How Much Does It Cost to Install a Flat Roof in Seattle?

Have you been told that installing a new flat roof is “no big deal,” only to be blindsided by hidden costs? Do you wonder why every roofing quote you get seems wildly different, even when they’re using the same materials?

Flat roofs are common across the Seattle area, but what most homeowners don’t realize is that doing one the right way is far more complicated and expensive than just rolling out some membrane.

At RoofSmart, we’ve installed hundreds of flat roofs all over the Greater Seattle area. Most of the time, these flat roofs have failed early because they weren’t properly insulated first. We know the importance of making sure each flat roof is installed correctly, even if it adds to the total project costs. 

Obviously, no one wants to hear that their roof job is going to cost way more than they thought, but we believe it’s important to educate Seattle area homeowners on how to prolong the life of their flat roof. 

In this article, we’ll break down all the components (from insulation types and taper systems to trim and decking) that can push your flat roof cost anywhere from $200 to over $500 per square foot. The more you know now, the less likely you are to face expensive surprises later. 

How much does it cost to install a basic flat roof in Seattle?

A flat roof installation can easily reach a cost of $200-$300 per square foot or more when factoring in necessary insulation, deck repairs, tapered systems, and other requirements

Whether it's torch down, TPO, or PVC, people only think about the roof covering itself when they’re trying to calculate the cost of installing a flat roof. But in a rainy, damp area like Seattle, that's only one part of the equation.

Where the cost really comes in is that a lot of flat roofs in the Seattle area have major condensation problems because they haven’t been insulated properly, so any moisture coming up has nowhere to go. 

And so to prevent condensation, you almost always need to add rigid foam insulation on top of the roof decking (on top of the plywood). This is a best practice suggestion, but it's not necessarily going to be required. 

So, how much does it cost to do a flat roof in Seattle? It depends on whether you're going to do it right or not. And if you're lucky, your roof was built correctly in the first place, and you already have all those layers underneath, and all you have to do is replace the skid.

But because of the cost, a lot of people don't do it.

If there is a lot that needs to be fixed or redone on your flat roof, then you might even need to spend $500 to $600 per square foot. That is not typical, but it is a reasonable upper bound.

What are the factors that impact the cost of installing flat roof materials?

Flat roof materials themselves aren’t usually too expensive, but there are other steps you need to take to make sure that your flat roof functions properly and will last for a long time. These added steps can increase the total job cost, but your roof will last a lot longer. 

Here are the factors that make your flat roof more expensive:

1.) Adding Insulation

If you need your flat roof replaced, it's usually because condensation has caused a bunch of damage that made your roof fail (it’s less likely to be because of a leak). But adding foam boards gives you insulation to prevent condensation. 

If you just put it back the way it was with no insulation, then you're going to get 5 to 15 years of life before the roof completely fails again.

Building code dictates how much insulation you need to use in your flat roof by giving a required R value. 

The R value is how you measure insulation value. It generally ranges from R0 to R60 (that's not an upper limit, but it's usually the max code). In the Seattle market, you need to have a minimum of R10 above the deck to prevent condensation.

So, how do you get the right R value in your insulation? Well, you've got three different types of foam: EPS, XPS, and PolyISO.

Here’s a quick comparison chart to show you the R value of each one, how much they cost, and what they’re made of: 

EPS is kind of like styrofoam, but it's not styrofoam. The most commonly used insulation type in the Seattle area is polyiso. It has the highest R value per inch (roughly R6 per inch). 

So if you only want your roof to meet the minimum R value (R 10), do 2 inches of polyiso, 2 inches of XPS, or 3 inches of EPS.

But what you really should do is have enough insulation that your building functions, right? So usually that's R38. What that usually looks like is two layers of 3.3-inch polyiso, which means almost 7 inches of additional height on your roof.

This can easily triple the cost of the project. 

2.) Replacing Rotten Wood

Usually, when there are condensation problems in your flat roof, that almost always means that the roof decking is rotten because it's been getting wet for years. 

And so your plywood is probably delaminating, rotten, and moldy. It's going to be around $5 or $6 per square foot to replace rotten sheathing.

If you're really unlucky, you've got car decking, which is going to be two by six tongue and groove planks. Or, on some buildings, it's three-by-six tongue and groove planks. And those have to be milled.

So that'll range for replacement costs, typically from $15 to $25 a square foot. If you just have plywood, of course, that's much less. It's going to be closer to like five or $6 a square foot to replace rotten sheathing.

3.) Tapering the Roof

What if your roof is dead flat and doesn't drain, and you've got pooling water? A lot of times, what makes sense is to build a taper system so the water actually drains. 

Depending on the amount of material you need and the size of your roof, this can add several thousand dollars or more to the total cost. 

If you have a roof that's flat or has been sagging, then you need to repitch it. The typical way to do that is with a custom taper system made out of foam.

Tapering the roof ensures that the water gets directed to the drains. You need a minimum quarter-inch per foot for drainage. Now, that's not a big deal if you've got to go 10 feet.

But if you've got to make it 30 feet, well, all of a sudden you've got quite a bit of additional height to try to get it to move in addition to your regular insulation requirements. This can add up quickly. 

One thing to keep in mind is that if you do have something that goes from a half-inch tall at the drain to eight inches tall at the peak, you have to have screws that go through all of that. So those are expensive screws.

4.) Adding New Trim Around the Perimeter 

Keep in mind that when you install new insulation, you're adding all this height to your roof. So, what about all the perimeter trim? If you’ve raised the height of the roof significantly (like when you add 7 inches of insulation), then you need to raise the perimeter trim as well.

You've got some options there. First, you can do pine. It's a terrible idea, because is pine rots quickly. But that's what most people do because it’s the more affordable option. 

Usually, what we'd recommend is cedar because it’s more durable. And then, of course, that's going to be primed, but it's not painted to match on the job. So now you've to paint as well, which adds to the cost. 

5.) Adding a Cover Board over the Insulation

It's really a bad idea (and it's not fire-rated) if you just put your membrane over the new layers of insulation. So, you have to put a cover board on top of it as well. 

That cover board is going to be either plywood (which is not recommended) or gypsum board (like DensDeck), which is a cement fiber. This is much more durable and waterproof than plywood. 

That goes down, and it gives your roof a Class A fire rating. When you put your membrane on, it's safe, and it's code-compliant. And if you walk on it, it's not like walking on a mattress.

That also means that, you know, if a branch falls and hits, it's not just going to spear all the way through. It'll more likely glance off because it's got a solid back. So it's safer, it's smarter, and it adds some cost.

How do you know if your roofer is actually qualified?

After reading about the many hidden costs and crucial steps involved in installing a flat roof in Seattle (like insulation, tapering, cover boards, and proper drainage), it’s clear that not just any roofing contractor can do this work well.

A lot of times, people get hung up on choosing between torch down, TPO, or PVC. But it's less about the material itself and more about how well it’s installed. 

It’s important to find a contractor who is experienced and understands the importance of insulating the roof. Hiring someone who cuts corners or doesn’t understand the technical complexities of flat roofing can cost you tens of thousands of dollars down the line.

The purpose of this article is to show you that if you see massive differences in cost when comparing proposals for your flat roof replacement, someone is probably leaving some things out.

So, how do you protect yourself from hiring the wrong person?

Read our article “Is Your Seattle Roofer Actually Good? Here’s How to Tell Before It’s Too Late” to learn how to verify that your roofer is qualified and competent.