What Is Synthetic Underlayment?
Synthetic underlayment is a woven or spun-bond polypropylene or polyethylene sheet installed across your entire roof deck before any shingles, tile, or metal panels go on. It is your secondary water barrier — the layer that protects your home if a shingle is ever lifted or damaged in a storm event.
Traditional felt underlayment (tar paper) served this role for decades. Synthetic underlayment replaced felt as the standard specification in Florida because it is stronger, lighter, more tear-resistant, and holds up better in high humidity — all critical properties for Haines City's climate. When underlayment is installed in the Haines City installation process →
Why Synthetic Underlayment Is Required in Florida
Florida's Building Code requires secondary water protection on all new and replacement roofing projects. The specific product must carry Florida Product Approval for the 135 mph basic wind speed zone that covers Haines City and all of Polk County. Felt underlayment (15 lb and 30 lb weights) can still meet this requirement in some applications, but synthetic is now the standard specification because it outperforms felt in every relevant metric for Florida's climate. Underlayment requirements for the 135 mph wind zone →
Synthetic Underlayment vs Felt — The Key Differences for Florida Homes
| Factor | Synthetic Underlayment | Traditional Felt (15 lb / 30 lb) |
|---|---|---|
| Weight per square (100 sq ft) | 0.5–1.0 lbs | 2–3.5 lbs |
| Tear resistance | Very high — holds shape in wind events | Moderate — tears in sustained wind |
| UV exposure tolerance | 180+ days without shingles | 24–48 hours before degradation |
| Slip resistance | High — walking surface integrated | Low — slippery when wet |
| Moisture resistance | High — resists absorption in FL humidity | Moderate — can wrinkle and buckle when wet |
| Florida Product Approval | Widely available from all major manufacturers | Available but being phased out in Florida specs |
How Synthetic Underlayment Works With Ice and Water Shield
Synthetic underlayment covers the full deck surface — but ice and water shield goes in the critical areas first. In Haines City, ice and water shield (a self-adhesive waterproof membrane) is installed in all valleys, along every eave, and around any area where water is most likely to pool or drive upward during storms. Synthetic underlayment then goes over the ice and water shield to cover the remaining deck area.
The two products work in combination — ice and water shield at the vulnerable points, synthetic underlayment as the full-deck secondary barrier. How ice and water shield works alongside synthetic underlayment →
For the full installation sequence including underlayment placement: Asphalt shingle installation using synthetic underlayment →