Flat Roof Insulation How To

flat roof insulationLike most Southern Californians, we use our garage for just about everything EXCEPT storing a car. It’s a luxury not all parts of the country have, but it affords Californians with some square footage that might normally be a basement in other parts of the country. In our case the garage is a HomeFixated proving ground, workshop and all-around man-cave (you do need to ignore the washer and dryer, which completely mess with the man-cave feng shui). Our garage has a flat and dark-colored roof, and in the summer it turns into an oven that will literally broil you if dare set foot inside it mid-day. When we recently needed to reroof the garage, I decided to capitalize on the opportunity and install some rigid foam board insulation as well.

While rigid foam insulation board isn’t the most impressive form of insulation on the market, I didn’t have a lot of options to work with given our garage design. I could have used spray foam or even batts of insulation from the inside, but our open ceiling and exposed joists are filled with banks of fluorescent lights and various items stored in the cavities between the joists. That left us with insulating from the top, and given our flat roof and torch-down roofing material, rigid foam insulation seemed the most practical. While the rigid insulation added to the cost of our roofing contract, our roofer (Sequoia Roofing in San Diego) didn’t rake us over the coals. The insulation project was yet another “while we’re at it” event that regularly keeps us dumping more money than expected into just about every house project that comes up. Ah, sweet homeownership! Before we dive into the details, keep in mind best practices and building code vary widely, and what worked in our case may not be ideal or up to code in your neck of the woods.

Before the roofing and insulating project could get underway, our old roof needed to be demo’d. If you are ever having a flat roof redone like ours, make sure you cover everything you care about in the garage. Definitely remove your car entirely if you actually park yours in the garage. If you don’t cover things up, you’ll find all your possessions covered with a thick layer of soot and grime that will take you months to clean. I thought about tacking plastic sheeting directly to the ceiling joists, but our roofer said when people do that, the weight of the dirt inevitably pulls all the sheeting down along with the dirt. Luckily our roofer was conscientious and laid plastic sheeting over everything in the garage. With that done, the old layer of torch-down roofing and underlayment was ripped up and disposed of.


Flat Roof Insulation Installation
replacing-flat-roof-perimeterThe rigid foam board insulation we used was 2″ thick and came in 4′ x 8′ sheets. Once the roof demo was complete and the flat roof was cleared of any remaining debris, some rotted 2x material around the perimeter of the roof was replaced. The 1x roofing boards (which also made up the “ceiling” of the garage) had nice gaps that you could see the ceiling joists through. The joists were marked around the perimeter to make finding them much easier later. The 4’x8′ sheets of rigid foam board insulation were then laid out in a staggered pattern across the entire surface of the flat roof. Some parts of the country necessitate a vapor barrier under the rigid insulation, however this wasn’t necessary in our case.

Adding the Roof Sheathing
Sheets of 4’x8′ OSB were then laid directly on top of the rigid foam insulation boards, leaving small gaps per the manufacturer’s recommendation to avoid buckling at the seams. Using the perimeter marks for the ceiling joists, chalk lines were snapped to indicate where to screw down the sheathing. Square shaped “washers” were then laid out on the chalk lines. These distributed the pressure of the screw-head to a larger area and also recessed the head so it wouldn’t protrude into the roofing layers to be added above. Long enough screws were used so that the screws would go through the sheathing, insulation, 1x boards and securely into the joists below.


Chalk lines snapped to indicate joist location below

Finishing the Roofing
base-layerOnce a nice smooth surface of sheathing was in place on the flat roof insulation, a layer of 28# fiberglass base sheet was nailed in place with large head roofing nails. A single-ply granulated modified bitumen roofing material was then torched-down as the final layer of roofing material.


The final layer of roofing being torched down

The end result is a man-cave that feels more like a temperate cave and less like a broiler. Although the R-value isn’t huge, the rigid foam insulation boards do a nice job of keeping the searing heat of the roof from making its way into the sanctified space of the man-cave. Between the flat roof insulation and our install of Reach radiant barrier insulation on our garage door, the shop is now downright comfortable, even in the warmer months. We expect the flat roof insulation project to help retain heat in the winter as well. Of course “winter” here in San Diego isn’t going to do much to tax the insulation.

Sequoia-Roofing-Angies-ListOur thanks to Sequoia Roofing, who did meticulous work on both our garage roof and on a portion of tile roof we needed replaced. It’s worth noting that I found Mike and Sequoia Roofing after careful research on Angie’s List. As you can see from the screenshot I took before hiring, some companies reputations really shine in a review system such as Angie’s List.


A typical tsunami-like deluge of rain thwarted by our newly insulated flat roof


  1. I am a Roof and Solar installer. My suggestion is to always use a smooth torch down single ply, then apply a Elastomeric roof coating for sun protection. Simple reason. when torching with smooth vs graulated you have a great advantage of working the material from top. when you trowel or otherwise work from the top of granular material it marks up or smudges. you can work-rework areas of with smooth, cant do that with granulated. The whole idea of a torch down is to make it a smooth seamless roof. every seam should be troweled smooth and seamless, working around pipe, vents, AC’s, or other needs to be worked to detail. granulated its not ideal for detail. The only place i use granulated is wide open roofing, no detail work, and where you get a consistent tar flow at seam. Elastomeric coating works great as top coating for smooth. if you need custom color just add color to white. i have put down 1000’s of rolls.

  2. Loved the way you showed how to insulate a flat roof, but I need a drip edge. Like 3″x3″. Home Depot and Lowes do not sell it. Any ideas where I could buy it? I’m in Covina CA. Thank-you

    • Try looking up sheet metal and/or gutter supply shops, or roofing supply stores. If they don’t have what you need, they should at least be able to point you in the right direction. Good luck!

  3. Good post! Did you consider a reflective radiant barrier? They are installed just before the roof sheathing (so its not visible). And they should be able to block a portion of the sun’s radiant energy.

    Here’s a helpful link from EnergySavers.gov for more info: http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11680

    • Hey Ethan! Well, we used a radiant barrier on the garage door, but the re-roof was done prior to me getting hip to radiant barriers. I suppose I could still do a barrier on the open ceiling now (from the inside), but the insulation we added did the trick. . . . at this point it would be overkill. But for warm climates like ours, radiant barriers can definitely be useful.

  4. Why not a lighter-colored roofing material? Would it make a difference?

    • Lighter would definitely make a difference and many similar roofs in our area are white, light grey or even silver. The color choice was an aesthetic one. The roof is very visible from one side of the house – a light color would show dirt conspicuously and wouldn’t tie in with the look of the house. We definitely considered it, but the insulation resolved the issue without having to change the color, and we got a more insulated garage in the process.

  5. Marc,
    I had to laugh because i recognized it was your garage roof before i even read a word of the story.
    Joe T

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