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Issue 1, Fall 2001

Why Slate Roofs Don't Need Ice and Water Membrane

Joseph Jenkins

Traditional roofing techniques have almost become extinct in the United States, fading away with the passing generations of roofing craftsmen over the decades. The generation gap between the traditional roofers of the past and the “modern” roofers of today has created a knowledge gap that at times seems almost unbridgable. This is starkly evident when the issue of “ice and water membrane” is raised.

At the International Preservation Trades Workshops near Washington DC a couple of years ago, during one of my “Slate Roof Restoration” presentations, a roofing contractor in the audience raised his hand to ask a question. “When do you use ice and water membrane on a slate roof?” he asked.

“Read my lips,” I replied. “N-E-V-E-R.”

The contractor stood staring at me in silence, jaw agape, dumfounded.

Weeks later I received a call from a roof restoration professional and consultant in New England. She wanted to know why I had included no ice and water membrane in the specs on a 52 square re-roof with slate on a residential job in Connecticut. During the process of answering her question, she became so flabbergasted that she hung up on me. She did not like hearing what I had to say. So hang on to your seat — if you’re an ice and water membrane junkie, as so many modern roofers and architects today are, you’re in for a surprise.

There is a photo of a 150 year old slate roof in this newsletter. This is a roof still in good functioning condition. Does it have ice and water membrane on it? No. There are millions of very old roofs in the United States and around the world, roofs 100 years old and some much older, still in good working order. Did they need or ever use ice and water membrane? No. These are traditional roofs constructed of traditional roofing methods. They do not, nor did they ever need ice and water membrane. Furthermore, these roofs can be duplicated today in the same style and perform with the same longevity without a square inch of ice and water membrane. So why do modern roofers and architects now think that ice and water membrane should be an essential part of every roof, as many seem to believe?

Well, aside from the roofing industry’s heavy ice and water membrane advertising pressure, we need to look at the evolution of roof design to get to the bottom of this. A generation or two ago, homeowners, architects, and roofers looking for cheaper, faster roofing methods began to use plywood for roof decks, covered with asphalt shingles. About 15 years into this modern development, roofers were shocked to discover that the plywood decking was delaminating along the bottom three feet of the roof, near the eaves. This was due to several reasons: low-slope roofs with slow water and snow run-off enabled ice and moisture to collect along the eaves; the plywood was susceptible to glue failure under hot, damp, and freeze-thaw conditions; and the roofs were afflicted with poor roof ventilation or a lack of “breathing” due to the plywood/asphalt combination smothering the roof and preventing airflow. Guess what came to save the day? You guessed it — ice and water membrane became employed to prevent delamination of the plywood roof decks — a band-aid solution to an inferior roof design. Inferior, that is, to traditional roofs.

Today’s modern roofers have become so steeped in the plywood/asphalt roofing systems that they have also become addicted to ice and water membrane, afraid to even think about installing a roof without it. However, traditional roofers who use traditional roofing methods avoid the use of laminated roof decking materials, and instead use natural wood boards or lath. They avoid non-breathing roof coverings and instead use natural slate or tile. Natural slate or ceramic tile combined with a natural wood board deck is a breathable roof. It is also a roof that does not need any ice and water membrane nor does it benefit from it. This may not be the sort of information that ice and water membrane manufacturers want you to hear, but it’s information that many roofers and architects need to know and understand. If you want to air your opinion on this matter, visit the slate roofing message board at and let’s hear what you have to say.

[Author's postscript: It should also be noted that peel and stick underlayments, and virtually all underlayments have a very short life expectancy relative to stone roofing. The idea that such underlayments will benefit a slate roof over the life of the roof is just incorrect. The underlayment will dry out, crack, disintegrate, turn to dust and fail long before the roof itself has reached its end. For some reason, this is a hard concept for many in the roofing and design industries to understand. If the underlayment material will fail long before the roofing fails, and the roofing will continue to service the building as a water-tight barrier for decades despite the lack of effective underlayment, then that should be seen as abundant proof that the underlayment's purpose is to keep the water out of the building until the final roofing is installed. Of course, this is assuming the slate roof is installed correctly with adequate headlaps, sidelaps, slope, and flashings. If the installation is poor, then the peel and stick underlayment may hide leaks long enough for the contractor to get paid and disappear. At the risk of being too candid, I must say that this may be the biggest reason why peel and stick underlayments are so popular today.]

Read an article about how to install ice dam resistant eaves on slate roofs.

Read an article about how to install a copper snow apron on a slate roof.



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