A roof under a roof – is it okay to install a roof on an old roof membrane?
Recently, we were working on a flat or low slope roof and we noticed that a neighboring roof had literally blown off, partially. A relatively large portion of the roof had been lifted up in heavy winds and been blown and now laid strewn across another neighbor’s rooftop.
We talked to the homeowner of the roof that had been damaged. They thought that the rear porch roof had been leaking but they didn’t even know that the entirety of the most recent membrane on top of that flat roof had completely been ripped away from the building and caught in the high winds of a recent windstorm and blown upon the neighbor’s roof. Little pieces of the damaged underlying substrate roof were scattered about and found in many different places.
The picture below shows the damage of the remaining wreck.
About 5 years ago, the owner of the building hired another roofing contractor to give him a new roof. He didn’t really think about the fact that the newest roof contractor was simply overlaying the existing Is multiple layers of roofing below. If that contractor had provided options, he would have been inclined or at least possibly considered paying extra to remove the substrate layers of roofing material. The building code technically requires that any roof system with more than one existing layer be fully stripped down to the substrate wood decking at the next roof membrane replacement. So in other words, you’re not allowed,by building code to apply more than a total of 2 layers of roofing material or roofing membranes to a rooftop. However, this happens so often that contractors, in an effort to save a buck or make their bids cheaper than other contractors, do not include removal of the single or multiple substrate layers of roofing. Hard bidding based on price alone is often referred to as a “race to the bottom”, and this example illustrates the negative effects of hard bidding based on price. We recommend that old layers of roofing are always removed and not overlaid, if the entirety of the roofing is to be done and built again. Repairs, or small areas of roofing are a possible exception, it doesn’t always make sense to remove the entirety of a roof if a repair is necessary. But if the roof is being completely built again then the substrate should just be simply removed. It makes more sense, it does cost additional labor and disposal fees, but it’s better for the long term.
In the next picture below, you can see the entirety of the old roof membrane where it has been lifted up by the wind and strewn across the neighbor’s rooftop.
How to Maintain A Flat Roof System
Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science. Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the roof and related systems of your building in Washington DC, contact us or fill out the webform below and drop us a line. We will be in touch if we can help.