the area and contact medical support right away if the burn area is significant.
Get Professional Help for Large Jobs
I ran a hot tar kettle on a massive commercial project in Florida where we pumped the hot tar up to the roof, pulled it across in a wheeled tank, and carried it to the mopper in buckets. Sometimes you see fire dancing on top of the tar in the bucket. I have put out my share of kettle fires successfully, thank God.
I would strongly advise the beginning or intermediate level do-it-yourselfer to hire a licensed contractor of good repute to do any job larger than a patch repair for obvious reasons of health and liability.
A kettle fire is a potential disaster that's close kin to an oil well catching on fire. Even for a contractor, but they are trained to keep it from getting to that stage and to deal with it in the event that it does happen.