Proper Installation Over Shingles- Metal Roof Noobie

I frequently am asked whether metal roofs should be installed on battens 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 solid decking. So, first and foremost, it’s critical to choose the right product and install it per manufacturer specifications. And, keep in mind, for residential applications, I never suggest installations over battens unless there is a solid deck beneath the battens. Installations over battens alone are an invitation to condensation issues on residences. Next, I highly advise the use of a layer of underlayment over the old shingles or decking whether or not battens are used. I interpret the International Building Code as requiring it and it’s just a good practice. I normally suggest a layer of one of the higher quality synthetic underlayments, and ice and watershield if required by code. If your home’s existing shingles are pretty curled, you could install a textured metal roof such as a metal shingle, shake, or tile directly over the curled shingles along with underlayment. You also could install a corrugated through-fastened metal roofing product and probably not have issues. 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 curled shingles. In that case, the old shingles should be removed or battens should be installed. Now … as to the potential benefit of battens. They do create a thermal break to help minimize heat transfer from the metal to the roof deck. That is a good thing. However, if you choose a metal roof that is either light in color or has reflective pigment in the paint, 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. It can also help avoid roof top ice dams in the winter. However, cross battening significantly raises the roof level. This means gutters will have to be re-hung. It also could cause the roof to interfere with skylights, clerestory windows, dormers, and even chimneys and plumbing stacks. And, again, if you have good attic ventilation and insulation, those things alone are very helpful and can be more than adequate in most cases. One thing to remember is that many of the products that are installed over battens have exposed fasteners. That can be something that some homeowners want to avoid due to possible issues developing over time with the fasteners. As a great option to the expense and bother of battens, there are concealed fastener 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 airspace in the hollow beneath these panels acts like the dead airspace between two panels of glass in a thermal pane window. This blocks conductive heat transfer and helps keep attics cooler. 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, etc. This is not good. While your home may 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 cause condensation in the attic that did not exist before. Additionally, I want to point out … if you install things in your home like tighter windows, house wrap and siding … those things will reduce the ability for moisture inside the home to exit through the walls. This can cause higher moisture levels in the attic. So, if you plan those improvements now, do not do something to the roof that could make it more prone to condensation when those other things are done. In conclusion, battens can sometimes be helpful but I 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 shingles, shake, slate, and tile products which have an integral dead air space between the metal and the roof deck, serving a function similar to that of battens.

Here are links to videos on ventilation: http://www.asktoddmiller.com/roofing-help-videos/buying-a-new-roof-part-4-ventilation/ http://www.asktoddmiller.com/roofing-help-videos/buying-a-new-roof-part-4-2-ventilation/ I often get asked what the importance is of underlayment beneath a metal roof. What does it do and is it really necessary? Interestingly, over the past several years, we have seen significant changes in the types of underlayments being used. The typical underlayment being used today is a low weight polymer based synthetic underlayment which lasts longer and is easier to install than the old asphalt based felts or tar papers. We also see ice and watershield underlayments being used where required by code, usually on the roof perimeter and up the valleys. In all cases, underlayments beneath metal roofs should have smooth, non-granulated surfaces so they do not scratch the backs of the metal roof panels. In warmer climates beneath metal roofs, especially metal roofs that have direct contact with the underlayment, high temp underlayments are commonly called for. And, even if the metal roof is installed on battens or purlins, there should still be an underlayment on top of the roof deck beneath the battens. So, why are underlayments necessary? First, they are required by building code. My interpretation of the code also requires them even if the old shingles are left in place. Next, they provide a good surface for the roofing installers to work on. They also protect the structure from water intrusion before the roof project is completed. And, while condensation on the back side of metal roof panels is not a common thing, they will keep that condensation if it does occur from reaching lumber or getting inside of the building. Finally, if the roof should have a flashing or something that occasionally in certain extreme weather conditions bleeds a little water, a quality underlayment will prevent that water from getting into the structure as well.

Synthetic underlayments should be installed with plastic cap nails, per the manufacturer specifications. Good question!

Thanks for the response. The contractor that quoted me said that he has always just screwed the metal directly to the decking on top of the shingles. From your response, the contractor is right with exception of some sort of underlayment. What do you guys perfer/ recommend for underlayment?

Is there anyway you can send me a link to a commonly used polymer underlayment? Everything I see on lowes website is very expensive. Thanks.

What is the best way to hold the underlayment down, roofing nails?