Roofs are designed to shed water and shelter structures from the elements. It’s not hard to figure out how pitched roofs—like shingles, tile, or metal—accomplish this, but one commercial building roof construction type can be more puzzling. It’s the flat roof. It’s common for commercial buildings to have flat roofs. At first glance, the term “flat” roof may seem like an oxymoron; however, flat roofs aren’t actually flat.
If you’ve ever wondered how a flat roof works, this article talks about a major component that makes flat roofs functional: tapered insulation.
What is Tapered Insulation?
Tapered insulation is installed in low slope roofing systems over flat roof decks in order to create a gently sloped roof surface. The mild slope helps direct water toward roof drains, scuppers, or gutters. Tapered insulation must be professionally designed and laid out in order to accomplish proper drainage and avoid ponding water on the roof.
Tapered systems are made up of various board thicknesses and slopes. Common slopes for tapered polyiso insulation include the following, from least pitched to most:
⅛” per foot
¼” per foot
½” per foot
Tapered insulation is made of the same materials as flat rigid insulation. However, instead of being a constant uniform thickness, tapered insulation has a varied thickness. For example, one end of a 4’x4’ board may be ½” thick and the other end may be 1”, 1-½”, or 2-½” thick, depending on the slope.
The Importance of Insulation
Insulation is required on all occupied buildings in order to support thermal comfort and energy efficiency. While some applications, such as structurally sloped roof decks or vertical walls, can achieve the code-required R-values with flat insulation, flat roof decks require tapered insulation.
Tapered insulation serves two functions:
Contributes to achievement of the desired R-value
Creates slope to direct water for proper drainage
Types of Tapered Insulation
There are two common insulation materials used to create tapered insulation panels:
Polyisocyanurate (Polyiso or iso)
Expanded polystyrene (EPS)
EPS tapered insulation requires a cover board for adhered membrane roof systems and can only be mechanically fastened or loose laid (not adhered) since this material is incompatible with adhesives. Polyiso, on the other hand, can be installed by adhering, screwing, or loose-laying because it has an adhesive-compatible facer on both sides of the polyiso core.Pros and Cons of Tapered Polyiso Insulation
Pros and Cons of Tapered Polyiso Insulation
One advantage to choosing tapered polyiso rather than EPS is the installation method. Installing tapered insulation with fasteners can become expensive and laborious since many different lengths of fasteners are required for the varying thickness of boards. When installing polyiso tapered insulation, foam adhesives take the guesswork out of using the right fastener lengths.
The cons of tapered polyiso insulation are that these systems are expensive, and usually need to be replaced when the roof is replaced. To combat this, designers should rely on structural sloped roof decks for large roof areas and only use tapered insulation for smaller areas.
When comparing the tapered polyiso insulation R-value to EPS tapered insulation, polyiso outperforms with a higher R-value per inch than EPS.
The R-Value of Tapered Insulation
Calculating the R-value of tapered insulation is not as simple as it is when dealing with flat insulation panels. For flat insulation that is at a constant thickness, you simply measure the thickness and look at the correlating data sheet.
For tapered insulation, the R-value varies throughout the roof. The best way to get an accurate reading of the overall R-value for tapered insulation systems is to measure the minimum thickness. This is called the minimum R-value and it can be found at the roof’s low points, like roof drains and gutter edges.
You can also ask professional tapered insulation designers to provide a calculated “average R-value” over the entire roof system. However, these values may not necessarily meet code requirements, especially if a continuous insulation R-value is required at the roof.
When to Choose Tapered Polyiso
If you’re contemplating using tapered polyiso on your roof, here are the factors to consider when comparing tapered EPS to tapered polyiso insulation:
R-Value Requirement
Attachment Method
How will you attach the insulation? If you do not want screws penetrating through the roof deck into the underside that is visible to the interior space, you either need to bring in rock (ballast) to hold the roof insulation down or side with polyiso to adhere the system.
Get a Quote for the Best in Tapered Insulation
Contact our experts to get a project-specific tapered quote. Our representatives can help you pick the right product and design the tapered layout so that your roof drains properly. Reach out to our reps today!