Metal Roof Framing Tips

Various types of metal are used in metal roofing. The roofing can be steel, stainless steel, aluminum, special alloys, or copper, with steel being the most common. It is possible to construct a roof using metal tiles in shingle or panel form, pre-manufactured panels, or rolled sheets.

Some manufactured metal roof panels do not require decking underlayment, since they are rigid enough to be considered structural members of the roof, while others require full sheathing or decking. In either case, installing metal roofing over OSB plywood decking and felt plus a layer of insulation will help deaden noise (rain, hail, etc.) and further eliminate condensation problems.

Another consideration is the shape of the roof surface to be covered; since standard manufactured panels comes in large panel sizes, which work best on simple roofs with large areas. Pre-engineered custom panels are also available, delivered to your construction site pre-cut and numbered, with a layout for installation. Metal shingles are appropriate for complex roofs with lots of hips and valleys entailing dormers, chimneys and other projections. Shingles are sold in both large panels and single shingles.

Weight

Weight is a big factor when designing and building metal roofs. Most types weigh from 50 to 150 pounds per 100 square foot, compared to 240 pounds 100 sq. ft for 3 tab asphalt shingles, 400 lbs per 100 sq/ft for wood shake, 900 lbs/100sq ft for concrete tiles, or 1,100 lbs/100sq ft for glazed clay tiles. Roof rafters or trusses for supporting a metal roof system can be of less strength than those required for slate roofing for example.

Most forms of metal roofing can be installed on minimally pitched roofs with no risk of leaks. The minimum roof pitch would be 3-in-12, in other words, the roof rises 3 inches vertically for each horizontal 12 inches.

Roof Decking Loads

Roof decking or a structural metal panel takes all the loads they are subjected to and transmits them to the support framing they are attached to and span. Deformations that frames may be subject to include bending forces (one side shortening under compression while another side lengthens under tension), thermal expansion/contraction differentials, lateral displacements due to settling or seismic shifts, and swelling or shrinking due to moisture.

Typical site-installed framing is made up of joists, purloins, beams and trusses or rafters. The spacing of framing members is determined by several factors related to the decking used; load carrying capability of the decking or structural roofing panels, it’s deflection allowables, and requirements at decking joints for support all come into play. The size of framing used is determined by allowable bending and shear stress for short spans, and by deflection allowables in long spans.

Metal Framing for Metal Roofs

When steel framing is used for a metal roof, typical joists and rafters are made of light gauge (20 – 12) cold rolled shapes, fastened together with welding or self-drilling self-tapping screws. Beams are usually hot-rolled steel shapes welded or bolted together and spaced at 6 to 14 feet apart. Trusses used as primary girder support can take the form of monoplane trusses, bowstring configuration, scissors truss, or sawtooth.