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Metal Roof Gauge, Steel vs. Aluminum, Galvalume and Kynar: The Four Words on a Quote That Decide the Next Forty Years
Roofing
Gauge, Metal, Coating, Paint: The Four Words That Separate a $12,000 Metal Roof From a $35,000 One
Two metal roofs can look identical on installation day and be twenty-five years apart in life. The difference is in four specifications that a good quote names and a cheap quote leaves off: how thick the metal is, what metal it is, what it was coated with at the mill, and what paint was baked onto it. Here is each one, what the numbers mean, and which combination you want.
Gauge: the lower the number, the thicker the steel
| Gauge | Thickness | Used for | What to expect |
|---|---|---|---|
| 29 gauge | about 0.36 mm (0.014 in.) | Sheds, barns, the cheapest exposed-fastener panels | Dents from hail and a dropped tool; oil-cans (ripples) in sun; fine on a shed |
| 26 gauge | about 0.45 mm (0.018 in.) | Residential panels, snap-lock standing seam, most metal shingle equivalents | The house minimum; stiff enough to walk on with care and to resist most hail |
| 24 gauge | about 0.61 mm (0.024 in.) | Standing seam done properly, mechanically seamed roofs, commercial | Flat panels that stay flat, hail-proof in practice, 30% more steel than 26 |
| 22 gauge | about 0.76 mm | Architectural and industrial; rare on houses | Heavy and expensive |
The gauge is also why a cheap quote looks cheap. Between 29 and 24 gauge the steel itself nearly doubles in cost, and the panel stiffness is what makes a long standing seam panel lie flat instead of showing every rafter in low afternoon light (oil-canning). For a house: 26 gauge minimum on panels and snap-lock seam; 24 if the panels are long, the roof is in full sun, or you simply want it done once. Aluminum is specified by thickness instead of gauge: 0.032 in. is the residential standard.
Steel or aluminum
Steel
Stiffer, cheaper, and the standard for standing seam and panels. It rusts if the coating is breached, which is why the substrate coating (next section) matters and why cut edges are hemmed or sealed. Fine anywhere inland; fine on most lakeshores if it is Galvalume.
Aluminum
Cannot rust. Lighter, softer, dents more easily, expands more with heat (so the fastening system has to allow for it), and costs 15 to 30% more. The choice within a kilometre of salt water (not an Ontario problem) and the common choice for interlocking metal shingles, where the small pieces hide its softness. Several Ontario metal-shingle makers are aluminum.
The substrate coating: Galvalume or galvanized
Bare steel would rust in a season. At the mill, the sheet is dipped in either zinc (galvanized, graded G60 or G90 by the weight of zinc) or an aluminum-zinc alloy (Galvalume, graded AZ50 or AZ55). Galvalume lasts two to four times longer than galvanized in the same conditions, resists the slow dull-and-pit of ordinary weathering, and is what every reputable roofing panel is made from now; galvanized G90 is acceptable on panels for outbuildings and is what the cheapest quotes are. The one place galvanized beats Galvalume is cut edges and scratches, where zinc “heals” the exposed steel a little better, which is why Galvalume panels get hemmed edges. If the quote does not say Galvalume (or AZ50), ask.
The paint: Kynar or SMP
| PVDF (Kynar 500, Hylar 5000) | SMP (silicone-modified polyester) | Plain polyester | |
|---|---|---|---|
| What it is | A fluoropolymer resin, 70% PVDF, baked on at the mill | A polyester resin toughened with silicone | Basic baked enamel |
| Fade and chalk | Very slow; the colour at year 30 is close to year one | Noticeable fade and chalking at 10 to 15 years, worse on dark colours | Fades and chalks in 5 to 10 |
| Paint warranty | 30 to 40 years on film, 25 to 30 on chalk and fade | 20 to 25 years on film; chalk and fade shorter | 10 or none |
| Scratch resistance | Softer; scratches in handling | Harder | Hard, brittle |
| Cost | The premium, roughly $1 to $2 per sq ft on the roof | Standard | Cheapest |
| Where it belongs | Any house roof you want to still be the colour you chose | Light colours on outbuildings; acceptable on a house in a pale grey | Sheds |
Dark colours in SMP are the ones people regret: a charcoal roof that is a streaky medium grey by year 12, with the overlaps a different shade from the exposed face. Kynar in the same colour is still charcoal at 25. The paint is 5 to 10% of the roof cost and it is the whole appearance for forty years.
The combinations
- The 40-year house roof: 24 or 26 gauge steel, Galvalume AZ50, Kynar 500 paint, concealed fasteners. Or 0.032 aluminum with Kynar for metal shingles.
- The sensible cottage or garage roof: 26 gauge steel, Galvalume, SMP in a light colour, exposed fasteners with stainless-capped screws.
- The 12-year roof that was sold as 40: 29 gauge, galvanized G60, polyester paint in dark brown, over the old shingles on strapping. It will fade in 8, the screws will weep in 15, and the strapping will show in every low sun.
On the quote, in this order: system, gauge, metal, substrate (Galvalume AZ50 or galvanized G90), paint system (PVDF/Kynar or SMP) with the paint warranty years and the mill’s name. Five words that a roofer who knows what they are selling writes without being asked. The metal roof cost page shows what each combination costs.
Questions people ask
What gauge metal roof is best for a house?
26 gauge minimum; 24 gauge for standing seam with long panels or in full sun. 29 gauge is for sheds and barns.
Is 26 gauge or 29 gauge thicker?
26. Lower number, thicker steel. 26 gauge is about 25% thicker than 29 and noticeably stiffer.
Is Galvalume better than galvanized?
For roofing panels, yes: two to four times the life against ordinary weathering. Galvanized G90 is acceptable on outbuildings; galvanized G60 is a shed. Galvalume with hemmed edges is the house standard.
What is Kynar 500?
A PVDF fluoropolymer paint baked onto the panel at the mill. It holds colour and gloss for decades where cheaper paints chalk and fade in 10 to 15 years. The premium roof paint; ask for it by name.
Does a metal roof fade?
SMP and polyester paints fade and chalk noticeably in 10 to 15 years, dark colours worst. Kynar fades very slowly; a 25-year-old Kynar roof is still close to its original colour.
Four words, on the paper
Gauge, metal, Galvalume, Kynar. If the quote has them, you are comparing roofs. If it does not, you are comparing prices for things you cannot see.
Sources and further reading
- Vicwest: Canadian metal roofing panels and standing seam; gauge and finish data sheets.
- Ideal Roofing: Ottawa-made metal roofing, panels and shingles, with the coating warranties on the site.
