#31 – Should a Metal Roof be Installed Over Battens?
We frequently are asked whether metal roof should be installed on batten’s or direct to the roof deck. By design, some metal roofs must be installed over battens, some must never be installed over battens, and some can be installed over battens or over solid decking. So first and foremost, it’s critical to choose the right product and install it according to manufacturer’s specifications. Keep in mind for residential applications, we never suggest installations over battens unless there’s a solid deck beneath the battens.
Installations over battens alone are just an invitation to condensation issues in the attic.
Next, we highly advise the use of a layer of underlayment over the old shingles or decking whether or not battens are used. We interpret the international building code as requiring this, and it’s just good practice. We normally suggest a layer of one of the higher quality synthetic underlayment and ice and water shield if required by code. Now, if your home’s existing shingles are pretty curled you could install text you metal roof such as a metal shingle, shake or tile directly over those curled shingles along with underlayment. You also could install a corrugated through-fastened metal roofing product and probably not have issues with that either. However, a true standing seam – especially one without any sort of striations or stiffening beads in the center of the panels – will probably show oil canning as the result of going over old shingles. In that case the old shingles should be removed or batten’s could be installed over those old shingles.
Now as to the potential benefit of batten’s, they do create a thermal break to help minimize heat transfer from the metal to the roof deck. That’s a good thing. However, if you choose a metal roof that is either light in color or has reflective pigment in the paint (meeting energy star standards) that will be very helpful as well. Your attic ventilation and insulation will also help with Summer Energy Efficiency.
One effective way to use battens is to cross batten. This involves putting down vertical battens first, attaching them through the roof deck to the rafters, and then you put down horizontal battens for the roof panel attachment. The resulting vertically-oriented Chambers can then be ventilated by bringing fresh air in at the bottom, and exhausting it out at the top. This has very positive results as far as keeping heat out of the attic, and it can also help avoid rooftop Ice Dams in the winter. However, cross battening significantly raise the roof level. This means gutters will have to be rehung, and it also could interfere with skylights, clearstory windows, dormers, even chimneys and plumbing stacks. Again if you have good attic ventilation and insulation, those things alone are very helpful and can be more than adequate for most homes. One thing to remember is that many of the products that are installed over battens have exposed fasteners. Those can be something many folks want to avoid. As a great option to the expense and bother of battens, there are concealed faster metal shingles, shakes, and tiles available. These products usually have an integral airspace or an insulated cavity between the metal and the roof deck. This creates a thermal break which is very similar to a vertically seamed metal roof installed on battens. The dead air space in the hollow beneath these panels acts like the dead air space between two panels of glass in a thermal pane window. This blocks conductive heat transfer and helps keeps attics cooler during the summer.
Finally, if a structure has very limited insulation and ventilation – especially ventilation in the attic – battens can be helpful. Especially if a vertically seamed metal roof is being installed. Again this is from a thermal break / thermal bridging standpoint. A home with limited attic ventilation is at high risk that moisture originating inside the living space can migrate to the attic, and cause condensation there. If that moisture hits a cool surface it will condense. Condensation will lead to mold etcetera, which of course is not good. While you’re home I have been fine with asphalt shingles and no ventilation, the addition of a vertical seam metal roof will reduce the roof deck temperature even just a slight bit. That can be enough to be the straw that breaks the camel’s back and causes condensation in the attic that never existed before. Additionally we want to point out if you install tighter windows, house wrap and siding, those things will reduce the ability for moisture inside the home to exit through the walls. So if you plan those kinds of improvements in the future, don’t do something now to the roof that can make it more prone to condensation once those other things are done.
In conclusion, battens can sometimes be helpful, but we find those occasions to be few and far between (limited primarily to homes with insufficient attic insulation and ventilation. And, a great alternative to battens are the metal shingle, shake, slate, and tile products which have an integral dead air space between the metal and the roof deck, serving a function very similar to that of battens.