As roofing systems come and go, there are a few that remain solid performers throughout the years. Based upon economics, building use and other factors, the type of roof systems can range drastically “simple” to “complex” or costly to inexpensive. Working on a wide range of projects from tilt up to high rise, etc., I have found that many people in our industry haven’t had a lot of experience with IRMA or PRM systems. The basic premise is simple. The term PRM says it all; Protected Roof Membrane. Now use the IRMA as in Inverted Roof Membrane Assembly and you see that that in lieu of having the roof membrane on top or above the rest of the assembly, you install it first then “protect” it. Like taking the an insulated roof system and inverting or flipping it upside down. How do you do this? It starts with the substrate. For this type of roofing system you would need a structural deck, typically concrete. Some have used lightweight insulating concrete, but most IRMA systems will be installed over a structural concrete deck. I have used this type of roof system over a plywood deck with success, however it was engineered specific to this system and it was a small equipment area. You wouldn’t normally install an IRMA roof over a plywood deck. Mainly because of the cost and when dealing with plywood decks, one would typically be dealing with a large roof area, which brings the cost way up based upon the extent of square foot area.  The second item that you would consider is the membrane choice.  The one thing about this system is having a myriad of options for your roofing/waterproofing membrane. You are not confined to one or two options. Use can use any of the following or even more if you so choose:

Hot fluid-applied rubberized asphalt;
Cold Fluid-applied urethane coating;
Single Ply;
Built-up asphalt roofing;
Modified Bitumen roofing;

Obviously, some are better than others. Some offer more advantages versus others. The main thing to understand is why you would utilize this type of membrane assembly.  In most roofing system, you install the insulation over the substrate, followed by the “waterproofing” membrane, with the surfacing (as in BUR or Modified roof systems).  With a single ply, the membrane is the first and final layer or surfacing. With the two fluid-applied systems, you would not see these in the more tradition roofing systems and they only come into play with the IRMA or PRM because of the covering or protection. Lets talk about what happens after the roof system is installed over the insulation. In time the building will have foot traffic, maintenance, equipment change out, typically atmospheric degradation such as sun, wind and rain. All of these are constant and cannot be taken out of the factors that affect the service life of the membrane. With the normal roof system, the traffic on the membrane eventually affects the service life. When water is allowed to enter through the membrane from any deficiency, then the insulation can now be affected.  Finally, finding the leak becomes problematic due to the fact that where the water enters the membrane and where it shows itself in the building can be two distinct locations. Maintenance or repair to the roof system may involve extensive testing via Infrared scanning or destructive testing by opening up the roof system to determine how wet or deteriorated the insulation is located. Both of these are costly and time consuming, not to mention the affect it has on the overall system. When leaks do occur within an insulated system, we have seen a lot of repairs performed without doing the homework to find out what has happened to the materials below. The roof may still leak, the wet insulation can still be holding water and worse yet, you have lost the insulating R-value due to the wet insulation thus rendering the insulation useless. The traffic also affects the overall service life. With a building that has a high level of equipment, maintenance and foot traffic the roofing membrane can suffer and the service life is shortened sometimes drastically.

So why use a IRMA or PRM system? Going back to the definition, we want to protect the membrane. How do you do that? Simple put it on the substrate (concrete in most cases) and cover it up so that it cannot be damaged. Start by installing the membrane over the structural deck. With all of the options listed above you want to “finish” the system meaning install the flashings, surfacing or protection sheet over the roofing/waterproofing membrane. This system can be worked over, walked on, even construction can occur after the roof membrane has been placed. In new construction, this allows the building to be watertight much earlier in the construction period so that interior work can move along faster than normal. As they say in the construction trade “Time is money!” Now that the membrane is in place, what is next? Now the insulation can be placed over the membrane. Except this is where the biggest change takes place. The typical roofing insulation is Polyisocyanurate insulation. To make it simple, this type is not suppose to be exposed to the elements. So you would not use it in an exposed assembly. EPS (Expanded Polystyrene) or bead board or “Styrofoam” is another typical roof insulation. This also should not be used in an exposed manner. So what can you do? When using an exposed insulation, you would utilize an EXPS or XPS or Extruded Polystyrene. One location to learn more about this type of insulation is www.xpsa.com.  You will learn that XPS insulation has more R-value per inch than traditional insulation. It is resistant to moisture, which is why it is used in this type of assembly.
It has greater structural capacity or basically is stronger. It is and can be used in many locations on your building. I recommend reading more about it as an option for insulation when designing systems.

So once you have the insulation protecting the membrane, what do you do then? Well the insulation has to be held in place. Otherwise wind can literally blow it away. As in traditional systems, you have to provide a surfacing. The surfacing of an IRMA or PRM provides two benefits. 1) Holds the insulation in place; 2) Provides a walking surface. Now there are other components such as flashing membranes and drainage board, filter fabric and insulation tape for joints, but a “Design” blog is for later. This is for bestowing the basic principle of the IRMA roof system. The two components used are typically rock or ballast and concrete as in pavers or poured. In both cases, the most important factor to consider after the membrane of course is Wind Uplift. With all roof systems, the wind uplift should be reviewed so that you meet the local requirements and do not allow a wind failure to occur. By providing the correct amount of ballast per square foot (Typically 15 pounds psf for the perimeter) you will meet or exceed FMG I-90 wind uplift rating. (FMG stands for Factory Mutual global). When using pavers, the typical paver size is 2′x 2′ x 2″, any larger and the handling of the paver becomes difficult. The paver is used in window washing locations, foot paths between access and equipment, etc. The ballast is used in the field areas where foot traffic does not occur or is not required.

Now you can see the one thing that stands out on this type of assembly. For me it is simple. If I get a leak, I don’t lose my insulation. I can remove the materials over the membrane, find the leak location due to the fact my membrane is fully adhered to the substrate, provide repairs and put the materials back into place. Finally, the wear and tear on the membrane is eliminated, thus reducing long term maintenance costs. Remember I also have a greater thermal envelope on my building due to the higher R-value per inch. There are so many advantages to this type of system I could on and on. However, it does have one thing that keeps it from become more widely used. The Cost! The ballast and pavers are costly, the insulation is typically more expensive and if you are using the hot rubberized asphalt
system, that too can be more expensive. We would recommend a Life Cycle Cost analysis for anybody considering the use of this type of assembly.

I first became interested in this type of system when working with SOM(Skidmore, Owings & Merrill) in Los Angeles, CA. Their firm has been around since 1936 and began in Chicago. The IRMA or PRM system is a staple of their designs. If you go up on a building that has ballasted insulation, membrane below and walkways of either pavers or concrete, you are most likely on a SOM designed building. Go to their website www.SOM.com and you can learn more about them.

So what do we know? If you protect the waterproofing or roofing membrane it will last longer. If you utilize an insulation that does not break down under water, you won’t have to worry about the weathering of the insulation. If you use a solid walking surface, you won’t have to worry about damage to the membrane or insulation. All of these add up to greater service life and lower maintenance costs. So protect your investment and begin protecting your roof. You can’t go wrong.

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