Is Through-Fastened Metal Roofing a Good Idea?
Yes, as long as it's well detailed it should last as long as a more expensive standing-seam roof.
Q:
I read “Martin’s Ten Rules of Roof Design” and found one part confusing: the preference for through-fastened metal roofing over standing-seam metal roofing. I understand that a through-fastened roof is easier to install and therefore cheaper, but I keep hearing that the screws will eventually back out of the panels and that their holes will leak. Is this true? I also see in the comments section that some people screw through-fastened roofs into 1x strapping installed over the roofing felt. Is that to minimize the consequences of these potential problems?
Nathaniel, via email
A:
Standing-seam metal roofs are great. If you can afford one, by all means install one. But I have no reason to believe that a well-detailed through-fastened metal roof won’t last just as long.
In the days before cordless drills were invented, through-fastened metal roofing was installed with nails. These nails would tend to back out and result in leakage. But that risk disappeared when roofers switched to screws with integral neoprene washers.
I like to install metal roofing over 1×4 or 2×4 purlins because this is a good way to handle any condensation that occurs when the outdoor air is warm and humid but the roofing is cold (either due to nighttime radiational cooling or from a layer of snow on top of the roofing). An airspace between the metal roofing and the roof sheathing allows condensation to drip harmlessly onto the felt paper before evaporating.
View Comments
Have you considered also using counter battens to allow for ventilation directly under the metal roof?
I live in the land of ice/snow and am concerned about sliding snow off of roofs loosing screws. Thoughts?
How about the snap rib metal profile?
I have seen lots of the sheet metal screw back out over time when the metal and wood under it move and contract at different rates. Also the neoprene washers can be damaged by the elements especially if the screws are overdriven some. You see some companies claiming lifetime metal roof the corragated metal might last a lifetime but the screws and washers will go first. Screw through metal sure has its place though.
I really like the purlins idea. That would help with installation on steeper roofs.
It's time for a Fine Homebuilding article on installing metal roofing.
Snap lock is a great product, but requires solid substrate underneath....
I've used both snap-lock and screw down metal roofing. To me the snap lock wins because the fasteners are covered and don't prevent the metal from moving-something it will do reliably. This can enlarge holes around through fasteners. Still, it takes a long time for any leaks to develop if installation is good, and a simple inspection every few years can head off any real problems if screws need to be replaced.
Our climate problem is going to continually drive our customers toward steel roofing. I am concerned that too many contractors/installers are not fastening the roof well enough to withstand high wind events. Customers will need to inform themselves, buyer beware!
Local demand is for the 36"wide corrugated sheet steel. Supplier taught me to always install over 1x4, or on steep slopes, 2x4 and to ensure you are nailing the strapping into the framework of the roof with galvanized ardox gunned nails. Just nailing into the sheathing is not good practice, especially at the eaves where the wind would first have to lift your roof.
I always drive my screws (2") on the ribs and only screw into the flat of the sheet across the first row of screws at the eave, this is to prevent mushrooming of the sheet bottom, which will ruin your sheet alignment. This row has a screw on each rib and each flat.
I have found it best to compress the joint between two sheets with my foot just below where the next screw goes in and fasten that joint rib all the way up first, so as to ensure sheet alignment as you go.
There are installers who will screw the entire roof on the flat, beside the rib; have they not considered what will happen when the sun destroys the rubber washer under the screwhead? Leak City. Hopefully, for those customers, there is a good reason for this that I have yet to understand.
I have a through fastened metal roof (not on purlins) on my detached garage and those washers leak if they are old. They must be periodically checked, all of them, and boy are there alot!
Additionally, I don't care how good the installer is, there's no way a thru fastened metal roof will be leak proof in my experience of having one. Those washers WILL fail. And good luck finding the one or two or three etc. that is the leaker.