You are here:

Roofing/Modified Bitumen verse Duro-last


Hi Bruce,

I'm sure you've been asked this a million times but we have had so many problems with our roof, any help would be greatly appreciated.

I live in a condo building in Chicago. It's a 6 unit, 3 level building that was built in 2006. We have had leaks since 2008 but a new roof has been approved by the association.

We have modified bitumen up there right now. It was not installed correctly. Roofers have said that the builders went as cheap as they could and nailed the roof right to the base sheet causing wrinkling and cracking.

I am currently getting water intrusion. We have had a bad winter out here so far with lots of snow and -17 degree temperatures.

The roof is very flat and does not have much of a pitch.

One roofer wants to do Modified Bitumen with a tear down of the existing roof. He actually did our twin building last summer.

Another roofer wants to do Duro-last placing it right over the existing roof. He claims that there isn't enough pitch for Modified Bitumen and it will fail and that Duro-last is made for roofs that pool.

Given our weather conditions, can you offer us any advice?


You ask a very tough question.  Both roof systems are successful, but can perform better under various circumstances.  Being that you are in Chicago, you have a wide range of weather conditions with very warm summer and very cold winter temperatures.  Obtaining a reflective roof surface can aid in reducing your summer time heat gain and cooling costs, however that same cool roof surface can result in unwanted condensation in the winter months.  Both membranes can perform effectively in the atmospheric range present in Chicago.

Given that your building was constructed in 2006 while not impossible, it is highly unlikely that the slope of the roof is less than 1/4"/foot as required by Building Code which is well within the capabilities of modified bitumen.  We have specified and overseen the installation of SBS modified roofs installed with hot asphalt and adhesive on dead flat roof decks, and one I have reviewed yearly for the past 20.  The roof is covered with ponding water over 50% of the area, it has never leaked, and should continue to perform for another 10 years.

For the modified option I suggest that you remove the old roof membrane and consider mechanically attaching a substrate board to the roof deck.  I am assuming that your building has a vented attic space and no over deck roof insulation is required or desired, so please respond if this is not the case.  Apply one layer of a self adhering modified base sheet and adhesively apply an energy star 'cool roof' modified top ply sheet with a granule or chip surface.  This roof should perform for 20-30 years.

For the Durolast option I would caution against the usual installation method of mechanical attachment since this leaves a greater potential for condensation to form under the surface of the membrane.  This condition does not always happen but it is difficult to predict. Sometimes minor changes in the interior atmosphere can have an impact on the propensity for condensation.  This roof membrane should also receive a mechanically attached substrate board and then the single ply PVC membrane should be fully adhered to the substrate board.  Durolast has recently introduced roll goods into their product line so they should be able to install this type of assembly.  Durolast generally pre-manufactures the membrane in the factory which reduces the number of field welded seams and installation time, but often comes at the sacrifice of aesthetics.  The old roof should be removed if you have the funds, but at the minimum the base flashing and cants need to be removed for the membrane perimeter attachment. The PVC roof membrane should not come into contact with Asphalt materials.  Durolast contractors generally secure work by being the lowest bidder, which as a building owner is appealing. This roof should be expected to last 20 years, but more is possible.

Both these types of roof membranes are subject to damage from hail, but a rigid substrate board should make both membranes more impact resistant.

I hope this helps you make your decision.


Bruce Ryan II, RRO
Professional Roof Consultants, Inc.


All Answers

Answers by Expert:

Ask Experts


Bruce A. Ryan II, RRO


I offer solid knowlege of all types of Commercial and Residential Roofing, Waterproofing, and, Building Envelope systems. Experience ranges across low slope and steep slope roof systems. I am also well versed in matters regarding condensation and ventilation. I enjoy donating some of my time and knowledge to the betterment of others.


Bruce Ryan has 20 years of roofing, waterproofing, and building envelope consulting experience with PRC, with 5 years of commercial roofing experience prior to joining the firm. He became Vice President of the company in 1998. Bruce Ryan plays a key role in the development of practical, long-term roofing and waterproofing solutions, along with implementation of on-site forensic studies. Bruce also has a high level of experience with regard to the impacts of roofing materials and construction for demanding clients with heavily occupied structures.

Oregon Construction Contractors Board Construction Specifiers Institute National Roofing Contractors Association The Institute of Roofing, Waterproofing, & Building Envelope Professionals Oregon Board of Investigators Installation Masters

University of Maryland BS Business & Administration Registered Roof Observer - RCI Private Investigator Certified EIFS inspector - Northwest Wall and Ceiling Bureau Certified InstallationMaster™ - The Installation Masters™ Training and Certification Program (developed by American Architectural Manufacturers Association)

©2017 All rights reserved.