Best underlayment for asphalt shingles in Vancouver (what actually works in rain & wind)

In Vancouver’s rain and wind, the best underlayment under asphalt shingles is a self-adhered SBS membrane at eaves/valleys plus a robust synthetic (polymeric) underlayment on field planes; on any shingle area below 1:3 (4:12) slope, switch the whole field to self-adhered. “Breathable” synthetics are optional, not required for most vented roofs. ( [RCABC][1] , [BC Publications][2] )

The fast plan I use on Vancouver homes

1. Self-adhered eave/valley membrane; generous laps and end dams. ( [RCABC][1] )
2. Field underlayment: polymeric synthetic, nailed/capped per spec.
3. If slope < 1:3 (4:12) → self-adhered across the whole field (no nail-applied). ( [RCABC][1] )
4. Ridge/hips sealed, walls stepped and counter-flashed, drip edges hemmed.
5. Keep pathways dry: soffit intake + ridge exhaust, baffles over insulation.

Quick comparison (Vancouver-specific)

Why underlayment choice is a climate decision first

Vancouver’s 1991–2020 climate normals show persistent cool-season precipitation and frequent wind events. That time-of-wetness stresses edges, valleys, and wall junctions—exactly where underlayment carries the load when rain gets beneath shingles. Build for shedding and controlled drying between storms. ( [Climate Data][5] )

Felt vs. synthetic vs. breathable—what actually matters here

Traditional felt (No. 15/30)

Felt meets code, but in practice it tears more easily, wrinkles when wetted, and offers short exposure windows. In Vancouver’s on-again off-again rain, that can complicate staging and detailing. Modern guidance emphasizes enhanced underlayment in demanding climates and slopes—exactly our reality. ( [National Roofing Contractors Association][3] )

Synthetic (polymeric) underlayments

These are my default on shingle planes steeper than 1:3. They resist tearing and foot traffic, stay flatter, and tolerate longer uncovered periods during changeable weather—meaning cleaner laps and fewer telegraphed wrinkles under shingles. NRCA’s steep-slope guidance treats synthetics as standard practice; the key is following cap-fastener schedules and manufacturer laps. ( [National Roofing Contractors Association][3] )

“Breathable” synthetic underlayments (vapor-permeable)

Do they help? Sometimes, but not for the reason most people think. Multiple research efforts show the overall asphalt-shingle roof assembly is low-permeance; swapping in a permeable sheet only nudges outward drying a bit, and often doesn’t change moisture outcomes in a vented roof. Use permeable sheets when the assembly logic calls for it (vented over-deck gaps, mass-timber protection) rather than as a generic “better” choice. ( [crca.org][6] , [Building Science][4] )

The code and standards guardrails I design to

• Vancouver Building By-law (VBBL) 9.26 spells out eave protection, acceptable underlay materials, and shingle installation basics (laps, fastening, low-slope rules). Even where eave protection exemptions exist, our coastal rain argues for self-adhered at vulnerable edges. ( [BC Publications][2] )
• RCABC Roofing Practices Manual (RPM): for shingle systems, nail-applied underlayment is not permitted below 1:3 (4:12) —those planes must use self-adhered; the accepted materials list includes SBS membranes, standard synthetics, and several permeable options. ( [RCABC][1] )

Bottom line: meet or exceed the VBBL, then follow RCABC’s steeper requirements to get assemblies that actually survive Vancouver’s wind-driven rain.

Where “breathable” really fits (and where it doesn’t)

• Vented roofs with standard sheathing (most Vancouver attics): Outward drying through shingles+underlayment is minimal; attic ventilation manages moisture. Choose robust synthetics for strength and staging; add permeable only if other layers (e.g., over-deck vent space) justify it. ( [crca.org][6] , [Building Science][4] )
• Over-deck ventilation (counter-batten air gap): Permeable sheets can help the sheathing dry into the ventilated space, but drying is still limited by the sheathing’s permeance and drainage plane above it. ( [Building Science][4] )
• Mass timber or recover scenarios: Some permeable, self-adhered products double as air barriers and temporary weather protection before shingles—useful on complex projects. (RCABC lists such products as accepted materials.) ( [RCABC][1] , [VaproShield][7] )

Slope decides a lot in our city

• ≥ 1:3 (4:12) shingle planes: Synthetic (non-breathable) is my default field underlayment; seams, caps, and laps per spec.
• < 1:3 (4:12) shingle planes: Treat like a “wet-risk” zone: use self-adhered membranes across the field, cemented courses, and conservative valley/wall details, per code and RCABC. (If a plane is flatter than 1:6, it may not be a shingle roof at all—design as waterproofing.) ( [BC Publications][2] )

Detailing that makes underlayment do its job

• Eaves: Self-adhered membrane lapped under drip edge at the eave, then over the rake drip—no shortcuts. ( [BC Publications][2] )
• Valleys: Full-width self-adhered plus metal valley flashing; widen on windward exposures.
• Walls/Chimneys: Step flashing with counter-flash and kick-outs; underlayment turned up the wall behind WRB.
• Fasteners: Plastic-capped nails on synthetics, in the field and laps where specified.
• Staging: Synthetics’ longer exposure ratings help in stop-and-start weather; keep edges pinned to prevent flutter and tears. ( [National Roofing Contractors Association][3] )

What I actually specify most often (Vancouver single-family)

• Eaves/valleys/penetrations: Self-adhered SBS (RCABC-accepted) for a continuous, sealed water-shed at stress points. ( [RCABC][1] )
• Field planes ≥1:3: Polymeric synthetic (RCABC-accepted), nailed/capped; breathable only if the assembly has a ventilated over-deck path or special substrate logic. ( [RCABC][1] )
• Low-slope shingle planes (<1:3): Self-adhered across the field, cemented courses, conservative details per VBBL 9.26.8. ( [BC Publications][2] )

Written by Harman, Roofer, Paragon Roofing BC (Vancouver).
I specify and install shingle and metal assemblies across Metro Vancouver’s wet, windy microclimates. My specs start with physics, then code, then craft—so your roof drains fast, dries fully, and keeps quiet through a November gale. For help, visit our Vancouver roofing services.

References (accessed Aug 17, 2025)

1. RCABC Roofing Practices Manual — Eave Protection & Underlayment
2. Vancouver Building By-law (2019), Division B, 9.26 Roofing
3. NRCA — Understanding Underlayment (steep-slope guidance)
4. Building Science Corporation — Field Testing Unvented Roofs
5. Environment and Climate Change Canada — Climate Normals
6. CRCA (2023) — Steep-slope technical update
7. VaproShield — SlopeShield Plus Self-Adhered
8. Canadian Climate Normals 1991–2020 (YVR)