Key Takeaways
Synthetic underlayment vs. old felt paper — why polymer-based underlayment resists tearing, holds up to foot traffic, and handles rain delays better.
Synthetic underlayment is a polymer-based sheet material installed directly on the roof deck beneath the shingles, and on most Illinois roofs today it has replaced traditional felt (tar paper) as the standard choice. It does the same basic job as felt — a secondary water barrier between the deck and the shingles — but performs noticeably better on every practical measure that matters during and after installation.
What's the Difference Between Synthetic Underlayment and Felt Paper?
Traditional felt paper is an asphalt-saturated organic or fiberglass mat — the older-generation material most roofs used for decades. Synthetic underlayment is woven from polymer materials (typically polypropylene or polyethylene), engineered specifically for roofing rather than adapted from older building paper technology. The material difference translates directly into better field performance in a few specific ways.
Why Does Tear Resistance Matter During Installation?
Felt paper is prone to tearing around fastener points and in windy conditions during installation, especially on larger commercial-scale jobs where a sheet may be exposed to wind gusts before shingles go down. Synthetic underlayment's woven polymer structure resists tearing far better, which matters for the integrity of the water barrier — a torn or fastener-pulled section of felt is a weak point that can go unnoticed once shingles cover it.
Is Synthetic Underlayment Safer to Walk On?
Yes, and this is one of the more practical differences on the job site. Felt paper becomes slick when damp and can be genuinely hazardous underfoot on a sloped roof deck. Synthetic underlayment is manufactured with a textured, slip-resistant surface specifically because crews need to walk across it repeatedly while installing ice-and-water shield, flashing, and shingles — a meaningfully safer surface during the installation process itself.
What Happens If It Rains Before Shingles Go On?
This is where synthetic underlayment separates most clearly from felt. Felt paper absorbs water, which can cause it to wrinkle, buckle, or lose integrity if a job gets rained on before shingles are installed — sometimes bad enough that soaked felt needs to be replaced before work continues. Synthetic underlayment is far more water-resistant and UV-stable, so it can sit exposed on the deck for days through a weather delay without the material itself breaking down. On projects where scheduling around Illinois weather is a real factor, that resilience alone can avoid a costly delay or rework.
Does It Matter for the Homeowner Long-Term?
Underlayment is a hidden layer, but it's part of the overall system that determines how well a roof performs over its life — alongside decking, flashing, and the shingle material itself. A tear or gap in the water barrier underneath the shingles is the kind of defect that doesn't show up as a leak for years, and then shows up as water damage that's disproportionate to how old the roof looks from the ground. Most contractors now use synthetic underlayment as standard practice on any roof replacement for exactly these reasons, rather than offering it as an upgrade.