Roof Underlayment cum Vapour Barrier?

A disturbing trend has taken root in southern Ontario over the last dozen or so years; migration to less permeable roof underlayment.

When I redid the shingled part of my roof in 2001, I asked many veteran roofers, “Why do you use tar-paper under shingles?” and the most common answers were:

  1. “I don’t know!”
  2. “Insurance; in case it starts raining during the re-roofing, the roof is will shed water.”
  3. “It’s easier to tear the old shingles off if there’s felt paper between shingles and roof sheathing.”

Not so on the last point with today’s peel and stick. The new generation of peel and stick roofing membranes are very difficult to clean off roof decks without destroying the wood sheathing and expensive.


The truth is, for centuries humans relied on shingles made of wood or slate to shed the rain. No underlayment. No problems. As humans started jacking up the temperature in their homes, ice daming became more prevalent and as a band-aid response, industry invented the thneed for roofing: membrane underlayment.

Just say “No” to underlayment

The Energy Nerd, Martin Holladay, has a bullet list of do’s and don’ts for roof design and he feels felt paper’s enough in a well designed roof. I concur. I would go further to say:

  • many of these new membranes will prevent air movement through shingles – if a solid lumber deck.
  • should not be used to solve ice daming, don’t put a band aid on it, air seal and insulate
  • it will cost you more the next time you reshingle, as you’ll likely have to re-sheath the destroyed roof deck
  • some membranes are vapour barriers/retarders and could lead to condensation issues

On this last point, the photo below shows a roof being slowly re-shingled in Toronto (i.e. cold climate):

This membrane has been exposed for more than 30 days, covers both walls, cathedral and ventilated attics and at 0.05 Perms Grace Ice & Water Shield is considered "Impermeable".

This cold-side membrane has been exposed for significantly more than 30 days on this project, is indiscriminately covering walls, cathedral and ventilated attics alike and at 0.05 Perms, Grace Ice & Water Shield is considered “Impermeable”.