By Paul Bianchina
Sheet metal - any of a group of metals thinner than 3/16 of an inch - is another material that is in common use on construction sites, but that may not be all that familiar to the home handyperson. It's a very easy material to work with, however, and one that has a lot of uses around the house.
The two most common sheet metals for construction use are sheet steel and sheet aluminum, though sheets of brass, copper and other materials are also commonly available. Sheet steel and aluminum are made by passing large pieces of metal, called slabs, through a series of huge rollers to gradually reduce their thickness. Sheet steel then often goes through a galvanizing process in which it is coated with zinc for better resistance to the elements. Sheet aluminum may go through an additional process called anodizing, in which a color coat is chemically bonded to the aluminum - a deep bronze color is the most common in construction.
Sheet metal is typically referred to by its gauge, which is the thickness of the metal - the smaller the gauge number, the thicker the metal is. Some sheet metal, particularly that used in the manufacture of steel studs and other steel-framing members, may also be referred to by its thickness in mils (1 mil = 1/1000 of an inch). Most of the sheet metal that you will find in use for construction purposes ranges from 12 to 22 gauge, or approximately 97 to 27 mils thick.
Cutting and Forming
Most construction sheet metal, such as that used for roof and wall flashings, is thin, light and easy to work with. For most cutting tasks, the only tool you'll need is a pair of aviation snips. Aviation snips are like big scissors for sheet metal, and are universally available as straight-cutting (yellow handles); left-cutting (red handles, which are best for a right-handed person); and right-cutting (green handles, which are best for a left-handed person). Start with a pair of straight cutters, then add as left or right as needed.
If you have a lot of sheet metal to cut there are electric shears available to buy or rent in both corded and cordless models, which cut quickly and cleanly with much less strain on your hands. For metal framing applications, an electric chop saw with a metal-cutting blade is very useful, and you can also cut it with a circular saw, reciprocating saw or jig saw that's equipped with a metal-cutting blade.
Professionally, sheet metal is bent in a large forming tool called a brake. The sheets are fed into the machine and then bent to the desired angle, leaving a sharp and accurate crease in the metal. Brakes can be found in sheet metal shops and many heating and air conditioning shops - you can purchase your sheet metal there, and they will custom bend it to your specifications for a nominal fee.
For small pieces, you can bend the metal yourself by clamping it between two blocks of wood or metal that are positioned along the desired line of the bend, and then bending the metal along the edge of the block by hand, or by tapping it repeatedly with a soft mallet. Heating the line with a propane torch will assist with bending thicker pieces.
Fastening
Sheet metal pieces such as flashings can be fastened down using roofing nails. Most flashings are thin enough that you can hammer the nails right through the flashing and into the wood below, but heavy gauges may require pre-drilling.
To fasten two pieces of sheet metal together, one of the most common methods is to first pre-drill a hole through both pieces and then join them using sheet metal screws. These screws have a sharp point and a fairly aggressive thread that cuts the hole larger as it's driven in, and holds the two pieces firmly together. For lighter gauge sheet metal, there are also self-tapping screws available that don't require pre-drilling. Sheet metal screws are available in several different lengths and gauges, and are especially useful for ductwork and other installations that may require disassembly.
Another popular joining method is the pop blind rivet. A hole is predrilled through both pieces, the rivet is inserted through them both, and then the shaft of the rivet is gripped with a pop blind rivet gun. Squeezing the handle on the gun pulls the rivet shaft and deforms the rivet until the shaft "pops" off, leaving the rivet to secure the metal. Pop blind rivets hold well, but are not removable.
Finally, remember that sheet metal is sharp! When working with it, you will want to wear gloves and other appropriate clothing to protect yourself from sharp edges and corners, and from the slivers and burrs left from cutting. Also, be sure you wear eye and ear protection when cutting it with a chop saw or circular saw.
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