Hy-Grade has evaluated no wood strapping, single wood strapping and double wood strapping. We choose to DOUBLE STRAP.
Let’s take a look at all options:
No Strapping – without strapping you’re left using a membrane on the roof deck. This membrane is there to protect against the condensation that will occur due to little or no airflow under the system. The condensation is caused when the warmer air above the roof decking, which is warmed from the attic space below, meets the cooler metal roof. The moisture in the air in between these two layers condenses due to the temperature difference (at the dew point). The membrane is used to try to keep the condensation from leaking back into the house.
Single wood strapping is better than no strapping, as it creates a wider gap between the existing roof decking and the metal roof, but doesn’t afford enough ventilation to resolve condensation issues. It also does not allow leveling of the roof, which corrects swells and swales on the roof.
Double wood strapping solves all these problems by lowering dew points (due to its self-ventilation capabilities) and leveling while adding to the overall strength and stability of the roofing system.
The 1×3 spruce battens are vertically attached to the existing roof with 3 ¼” framing nails that are “dovetailed” (nailed on angles) into the roof deck, most often through the existing shingles.
Dovetailing the nails into the roof deck increases the strength of the roofing system attached to the strapping because the angled nails are almost impossible to rip out of the roof deck; this is important during high winds.
2×2 spruce battens are nailed horizontally to the 1×3 battens also with 3 ¼” framing nails. Then the Hy-Grade steel roof shakes are nailed to the 2x2s with 2” “hot dipped” (galvanized) roofing nails. Since the nails and shakes attached are both made of galvanized steel, there is no chance of corrosion due to dissimilar metals. Again, both steel, both galvanized, no corrosion!
Hy-Grade’s double strapped steel roofing system helps remove hotter air from your attic space(s) due to its self-ventilation capabilities which lowers your energy bills in the summer. Along with proper attic insulation and ventilation, it can lower your heating bills in the winter too.
A 2012 study prepared for the U. S. Department of Energy Buildings Technology Program and conducted at the Oak Ridge National Testing Laboratory in Oak Ridge, TN showed that “Above Sheathing Ventilation” or ASV, which is exactly what double wood strapping creates, saves more than 30% in energy consumption when compared to direct-to-deck roof systems.
The most surprising find; however, was that a black metal roof attached with AVS (double wood strapping) affords as much energy savings as a “cool roof” coloured direct-to-deck roof system with a solar reflectance of 0.25. This is because the convection air currents created with a black steel roof does as good of a job removing/keeping heat from your attic spaces as a “cool roof” does.
Double strapping with wood is the only way to go!