How to Install Galvanized Metal Roofing On a Shed


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Metal roofing is a popular choice for sheds. Metal sheets are fairly easy to install, are durable and relatively inexpensive. Metal roofing is long-lasting, depending on the thickness of the metal sheets and any coatings that may have been applied.

They can take a lot of stress from weather, including being able to withstand wind when installed correctly. They are resistant to fire. Metal sheet roofing is also recyclable. In fact, the metal sheets you purchase may well be partly made of recycled material.

In this article I’m going to answer most questions about metal roofing and go into details for installing a metal roof.

Metal roofing can keep your shed cooler in summer. While this might seem to make no sense, as metal gets hot, the metal actually dissipates heat by having it spread throughout the roofing material and reflecting it outward. Shingles, on the other hand, actually absorb heat, especially the darker colored ones.

There are some drawbacks, however. Metal roofing is subject to dents and scratches from falling limbs or other debris. Scratches can damage your roofing color and any protective coating that was applied, leading to eventual rust. Metal roofing is more expensive than other forms of roofing, such as shingles, but it usually makes up for this extra cost in longevity.

The roof can be noisy in heavy rain, or especially hail, but the wood sheathing that is usually installed underneath will lessen the noise. Lastly, some people just don’t like the look of metal roofs, especially if they want their shed roof to match their house roof, or at least not clash with it.

While metal roofing is durable and doesn’t require much maintenance, you should still inspect it for any deep scratches or gouges which may end up rusting. Any damage, such as dents, can’t be easily repaired. The whole panel will need to be replaced.

When investigating metal roofing options, you may be surprised to learn how many there are. There are various types of thicknesses and coatings. While metal roofing can be made of various metals, galvanized steel is the most commonly used and is the least expensive by far.

Some of the lighter-weight metal panels are touted as being suitable for sheds but remember that the heavier the metal, the less likely it is to dent. Weigh the greater cost of heavier grade metal against having to buy and install a new panel to replace a dented one.

How your metal panels are protected against the elements is a very important consideration. The galvanization process involves bonding a zinc coating to the steel. However, this zinc coating gradually wears away with exposure to the elements. Once this zinc coating starts to deteriorate, it allows rust to form.

The paint used to protect the metal can extend the life of the panels and also decrease the time between needed maintenance projects. Many metal roofing paints are based on polyester. These increase the time to the first required maintenance by as much as ten years, even in a coastal environment where the metal is exposed to salt in the air humidity.

A PVC Plastisol coating is even better, extending the time to first maintenance by up to 15 years. Color plays a part also, as the lighter paint colors tend to last noticeably longer than the darker ones, and some paint for metal roofing contains a reflective agent. A good tip is to ask your supplier about the time to first maintenance with the various choices. There are several main types of metal roofing, for more information on what’s best for your shed read my article What’s the Best Metal Roofing for a Shed?

Is There a Minimum Slope For Installing Metal Roofing?

Corrugated metal roofing is recommended to be installed on a higher roof pitch than conventional metal roofing on a house. The minimum pitch for conventional metal roofing is 3:12. This means that the roof rises three inches for every foot of roof material. The recommended minimum slope is 5:12. This ensures that the roof more easily withstands heavy rain and snow can more easily slide off. Check out my article The 7 Most Popular & Practical Shed Roof Designs for more information on types of roofs.

Can I Install Metal Roofing Over Shingles?

It is certainly possible to install metal roofing over shingles. You should check with your building authority, as some localities specify how many layers of roofing are allowed on a building. Since you are roofing a shed, the restrictions may be less than those for a house, but they should be checked before you start.

In addition, your choice of roofing panels comes into play. Some forms have to be installed over battens, some should not, and others can go either way. Shingles will act as a good underlayment, providing insulation and good moisture and sound barrier. One caution is that the shingles need to be in relatively good condition, which means no missing shingles, and shingles that are not deteriorating to the point that pieces are cracking and breaking off.

How Do You Attach a Metal Roof to a Shingle Roof?

