Installing Solar Panels on a Flat Roof: Is It Possible?

Solar is a smart energy decision for homes and businesses with flat roofs, but there are a few considerations you’ll need to keep in mind as you go through the installation process.

You’ve been thinking about getting a solar system for your home. You’ve heard about all the ways it can save you energy, and it sounds like a great opportunity. There’s only one problem: you’ve always heard that solar panels need to be placed at an angle, to catch the sun’s rays. And your roof is completely flat. Are you still able to install solar panels on your home?

Our answer is YES……but there are choices and issues to be considered that aren’t necessary for solar panel installation on a pitched roof.

If your roof is in good repair and doesn’t have large obstructions around, such as trees, which can block the sun and diminish your energy output, you can certainly install solar panels on a flat roof.

In fact, in some ways, a flat roof can be preferable to a slanted one when it comes to solar installation.

For one thing, the panels are harder to see from the street, preserving your home’s overall aesthetic value. There are other benefits too.

When planning solar panel installation on a flat roof, one of the decisions you have to make is whether to mount the panels flat or to use tilt-up mounting equipment to raise the panels to a more normal 25°.

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 it 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 the 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 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 is ballasted racks. Ballast is the weight used to stabilize something – in this case, it’s the 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.

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 may require that ballasted systems be fastened to the roof.

There are a couple of 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. 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 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. 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.

Is output impacted with a flat roof?

On a typical slanted roof, the angle of the solar panels depends on the angle of the roof and the direction the roof faces.

On a flat roof, you can face your solar panels in any direction and tilt them to any angle you want. This freedom means you can optimize your energy output. By facing your solar panels south and adjusting them to an angle equal to your home’s latitude, you can maximize their sun exposure and produce more energy.

Do flat roofs require specialized installers?

Not every company is equipped to install solar panels on a flat roof.

However, if that style of roof is prevalent in your area, then it’s likely that many of your local solar installation companies will be prepared to accommodate you. And even if it’s not, there are undoubtedly some installers in your area who can help. Find a solar company near you that can accommodate your needs!

Are there risks involved in installing a solar system on a flat roof?

As we’ve already discussed, installing solar panels on a flat roof is safer than on a slanted one.

But once you have them installed, there are a couple of issues that can arise. One is the issue of dirt. Tilted solar panels are self-cleaning: water runs off, taking most dirt and debris with it. Horizontal panels don’t offer this benefit.

The main problem that comes from installing solar panels on a flat roof is leakage. If your panels are installed flat, water can pool on and around them when it rains, causing damage to your roof.

Fortunately, there are plenty of ways to avoid water pooling.

The best solution is to tilt your panels at an angle. Even a tilt of just 3 or 4 degrees can help prevent pooling and keep your system clean.

There are also steps you can take during installation to combat pooling.

By drilling the panels directly into your roof, you can anchor the system into place, which reduces the pooling risk.

You can also use a ballasted system, which anchors the panels using weights instead of drilling. There are hybrid systems, which incorporate both methods.

Having a flat roof doesn’t preclude you from having a fully functional solar system in your home.

It can even provide a better energy output than a slanted roof (because you can control the tilt) for roughly the same price. So you can install solar panels on a flat roof today!

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