Can solar panels be installed on a flat roof?

Most houses have sloped roofs, but flat roofs tend to be popular in mild climates that don’t need peaked roofs to deal with snow loads, such as southern California and the southwest.

If you’ve seen photos of solar panels on a typical house with a sloped roof, you may have noticed the racking system holding the panels in place. In the most common case where asphalt shingles are used, the racking is attached to the roof deck with bolts that are drilled through the shingles and attached to the deck underneath. This keeps the panels in place through the strongest winds, rain, and snow.

But how do you attach solar panels to a house with a flat roof?

There are two types of solar racking systems for flat roofs. One type is bolted or screwed to the roof, just the same as with a sloped roof. Another is a ballasted rack. This type uses weight to hold the array in place without putting holes into the roof. Ballasted racks have the advantage of a penetration-free installation, but place a greater load on your roof.

The choice should be made by your installer in consulation with an engineering review, but in this article we’ll review how these systems work so that you can have a knowledgeable discussion with your installer. It’s always important to know what equipment is being chosen for your home solar system, and why.

Flat roofs aren’t actually flat

First, a quick point should be made: flat roofs aren’t completely flat. They always have a slight pitch, or slope, which is necessary to allow rainwater to drain off the building. A completely flat roof would allow water to pool, which wouldn’t be a good thing at all.

A roof is considered to be flat if it has a slope of about 2/12 (16.7%) or less. The pitch will be toward the gutters, preventing damage caused by pooling water.

I mention this because people sometimes worry about how the addition of solar panels might affect drainage off a flat roof. The answer is that is doesn’t have a negative impact because your roof is already designed with drainage in mind.

The pitch of the roof prevents water from pooling on the solar panels too, preventing damage.

I actually have a section of flat roof on my home in Buffalo, with one row of solar panels on it. After several years of ownership, I haven’t had any problems with those panels, even though they will be covered in snow for several weeks out of the year. The front glass of a solar panel is designed to very durable.

Attached solar racking systems for flat roofs

The first type of solar panel rack that can be used on a flat roof is an attached racking system, similar to those used with sloped roofs. They’re called attached because they are fixed in place by fasteners, usually bolts or screws.

With this system, the racks are secured to the roof with mounts that are fastened to the roof deck. This means drilling through the roofing material (often asphalt shingles) and then attaching fasteners to the structural surface underneath, which is a layer of plywood or oriented-strand board (OSB) that is called sheathing.

Once properly secured to the sheathing this way, the solar panels are very secure and won’t blow away even in the highest winds.

The main installation detail that’s important is to ensure that the mounting points are made waterproof - after all, holes are drilled into the roof. If these holes aren’t waterproofed properly, you’ll have leaks and an expensive repair to deal with later on.

The example above is the IronRidge XR Tilt Mount. The highlighted area shows the footing, which is screwed into place on the roof. It also comes from the manufacturer with a waterproofing membrane. The manufacturer also recommends an additional sealant if the home is in a climate that experiences freezing.

This system also adds increases the tilt to the array to capture a little more sunlight.

There are other manufacturers with competing products, and they have similar designs.

Ballasted racking systems for flat roofs

Another type of solar racking system for flat roofs is a ballasted rack. Ballast is weight used to hold the racking system in place.

This means that if you use a ballasted rack, heavy weights such as concrete blocks are placed on the rack to hold it down. The ballast is the primary force that holds the system in place, but a few fasteners may still be required.

The photo above is an example of a commercial installation, but the same type of system can be used on a residential building too.

If the roof has a very low slope, there might be no screws or bolts at all required to hold the system in place. But if the roof has a slightly higher slope, some fasteners may be still be required. In addition, local code in earthquake-prone areas, such as California, may require that ballasted systems be fastened to the roof.

There are a couple important advantages with a ballasted rack, which include faster installation and sometimes no penetrations in the roof deck. But even if fasteners are needed, it will require fewer than with an attached system. This means less labor, fewer holes to waterproof, and less chance of a leak down the line.

However, a ballasted system can weigh considerably more, so understanding the load bearing capacity of your roof is an important consideration. We’ll get more into that in the pros and cons section below.

Tilted racks

To capture the maximum amount of energy, solar panels on fixed racks will ideally be tilted so they are perpendicular to the midday sun. For locations in the continental United States, that means a tilt anywhere between about 22° and 38° (where 0° is pointed straight up). Solar racking systems for flat roofs will normally have a small tilt, but not enough to be at the ideal tilt for most locations in the US.

Tiltable racking systems were created to address this problem. Available with both ballasted and attached mounts, these systems allow the installer to adjust the tilt of the solar panels so they sit at the ideal angle to capture the most solar energy throughout the year.