Usually, roofing felt is attached over the shingles. This makes for a good, straight surface for your metal. Start putting the panels on, making sure to keep the edges straight. The screws may be attached either on the flat surfaces of the roofing or the raised ridges. Whichever you choose, keep it uniform throughout and keep your screws in a straight line. Be very careful not to scratch your metal. Attaching the ridge vent is the last step.

Do I use 1×4 or 2×4 Strapping for a Metal Roof?

Strapping is also referred to as battens or purlins. Wood strapping allows for easier attachment of the panels and also provides needed airflow under the roofing itself. This airflow prevents condensation damage and also provides some insulation effect, keeping your shed cooler in the summer. Remember that any time your shed roof is cooler than the air and there is humidity present, the moisture will tend to condense on your roof, especially in early morning.

Another choice is to use the wood strapping directly on the joists or truss chords without any sheathing. Depending on your climate, this may be a viable choice, but most people prefer the added stability of the sheathing. However, you will need to attach the strapping to the joists or chords through the sheathing, so be prepared to do some measuring to locate the studs.

Many metal roofs have been installed with 1×4 strapping in the past. However, this was in the days of the old “tin” roofing, usually on farm buildings. These days, 2×4 strapping is preferred, especially since a 1×4 may be much closer to �” wide than one inch. In addition, installing strapping vertically first, then another layer horizontally to attach the roofing panels will add to the insulation properties and airflow. Are there any advantages to a truss roof? Read my article Are Trusses a Better Choice for a Shed Roof? to find out.

Can I Install Metal Roofing to Plywood?

You can install metal roofing over plywood sheathing, and some sources advocate this, but attaching the roofing directly to the plywood is probably taking unnecessary chances. Plywood simply will not hold the screws for very long. Even some of the screw manufacturers discourage this and state that there must be at least one inch of solid wood to attach the screws.

This is one reason that metal roofing is usually installed over strapping. The strapping allows a solid attachment while also permitting inserting more screws than would be allowed if you were inserting the screws into the joists. Your roof is not a place to scrimp on materials or installation if you want it to last.

Do I Need to Use Black Paper Under My Metal Roof?

You do need to use some sort of underlayment under your shed roof, especially if you’re using plain galvanized sheets, commonly referred to as “tin” roofing. If you don’t, your roof sheathing won’t be protected properly from moisture and will eventually start to rot. Black or tar paper is a good choice to prevent damage from moisture condensation. Tyvek wrap has also been used successfully under the metal.

How Many Inches Do You Overlap Metal Roofing?

With a corrugated roof, each sheet should overlap the previous sheet by four to six inches, and a full corrugation on the sides. For other types of metal roofing, some require less overlap, such as two inches. The manufacturer of the roofing panels you choose should provide instructions stating how much overlap is required for their particular product and how it should be accomplished and fastened.

How Far Apart Do You Put Screws in Metal Roofing?

The general rule of thumb is to place the screws one and a half feet to two feet apart. You don’t want them any farther apart than that. There are usually four screws per line of screws. In addition, screw placement varies with the type of roofing chosen.

There are two schools of thought as to placing the screws on the high parts of corrugated metal roofing or the low parts. Installing screws in the peaks means that the screws are less exposed to water and debris, but the connection may not be as secure. There is the risk of overtightening the screws and causing small dents in the roofing.

Using the valleys for the screws may keep smaller debris from sliding off the roof. In addition, the screws will be subjected to water flowing off the roof, allowing more possibility of leaks if any screws were not installed correctly. While there is less chance of distorting the metal when tightening the screws, some sources urge placing the screws on the hills.

Rows of screws are generally placed every third corrugation. Your roofing manufacturer has installation instructions that will provide information as to how to fasten the screws according to the material.

Can I Use Nails to Attach Metal Roofing?

Nails should not be used to attach metal roofing. There are roofing screws specially designed for attaching metal roofing. These usually have a hex head and an attached metal washer. Some have a rubberized or neoprene base to the metal washer to ensure that the attachment is watertight. The length varies according to the distance needed. Generally, screw length should be chosen so that the screw goes at least one inch into the wood base.

How Much Should a Metal Roof Overhang on a Shed?

