Most metal roofing systems installed in Canada are typically installed over 1 layer of existing asphalt shingles. Many companies cut costs by applying impermeable vapor barriers over the existing asphalt shingles. When this happens, residual moisture in the shingles becomes trapped between the shingles and the impermeable vapor membrane and can cause mold to develop. For this reason it is important to ensure that a permeable vapor membrane with a diffusive air layer is installed which prevents condensation build up between the vapor barrier, the shingles and the roofs sheathing deck, thereby enabling the existing roof to both "breath" and remain 100% watertight.
Why Double Strapping and Dual Ridge Ventilation Eliminates the Condensation Buildup Problems Associated with Traditional Metal Roofing Products
The image above shows a correctly constructed double strapped roof deck that provides both proper drainage and ventilation
THE QUESTION COMMONLY ASKED BY METAL ROOF BUYERS
Will a double strapped metal roof lead to condensation forming beneath my metal panels? Great question.
The inherent flaws of single strapped metal roofing are well documented and have given self ventilating metal roofs built on double strapping an unwarranted reputation of trapping moisture beneath the steel roof panels. Condensation occurs because warm, moist air collects in an air space which has contact with a cooler metal roof surface. Whenever temperatures drop, the temperature of metal roofing drops simultaneously . If warm, moist air contacts this cool metal roof surface, its “dewpoint” is reached and condensation develops.
There are no exceptions to this rule, condensation forms beneath both metal shingles and continuous sheet steel panels.
Metal shingles have no way of dealing with this condensation other than the protection of the vapor barrier system applied beneath the shingles.
While single strapped and double strapped roofs both provide ventilation, only a double strapped roof provides both the drainage and ventilation needed to deal with condensation build up behind metal roofing.
THE CONDENSATION RISKS ASSOCIATED WITH SINGLE STRAPPED METAL ROOFS
Condensation naturally forms behind all steel roofing. Single strapped metal roof systems provide no drainage which can eventually rot out both the wood strapping and the sheathing deck that holds the strapping and metal roof panels in place.
Roof failure caused by single strapping is more common with interlocking metal shingles because they allow small amounts of wind driven water to collect behind the single strapped roof. This happens because the strapping is only mounted horizontally directly to the roof deck.
NEVER LET A CONTRACTOR INSTALL ONLY SINGLE STRAPPING BENEATH YOUR NEW METAL ROOF. EITHER INSTALL 'DIRECT TO DECK' & FORFEIT ENERGY SAVINGS, OR DOUBLE STRAP YOUR ROOF. SINGLE STRAPPING WILL CAUSE MOISTURE BUILDUP AND ROOF FAILURE
A final word on metal roof condensation buildup from the President of the MRA
A double strapped roof provides a good solution for addressing the problem of condensation forming beneath metal roofing according to Todd Miller, President of the Metal Roofing Association of North America.
Read more about this on the MRA website (Metal Roofing Association )
The illustration above shows the ventilation characteristics of a single strapped interlocking metal roof tiles.
The flaw of single strapped metal roofs is that the horizontal battens mounted to the roof deck trap condensation and eventually rot out the strapping and the roof deck