Can solar panels be installed on a flat roof?
Some homes and many businesses have flat roofs. Solar panels can absolutely be installed on these, but it requires a different kind of racking system.
Most houses have sloped roofs, but flat roofs aren’t uncommon, especially with Modernist designs. These tend to be more popular in milder climates that don’t need peaked roofs to deal with snow loads, such as California.
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 what about a house with a flat roof? Can solar panels still be installed?
Yes they can! There is no problem with placing solar panels on a flat roof, and there’s a couple types of systems available. The main difference is in how they are secured to the 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.
So, if you have a house with a flat roof, it will have a slight pitch toward the gutters. This will prevent 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.
Attached racking systems
The first type of system that can be used on a flat roof is an attached racking system - the same type as used with more common 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.
Ballasted racking systems
Another type of solar racking system for flat roofs are ballasted racks. Ballast is weight used to stabilize something - in this case, it’s weight used to hold the racking system in place.
This means that if you use a ballasted rack, heavy ballast - usually concrete blocks - is 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.
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 may 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 balllast 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. To protect against leaks, the normal way that the footings for attached racks are waterproofed is with a material such as butyl tape, followed up with caulking.
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.
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 links below to calculate the solar incentives available in your area, and to get quotes from multiple qualified solar installers near you.