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How To Join Aluminum Roof Flashing Using Aluminum Brazing Rods

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Aluminum flashing is great for sealing the edges of chimneys, vents and other objects that protrude through roofs. Its corrosion resistance, light weight and reasonable cost make it an ideal material in this role. However, if you have a need to join aluminum flashing together for an extra-long edge or another reason, then you should understand how to use aluminum brazing rods to create a strong, waterproof seal. Below is what you need and how to do it:

Brazing aluminum flashing – tools and materials needed

  • Methyl acetylene and propadiene disposable gas cylinder
  • Handheld torch for use with disposable cylinders
  • Aluminum brazing rods
  • Acetone
  • Shop towel
  • Stainless-steel wire brush
  • Heat-resistant gloves
  • Eye protection
  • Fire extinguisher
  • C-clamps
  • Hammer
  • Pliers
  • Laser thermometer

Brazing aluminum flashing – a step-by-step procedure

1. Keep yourself and home safe – by far, the most important considerations when brazing aluminum flashing are protecting yourself from injury and keeping your home from being damaged. It is best that brazing take place on level, flat ground whenever possible, so you can have a stable location from which to work. However, if you must braze aluminum flashing on your rooftop, be sure to always have a fire extinguisher close-at-hand. In addition, wear eye protection in case of splashing molten aluminum, and also be sure that you wear appropriate gloves to protect your hands from burns or cuts.

2. Clean the flashing – a dirty section of flashing will prevent the brazing process from working. Aluminum normally carries a thin layer of aluminum oxide, and this serves as a barrier against your efforts to braze it. However, pouring acetone on the aluminum pieces and wiping it away with a dry cloth can remove most of the flashing contamination. Be sure that you allow the acetone to completely evaporate before using a torch, and seal the bottle to prevent a flash fire from the fumes.

After using acetone to break down contamination and aluminum oxide formation, vigorously brush the surface of the aluminum with a stainless steel brush; the brush will further remove traces of aluminum oxide and provide a virgin surface for brazing.

3. Prepare the torch and cylinder – when brazing aluminum, the metal's temperature should be hot enough to melt the rods; also, keep in mind that aluminum is an excellent conductor of heat, and it will quickly dissipate the torch's energy. A cylinder containing a mix of methyl acetylene and propadiene gases produces a flame several hundred degrees hotter than propane and is ideal for brazing aluminum.

After screwing the torch into the cylinder, ignite it using a spark igniter and form a strong, blue flame. Don't permit the flame to flicker wildly or excessively.

4. Align the pieces of flashing – position your two pieces of flashing so they overlap by a few inches. Next, clamp the pieces together using several C-clamps to hold them firmly in-place.

5. Heat the joint to the aluminum rod's melting point – apply the blue tip of the flame from the cylinder torch to the aluminum flashing, and periodically check the temperature of the metal using the laser thermometer. When it reaches around 700 degrees, quickly position and apply an aluminum brazing rod to the joint; if the flashing is heated sufficiently, the brazing rod will melt and begin to flow into the joint between the two pieces.

When applying aluminum, keep the work piece level and straight. An inclined piece will permit the melted aluminum to roll off and spill to the floor, bench or even you. If you must work on your rooftop, be extra cautious with the metal, and use scrap lumber to construct a temporary "bench" for resting your work pieces.

6. Continue heating and brazing – after placing the initial bit of aluminum, continue alternating the torch flame and the aluminum rods. Don't overheat the aluminum flashing, or you may scorch or melt it. Continue brazing until the joint between the two pieces of flashing is thoroughly covered, neat and even.

7. Test your work – once you have finished brazing, you can test your work using a hammer and pliers. Tap on the seam with a hammer, and flex the flashing slightly using a pair of pliers. The seam should hold up against most abuse and be waterproof, if properly brazed.

These instructions can help you as you perform roof repairs.