When it comes time to replace a roof, homeowners will likely be looking at several different products, and different roofing companies to perform the work. Often, a roofing contractor gives a quote which is much lower than anyone else. This is often a red flag; they may install an inferior material, surprise you with a sudden jump in charges, or simply do the work wrong. One way I see this happen is “cutting back on costs” by not removing the existing roof first – they install the new roof over the old one.
In my career, I have never seen a satisfactory roof that has been installed over an existing one. There are a number of reasons for this…
Double-layer roofs will look good and perform well at first, but often for only a short amount of time. The photo above is from a roof that was only ten years old – far too young to fail this badly. This roof should have lasted 15-20 years.
Due to the poor ventilation of the underside of the new shingles, the shingles overheat in warm weather and deteriorate. This also creates problems with the attic structure/sheathing, which may become damaged.
Certain manufacturers prohibit double-layer installations, and will dishonor the warranty if a roof is incorrectly installed.
And finally, the most important reason not to double-layer a roof:
When the new roof inevitably fails, the roofers will need to remove twice as much material, meaning double the labour and double the disposal cost.
I strongly recommend against double-layer roofing – if you have a property that you think may have a problem roof, a professional home inspection is your best protection against unwanted surprises.
Author David Fairbairn is a certified, licensed home inspector serving Vancouver and the Lower Mainland. He has been featured in the media and has contributed to "24" Newspaper, and Global TV. He has spent years working with residential and commercial building projects, and holds a Power Engineering License in BC. Why not give him a call for your next Home Inspection? Call 604 395-2795 or email firstname.lastname@example.org today!