Roofing Southwest Blog > The Solution to Flat Roof Ponding

The Solution to Flat Roof Ponding

November 25, 2017

Commercial and industrial buildings are commonly covered by “flat” roofs. Keeping flat roofs properly drained can be tricky. In fact, while such roofs are often referred to as “flat”, they should not be completely flat — they need to have a gentle, yet well-planned slope.  Should you happen to find water sitting on your roof for days after a rain, it’s likely due to inconsistent slope, or no slope at all.  Moreover, the water wears away at coatings, and eventually creeps through the roofing material and underlayment, weakening the roof and possibly compromising the space below.  

In cases where a roof is totally flat, or the slope is inconsistent and causes water to pond, we are able to use roof insulation products that are specifically made to add sloping. 

A quick background on flat roof anatomy, and popular insulation products:  

Elements of a Flat Roof

There are three major elements that make up a flat roof structure…

  • Flat roofing material comes in all different shapes and sizes – foam, singly-ply, built-up and more. The various layers are bonded to the underlayment and to each other with some form of adhesive and or fastener.
  • On top of that are various different types of coatings that will add fire resistance, durability, shed heat or snow, add resistance to environmental chemicals and more. 
  • And finally, flat roofs also require some form of insulation below the roof material and coatings. 


More About Insulation

There are various types of insulation available for flat roofing:

  • wood fiber -  organic insulation board composed of wood, cane, or vegetable fibers mixed with fillers and binders.
  • perlite - composed of inorganic, expanded silicaceous volcanic glass and combined with organic fibers and binders.
  • Polyisocyanurate - a closed-cell foam plastic insulating core sandwiched between organic or inorganic felt facers, glass-fiber mat facers, or glass-fiber-reinforced aluminum foil facers.
  • Expanded or extruded polystyrene - expanded polystyrene consists of the polystyrene polymer impregnated with a foaming agent, while extruded polystyrene consists of a blended polystyrene polymer heated and run through an extrusion process. The material is exposed to atmospheric conditions, which causes it to expand and create its closed-cell structure. 
  • cellular glass - composed of crushed glass combined with a foaming agent.
  • gypsum board - non-combustible, water-resistant, treated gypsum core panel.


An Elegant Solution

As you can see, there are a lot of elements that go into a flat roofing structure. If any one of these elements is not properly planned or installed correctly, you will often end up with inconsistent sloping and drainage problems.

Thankfully there is a solution called “tapered insulation”. Tapered polyisocyanurate (or “tapered iso” for short), is a foam material created in sheets of several different thicknesses. Rather than making the sheets of insulation flat, they are intentionally made with various taper measurements - often 1/8” or 1/4” per foot. The various height sheets lock together and create a gentle slope that causes water to move downhill on what would otherwise be a flat roof surface. The taper can be designed to move water toward roof drains, scuppers or gutters.

Tapered iso has great insulating properties, guides to show which sides lock together, plus it is easy to install. While it shouldn’t be used for high-traffic areas (unless properly protected), it is a great option to help shed water from flat surfaces. Best of all, it can be designed for all roof dimensions.


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