The Roofing Resource Blog

Eliminate Flat Roof Ponding Water with Tapered Roof Insulation

Tue, Feb 24, 2015 @ 08:00 AM | Perry

sing Tapered roof insulation is an efficient way to eliminate ponding water on a flat roof. It can be cut, fit, and installed in a way the moves ponding water towards existing drains, scuppers, or gutters. But for all the great things tapered roof insulation is, cheap is not one of them. Adding a tapered insulation package will substantially increase the price of a flat roofing project.  

Understanding what tapered roof insulation is, how it works, and the basics of installation will help you make the most of your flat roofing investment. 


What is Tapered Roof Insulation?

tapered_insulation_for_flat_roofsTapered polyisocyanurate roof insulation or tapered iso for short, is made from the same ridge foam material that normal polyiso roof insulation is made from. Regular iso board comes in flat sheets and in several different thicknesses. Tapered iso is not flat but is made with slope or “fall” and also comes in several different thicknesses but with slope. The slope causes water to move downhill on what would be an otherwise is a flat roof surface. 



How it works

Tapered polyiso is sold in 4’ x 4’ pieces or panels that change thicknesses over 4’ distance. The amount of the change in thickness in each panel is a 1/4” per foot. This means that for every foot of distance the iso get 1/4 “thicker. For example an X panel begins at 1/2” thickness and increases a 1/4” in thickness every foot. Since the piece of iso is 4’ long, the thickness increase will be equal to 1/4” times 4 or a rise of 1” over the 4’ distance ( 1/2  to 1 /12” )Flat roofers can then arrange the sloped insulation to move water towards roof drains, scuppers, or gutters. 


X, Y, and Z Panels 

There are three main pieces of a tapered roof insulation package that are designed to work in conduction with one another. The names of the panels are after the last three letters of the alphabet X, Y, and Z. The size of the insulation is as at the following:

X panel measures 1/2” to 1 1/2 “in thickness 

Y Panel measure 1 1/2” to 2 1/2 “in thickness 

Z Panel measure 2 1/2 to 3 1/2 in thickness 


These three pieces of roof insulation can be installed back to back to for form a continuous slope that begins at 1/2” and ends twelve feet later at 3 1/2 “.  If the slope needs to be longer than three panels or 12’ you can continue this same pattern but you will have to add a base layer of 3” flat regular iso behind the Z panel. Remember the Z panel finished height was 3 1/2” so when you add a 3” base behind the 1/2” of the next X panel continues the slope. You can repeat this processes as needed to reach the height or distance you need to remove the ponding water from your flat roof.