Roofing Underlayment Compared: Synthetic vs Felt vs Self-Adhered
Underlayment is the layer you never see and can never replace without taking the roof off. Here is how felt, synthetic and self-adhered membranes actually behave under a metal roof.
Underlayment is the cheapest layer in a roof assembly and the one most likely to be treated as an afterthought. That is backwards. Under asphalt shingles, the underlayment gets replaced every time the roof does — every fifteen years or so, whether it deserved it or not. Under a metal roof, the underlayment has to survive as long as the panels do, because the only way to change it is to strip the roof. Choosing it on price is a decision you live with for a very long time.
What the Underlayment Is Actually For
It is not the waterproofing layer. The panels are. The underlayment is a secondary barrier that handles the water the panels are never meant to see: wind-driven rain forced back under a lap, snow blown in at a ridge vent during a whiteout, meltwater backing up behind an ice dam, condensation forming on the underside of a cold panel. It also protects the deck during construction, gives the crew a working surface, and acts as a slip sheet so the panels can move against the deck without abrading.
Asphalt-Saturated Felt
Felt is the traditional product — an organic or fibreglass mat saturated with asphalt, sold as No. 15 or No. 30 (numbers that once described the weight of a hundred square feet of the stuff and now describe very little). It is inexpensive and it is what most older Ontario roofs have under them. It also has real problems under metal.
- ✓It absorbs water. Get a roll wet before the panels go on and it swells, then wrinkles as it dries. Those wrinkles telegraph through a flat metal pan and show up as ridges in the finished roof.
- ✓It tears. A dropped tool, a boot heel on a hot afternoon, a gust catching a loose edge — felt gives up easily, and every tear is a hole in your secondary barrier.
- ✓It degrades fast in sunlight. Left exposed, felt dries out, curls and loses its asphalt in a matter of days, not weeks.
- ✓It softens with heat. Under a dark panel in July it can go tacky and stick to the underside of the metal, which fights the panel's thermal movement.
- ✓It does not seal around fasteners. Every clip screw is a permanent hole in the barrier.
Synthetic Underlayment
Synthetics are woven or spun-bonded polypropylene or polyethylene, usually with a scrim reinforcement and a coated top surface. Compared with felt they are dramatically tougher — you can stand on a synthetic sheet, walk it, and drag material across it without opening it up. Rolls are longer and much lighter, so there are fewer laps and less handling. They do not absorb water, so they do not wrinkle, which matters enormously when the finished surface is a flat metal pan that shows every irregularity beneath it.
The variables to check on a synthetic are the stated exposure window (how long the manufacturer permits it to be left uncovered), the traction of the walking surface — some are genuinely slippery when damp, which is not a small thing on a 9:12 — and whether the product is approved for use under metal. Not all of them are. Some synthetics are only rated for shingle assemblies and will not tolerate the temperatures that build up beneath a dark panel.
Self-Adhered Membranes
Self-adhered underlayment — peel-and-stick — bonds directly to the deck and, critically, seals around the shank of every fastener driven through it. That is the whole point. Every clip screw in a standing seam roof is a penetration, and a membrane that closes around the screw is a fundamentally different level of protection than one that just has a hole in it.
Two broad families exist. SBS-modified bitumen membranes use rubberized asphalt adhesive and are the familiar ice-and-water product. Butyl-based membranes use a non-asphaltic adhesive and generally hold up far better at the temperatures a metal roof reaches. Both come with a range of top surfaces — granular, film, or a woven polyolefin facer. For metal work, a smooth or film-faced sheet is preferred: a granular surface abrades the panel coating from below as the roof moves.
Compatibility matters. Asphalt-based products in direct contact with bare aluminum or with certain panel coatings can cause problems over time. Before you commit to a membrane, confirm with the panel manufacturer that it is approved beneath their system, and follow their installation instructions rather than the membrane maker's generic advice.
How the Three Get Combined on a Real Roof
In practice these are not competing choices so much as layers of a strategy. A typical, well-built Southern Ontario metal roof uses a self-adhered membrane in the places water is most likely to be pushed backwards — the eaves, the valleys, around chimneys and skylights, at sidewalls and headwalls — and a high-quality synthetic across the balance of the field. Some assemblies go fully self-adhered, which is the most protective option and the right call on complicated, low-pitch or heavily shaded roofs where snow lingers.
Where This Goes Wrong
The failures we see are boring and repetitive. Felt left exposed through a rain delay, then covered wet, and the wrinkles show through the panels for the life of the roof. A slippery synthetic chosen for a steep roof and the crew ends up working carelessly. A standard ice-and-water membrane used across a full unvented deck under dark panels, where it softens, bleeds, and grabs the panel underside. And the classic: no membrane at the eaves on a house that ice dams every February.
Condensation, Not Just Rain
In an Ontario winter, the underside of a metal panel can sit well below the dew point of the air in the assembly. If moist indoor air reaches that surface, it condenses and runs. A solid deck with a continuous underlayment, a properly detailed air barrier at the ceiling, and balanced ventilation are what prevent that. No underlayment on its own solves a condensation problem — but the wrong one, in an unvented assembly, will hold moisture against the deck and rot it quietly for years.
Frequently Asked Questions
Can the existing underlayment be reused when re-roofing in metal?
No. Old felt has already had its working life, it will be full of nail holes from the previous roof, and its condition under the shingles is unknown until the tear-off. Anything going under a roof intended to last decades should be new and installed on a clean, inspected deck.
Is two layers of felt as good as one layer of synthetic?
No. Two layers of felt is still felt — it still absorbs water, still wrinkles, still tears, and still has an open hole at every fastener. Doubling it adds bulk and cost without addressing any of the actual weaknesses.
Do I need underlayment if the metal is going over open purlins?
Over open framing there is nothing to lay an underlayment on, so the condensation control has to come from somewhere else — a condensation-control blanket, a felt-backed panel, or a properly designed vapour and ventilation strategy. That is a different assembly with its own rules, and it is not a place to improvise.
How long can underlayment be left exposed before the panels go on?
It depends entirely on the product, and the manufacturer states the limit. Felt is measured in days. Good synthetics are measured in months. Either way, the practical answer is that the roof should be dried in and panelled as promptly as the schedule and weather allow.
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