Roof strapping refers to wooden strips to which the metal roofing material is attached. The nails/screws attach to these strapping strips. The strips, also referred to as furring strips, nail strips, purlins or battens, provide a smoother surface when installing a metal roof over an existing roofing material, such as shingles.
It also provides some space between the roofing and the underlayment, providing additional insulation, airflow to prevent moisture problems, and some sound deadening.
Installing roof strapping is simply attaching either 1x4s or 2x4s to the roof rafters and then you attach the metal roofing to the strapping.
Do You Need to Put Strapping Under Metal Roofing?
This is an ongoing argument among roofers and do-it-yourselfers alike. Some say they never use furring strips; some say it’s only necessary when installing a metal roof over existing shingle roofing, others say metal roofs should always have furring strips. The manufacturer of your roofing should have recommendations on this.
For instance, standing seam metal roofs require furring strips or battens. Some people like to install vertical strips first, then the horizontal ones across the roof for attaching the metal panels. This allows for further ventilation or installing insulation between the vertical strips before nailing the horizontal strips over them.
One good reason for strapping is to ensure that the panels are firmly attached and will stay that way. If you have plywood roof sheathing, which is very common, it’s almost a given that you will need furring strips. Plywood just will not hold the roofing screws for long.
They tend to work their way out of the plywood during the normal expansion and contraction of the metals and the force this exerts on the connections. Even the manufacturers of some roofing screws state that their product has to be attached to at least one inch of solid wood, which leaves out most plywood.
This last fact brings up another point. Most strapping has traditionally consisted of 1 x 4s. However, due to the way wood is milled, boards are usually a bit smaller in width than stated, for instance, a board labeled as one inch wide is much closer to 3/4 inch thick, which doesn’t meet the screw manufacturers’ guidelines. More people are using 2 x 4s these days for a sturdier attachment. Check out my article What’s the Best Metal Roofing for a Shed? if you need more information on the types of metal roofing.
Can You Put Metal Roofing Directly on Shingles Without Furring Strips?
It’s not a good idea to try to install metal roofing over shingles without the strips. The strips provide an even, level base for the metal panels. In addition, the strips provide space for airflow. Be sure to check with your local building authority first. There may be restrictions on putting one roofing layer on top of another.
Inspect your shingles for missing or cracked ones. If your shingles are generally in bad shape, it’s probably better to strip off the shingles and start fresh. Check under damaged shingles for moisture on the sheathing. You may find that some of the sheathing needs to be replaced if moisture has been getting under the shingles for a time without your knowledge. Usually, roofing felt is applied over the shingles. Then your strips are installed over the felt. Be sure to read my article What’s the Minimum Slope for Metal Roof on a Shed? to be sure your not going to have a leaking problem.
Does a Metal Roof Need a Vapor Barrier?
If you do happen to be installing the metal over shingles, you won’t need a separate barrier, as the shingles will act as a vapor barrier. If not, you’ll need one. Once metal roofing is cooler than the outside air, moisture will start to condense on it. This happens more quickly with metal than with other materials.
You’ll be able to see this especially in the early morning when there is dew on the ground. The roof will often be covered with water beads. A vapor barrier is important to protect the sheathing from moisture collecting on the metal roof.
There are more options available these days besides asphalt roofing felt or tar paper. There are synthetic barriers made of various kinds of plastic-type materials bonded together. These last longer than roofing felt and is better at keeping out moisture. There is also a self-adhering membrane made to stick to the roof sheathing. It’s made with a peel-off backing which is removed to attach it. It survives a great variance in temperature.
How to Install Roof Strapping for Metal Roof
Locate the rafters or truss chords and mark the locations all the way down on the sheathing. A chalk line is great for this. Use the chalk line to mark the track of each horizontal furring strip to keep it straight along the whole length of the roof. Some people choose to start measuring for strip placement from the roofline, starting about six inches to two feet from the top for the first strip.
Others choose to start from the bottom, two inches above the roof’s edge. Whatever way you choose, the strips should be around 16 inches apart, and no more than 24 inches, with a couple of strips on either side of the peak for the cap installation. If you experience heavy snow in your area, you may want to use the closer measurement for extra support. Use eight-penny nails to attach the strips to the rafters.
If you wish, you can start with some vertical strips nailed along the tracks of the rafters or chords, then nail the horizontal strips to them. The vertical strips allow for putting foam board or other types of insulation between them before nailing on the horizontal strips. This is easier than installing insulation on the inside of the roof.
The only problem you might encounter with this method is if you are replacing your roofing or installing over an existing roof and you already have a gutter system installed. The new roofing will be higher, and the gutters may need to be removed and reinstalled. If you need more information read my article How to Install Galvanized Metal Roofing On a Shed.
Conclusion
Installing furring strips is fairly easy once you have your rafters or trusses marked out and chalk lines where each strip will go. They will make installing your roofing fairly simple and provide a good space for airflow to encourage moisture to evaporate should it get under your roofing.
It also forms a further insulation space, keeping your shed a little cooler in the summer. It even provides a little support for your shoes while working on the roof. Once you start on their installation, it should go smoothly.