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Home / Code Guides / Water-Resistive Barriers

House Wrap & Water-Resistive Barrier Requirements — Laps and Flashing

Siding is not waterproof and was never meant to be. The water-resistive barrier behind it is the surface that actually keeps bulk water out of the wall, and it only works if every lap sheds downward.

RequirementModel Code StandardSection
Barrier requiredOne layer of No. 15 asphalt felt or an approved equivalent water-resistive barrier must be applied over studs or sheathing behind the exterior veneerIRC R703.2
Horizontal lapsApplied horizontally in shingle fashion, with the upper course lapping the lower not less than 2 in.IRC R703.2
Vertical jointsWhere vertical joints occur, the barrier is lapped not less than 6 in.IRC R703.2
Flashing integrationFlashing at window and door openings, penetrations, roof-to-wall intersections, decks, and material transitions must be integrated with the barrier so water drains to the exterior — the barrier laps over a top flange, and a bottom flange laps over the barrier below itIRC R703.4
DrainageThe wall assembly has to provide a means of draining water that gets behind the cladding; for most claddings the barrier plus a drainage path satisfies it, and some assemblies need a defined drainage spaceIRC R703.1.1
AttachmentFastener type, spacing, and seam treatment follow the barrier manufacturer’s installation instructions, which are enforceable as part of the product listingIRC R703.2; R102.4
Two layers behind stuccoCement plaster applied over wood-based sheathing requires two layers of water-resistive barrier rather than oneIRC R703.7.3

Worth knowing

  • Laps are directional, not just dimensional. A perfectly measured 6 in. lap installed with the upper sheet tucked behind the lower one funnels water straight into the wall.
  • House wrap and air barrier are different jobs. Taping seams is what turns a water-resistive barrier into an air barrier, and only some products are listed for that use.
  • UV exposure degrades most wraps in a matter of months — check the manufacturer’s stated exposure limit any time cladding will be delayed.
  • Window flashing sequence is where most wall leaks begin: sill pan first, jamb flashing over the sill flange, head flashing tucked under the barrier above.
  • Self-adhered and fluid-applied barriers are approved alternatives, but they follow their own evaluation reports rather than the felt lap rules.

Common questions

What does the code say about barrier required for Water-Resistive Barriers?

One layer of No. 15 asphalt felt or an approved equivalent water-resistive barrier must be applied over studs or sheathing behind the exterior veneer. (IRC R703.2)

What does the code say about horizontal laps for Water-Resistive Barriers?

Applied horizontally in shingle fashion, with the upper course lapping the lower not less than 2 in.. (IRC R703.2)

What does the code say about vertical joints for Water-Resistive Barriers?

Where vertical joints occur, the barrier is lapped not less than 6 in.. (IRC R703.2)

Adopted edition by state

Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.

Related sections and guides

This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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