Metal roofing without a gutter system usually overhangs the eave sides up to three inches. The rake ends of the roof should not have an overhang to prevent wind damage. If you plan to install gutters, the overhang would only be an inch or so, enough so that water flows into the gutters. The overhang is usually planned for when installing rafters or roof trusses, so it’s not something that you would have to figure out when actually installing the roofing.

How to Install Metal Roofing

For corrugated metal roofing, you’ll need thick gloves and eye protection, a polyurethane sealant, ridge peak, roofing felt, roofing cement, tin snips, staple gun and roofing staples, screwdriver (battery-powered or screw gun is best), and measuring tape.

While you can use a circular saw with blades designed for metal cutting, this type of cutting tends to leave burrs on the panel edges, which must be treated later. Any hot filings coming off the blade are hot enough to stick to the panels and eventually rust. There are sheet metal shears that operate either by electricity or are battery-powered available if you think you’ll have a lot of cutting to do. Otherwise, tin snips with long blades or aviation snips will do just fine.

For materials, you’ll need your roofing panels and ridge cap. You can buy large metal panels and cut them down to the size you want. Many home improvement stores stock shorter metal panels that are 10 to 12 feet long. You may also choose to install eave flashing. You also will need roofing screws at the needed length, depending on whether you are installing screws on the hills or valleys of the metal panels.

If your roofing screws didn’t come with rubber or neoprene bonded to the metal washers built onto the screws, you can get neoprene washers separately. Neoprene washers are especially good at keeping screw openings from leaking, and neoprene is slow to deteriorate.

Cut your panels to the needed length, allowing sufficient extra material for the overlap. If you don’t have instructions on the overlap from the manufacturer, plan to overlap at least one full corrugation on the sides and around four to six inches on the length, if needed. Layout your roofing felt next, fastening it to the sheathing with roofing staples.

The felt should overlap by at least four inches. If you are installing eave flashing, it goes on next over the felt. This flashing helps seal out moisture when rain runs off the roof.

Start your horizontal strapping about six inches down from the roof peak. Repeat every 16 to 24 inches, making sure to attach the strapping to the joists or truss chords. The strapping should go all the way across the roof.

Start installing your metal panels at a bottom corner. Roofing screws are made to be self-tapping, so you won’t need to drill starter holes for them. Put your first panel in position so that it overhangs the edge your desired distance and is square to your frame.

Install your first screw at the outside bottom corner. You want to make sure it’s tight enough to allow the washer to make a watertight seal, but not so tightly that you dimple the metal. Continue along the bottom edge, inserting screws into the joists. On the side edge, screws should be inserted every eight to ten inches along the rafter or roof chord.

When installing the next panel, make sure it is even and square to the last one, overlapping the last panel by one full corrugation. For added moisture protection you can use a bead of silicone sealant near the edge of the lower sheet to seal the edges of the overlaps. Some manufacturers urge the use of mastic tape instead of fluid sealant. Be prepared to spend some time with this, as this tape is said to be somewhat difficult to work with.

Once you get to the top, start at the bottom on the other side and install the panels the same way. When you are finished, the ridge cap goes next. This is a strip of metal angled to fit the slope of the roofing and designed to overlap and cover the edges of the roofing panels. It is attached with the same screws that are used with the panels.

It usually overlaps each side of the gable by six inches. The ridge cap may come with special sealing instructions, such as using tape, foam and sealant. The tape is applied along the length of the gable on either side just above where the edge of the ridge cap will sit. Then the foam is installed on the tape and finally, the sealant is applied.

At the ends of the cap, make two angled cuts an inch long in the cap opposing each other so that you end up with a triangle. You can use a speed square to figure out the proper angle according to the roof pitch.

Using sheet metal pliers, bend each side so that you have two flaps that will fit over the edge on either side of the gable. Seal all your overlap seams with roofing cement or roofing caulk. This is best done in dry weather, as the cement needs to dry for at least 12 hours after application. If your looking to save money read my article What is the Cheapest Type of Roof to Build?

Conclusion

A properly installed metal roof is a lot more work than just nailing a few metal sheets to your roof. However, the proper installation using the proper materials will help protect your stored items from moisture and excessive heat and will ensure that your roof will last a long time, perhaps longer than you will live in the house.

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