Removing old material improves roofing install

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This article was published 04/05/2019 (2318 days ago), so information in it may no longer be current.

QUESTION: The roof membrane on my home is old and worn. I am wondering if I can put new membrane on top of the old roof membrane, or if I should remove the old membrane before installing a new one.

Your advice would be appreciated.

Thank you,

— Victoria

 

Answer: The proper course of action for installation of new roofing may depend on the type of roofing on your home. Regardless, removal of old roofing prior to reinstallation of new material is always the best practice and has several benefits.

With the snow gone, this is an ideal time of year to address your question, as many homeowners ponder whether to replace their roofing, or delay for another year. If your roofing is nearing or beyond the end of its normal life, replacement earlier rather than later can prevent a problematic and costly leak. Also, the cost of roofing materials may rise slightly every year and labour charges can also increase over time, especially if there are weather-related events that cause damage to multiple homes. Risking serious water damage, from a summer thunderstorm or windstorm, may not be worth the initial savings in delaying the inevitable roofing upgrade.

Removing older roofing before reinstallation of new material is always the best course of action, but is not absolutely necessary in all cases. In the past, it was quite common to apply up to three layers of roofing without getting down to the sheathing. This was often done with older wood roofing, due to the time-consuming nature of removing old cedar shingles or shakes and all their fasteners. One downside to that method is that the new shingles did not have a smooth surface to sit on. That could cause the shingle tabs to seal poorly and, cosmetically, the appearance would have been poorer than necessary. Many manufacturers will also not warranty their shingles unless they are installed with a proper layer of roofing paper or synthetic underlay, or if they are applied over older roofing.

The previous comments apply to typical asphalt-based shingles, but you have inquired about a “membrane,” which may be an entirely different type of roofing. Most roofing membranes are used on roofs with a shallower pitch, or lower slope, than traditional roofs. Because most membranes have a larger overlap than shingles, or are fastened without the use of nails, they are more suitable to flatter roofs. The types can vary between lower-quality rolled roofing, modified bitumen, PVC or other materials. For simplicity, I will assume your roof has either older rolled roofing or single-ply modified bitumen roofing, which are the most common for colder climates.

The first thing to investigate prior to making your decision is the condition of the original membrane. If it is excessively worn, has missing granules and visible felt, or is partially missing in several areas, then removal will be a must. Damaged roofing, especially single-ply membranes, can cause newer roofing to conform to its shape if applied above. If the older material has dips, gaps or raised seams, those will partially transfer through the smooth surface of the new membrane. This can cause premature failure, especially if the roof is walked on. Also, if there are any soft spots when the roof is inspected, it could mean moisture damaged sheathing below. The only way to address that is by removing the current roofing to examine the condition of the roof deck below.

Since you have stated that your older roof membrane is worn, removal should be planned, for sure. Not only will that allow for a superior job from both a structural and cosmetic point of view, it will let the roofers get a good look at the sheathing. Once the old roofing is gone, the old deck boards or plywood can be fully inspected for moisture damage or deterioration. If some sections are noticeably damaged, cutting out and replacement will be critical to achieving a long-lasting, leak-proof roof after completion. If the majority of the sheathing is damaged, spot repairs followed by installation of an additional layer of sheathing will be the way to go. That will strengthen the entire roofing system, as well as provide an excellent, smooth surface for application of the new membrane.

If the older membrane is removed and the overall condition of the decking is satisfactory, that will be a good sign that the insulation and ventilation is sufficient in the roof system below. If there are defects in those areas, removing the older roofing completely will allow better access for upgrades. If the roof is low pitch or flat, there may be minimal room for ventilation and the roof cavity could contain older insulation that is damaged or insufficient. Partial removal of the sheathing may be a practical way of replacing the current insulation with blown-in high-density polyurethane foam or equivalent products. That material will provide an excellent air seal, as well as good thermal insulation.

Even with a steeper-pitched roof, removal of the old membrane will allow for easier cutting of inspection ports, to inspect the attic for adequate insulation.

If limited, more insulation could be blown in through the ports, prior to installation of the new roofing.

While it may not be absolutely necessary to remove the old roofing membrane prior to upgrades, it is always the best choice.

Getting the old, damaged membrane out of the way will allow for proper inspection and repair to damaged sheathing and insulation, and will provide a better surface for installation of the new membrane.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

 

trainedeye@iname.com