Note that these are fixed tilt, and not motorized tracking systems that follow the sun. While single-axis tracking systems are available for residential installations, they are almost never worth the added expense.

One thing to be aware of is that by tilting the solar panels, you won’t be able to place multiple rows of panels as close together because of the shadow that tilted panels cast. This might be a problem on a rooftop with limited space.

Fixed tilt systems may add some cost, so talk with your installer to find out if the increased energy harvest is worth it.

Ballasted vs fixed racking: pros and cons

There are pros and cons to both attached and ballasted systems, and which one you choose for your house will depend on cost, the structural load bearing capacity of your roof, and the products with which your solar installer is familiar and prefers working with.

While a ballasted system might seem like the clear winner because it may require no holes in the roof, the additional weight is a disadvantage. While a new roof should have no problem supporting the higher weight, keep in mind that solar panels usually come with a 25-year warranty, and systems could be expected to last even longer than that.

This means that while a newer roof might have no problem supporting the weight of the ballast, that ballast will be sitting there for decades. The compressive force of that weight, over the course of a couple decades, could accelerate the aging process of your roof, leading to localized sagging.

Because of this, before a solar installer chooses to go with a ballasted system, it’s important to get a structural engineering review. Depending on your local code, this may be required for any solar installation, but it’s especially important with a ballasted system.

Keep in mind that the roof may need enough capacity to support both the panels and other loads, such as snow loads.

On the other hand, an attached racking system requires a lot more fasteners to be placed into the roof deck. Manufacturers like IronRidge often include waterproofing features built-in, but sometimes additional materials like caulking or butyl tape are recommended to keep things leak-free.

This requires more labor and, depending on the construction of the roof, may require the solar contractor to have a roofing expert on hand. This can further increase costs.

But the lighter weight of attached racks might mean fewer maintenance worries later on.

Calculating solar energy production for a flat roof

Depending where you live in North America, the ideal tilt for a racking is somewhere around 35° (where 0° means the panel is flat, facing up) give or take a few degrees.

A flat roof with a tilt of around 16° or less is quite far off from that ideal. Does this mean that you can expect to lose a lot of power generation with a flat roof, or should spend money on a more expensive tilted rack?

For a homeowner, it’s probably not worth it. We wrote an article explaining that the tilt of your solar panels matters less than you might think. The reason is that the sun, as a point in the sky, isn’t the only source of sunlight in the sky. Atmospheric scattering (the blue sky) and clouds also are sources of much of the light that hits your panels.

You can calculate this yourself using the PVWatts calculator from the National Renewable Energy Laboratory.

It’s easy to use, and just requires you to plug in a few numbers. As an example, I used 90210 as my zip code. In Beverly Hills, a tilt of about 33% will generate the maximum energy over the course of the year.

For my theoretical system in 90210, a 33% tilt would generate 10,318 kWh over the year. But solar panels on a flat roof with a tilt of 10% would generate 9,655 kWh, or about 93.6% as much as the panels with the ideal tilt.

That’s a pretty small loss! For a homeowner with only a couple dozen solar panels or so, it’s probably not worth it to spend extra money on a more expensive racking system to squeeze as much power out of your solar panels. Instead, it probably is more cost effective to install more panels if you want more power.

This is especially true of sophisticated motorized racks that automatically move in one or two axes to keep panels pointed at the sun. These do make economical sense for large solar farms, but rarely do for residential installations.

Bifacial solar panels might be a good idea

Conventional solar panels are monofacial: they collect sunlight only from the front.

Bifacial solar panels, on the other hand, are transparent at the front and back and can generate electricity from sunlight that hits the rear of the panel.

These panels have a cost premium, but depending on the situation, they can generate 5-30% more electricity than conventional panels.

One situation where bifacial panels might make sense is a flat roof, especially with a tilted rack, which can allow enough sunlight to hit the rear of the panel for these panels to make sense.

You can read my article “What are bifacial solar panels?” to learn more.

Choosing the right system for your flat roof

As you can see, flat roofs are no problem for people who want to install solar panels. It does mean that you, the homeowner, will need to ask a few additional questions of your solar installer:

• Does the installer recommend an attached or ballasted system? Why?
• Review the structural engineering report of the roof with your installer. If they propose going with a ballasted system without doing an engineering review, this is a red flag. Find a different installer.
• What angle will the panels be at?
• Does the installer recommend a tilted system? Why, or why not?

As you’ve hopefully learned from this article, there’s no one right choice for all situations. Instead, work closely with your installer to understand the tradeoffs of different types of equipment. A good installer will take the time to carefully explain everything.

If you’re ready to make the leap and go solar, use the link below to get quotes from multiple qualified solar installers near you.