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Underlayments, membranes and WRBs · Lesson 3 of 4

Underlayments, membranes and WRBs: methods and worked application

How the layers are installed: a dry-in sequence from eave to wall, the eave order in the code and in makers' instructions, three worked calculations, the roof-wall lap order and the wall WRB laps.

You will be able to: Follow a documented dry-in order, resolve the eave order against the product instructions, calculate ice barrier extent and two-layer coverage with units, and install the laps at roof-wall corners and walls.

13 min · 12 slides · 9 questions

All lessons in this module
Slide 1 of 12

Read the documents first

Before any layer goes down, record:

  • the code edition your code official enforces
  • the roof slope, and whether it is a wind-design area
  • whether the local Table R301.2(1) calls for an ice barrier
  • the product instructions or evaluation report for each layer

Sources

Videos for this lesson

What this video shows: A manufacturer's sequence for drip edge at eaves and rakes, ice and water protector and synthetic underlayment; its description gives both eave orders, and how IKO says to do the newer one.

Video: How to install Drip Edges, Eave Protection & Underlayment - Blueprint for Roofing by IKO North America, on YouTube. AFS checked its description against this lesson.

What this video shows: Self-adhering underlayment at the eaves, carried at least 24 in past the warm side of the wall when code requires it (a more conservative reference point than the exterior wall line in the code text, which the worked example uses); the lap values shown are those of the product used.

Video: Installing Self-Adhering Underlayment at the Eaves by Fine Homebuilding, on YouTube. AFS checked its description against this lesson.

What this video shows: A roof-wall corner built in order: membrane and underlayment up the wall, the kick-out in place of the bottom step flashing, step flashing laps and house wrap integrated over them. Its eave puts the membrane before the drip edge, one of the orders makers publish.

Video: Kickout Flashing by ProTradeCraft, on YouTube. AFS checked its description against this lesson.

What this video shows: Home Innovation Research Labs' recommendations for installing non-perforated, non-woven house wrap (2011); its description gives no lap values.

Video: Moisture Management in Residential Construction - Exterior House Wrap Installation by Home Innovation Research Labs, on YouTube. AFS checked its description against this lesson.

Glossary

Dry-in
Getting the underlayment on so the deck is protected before the roof covering goes on; Wilen (NRCA) lists weather protection for a limited time as one of underlayment's jobs.
Kick-out diverter
The first piece of step flashing at the bottom of a roof-wall intersection, set flat on the roof and flush with the wall, not fastened to the wall, to turn water away from the wall (PNNL).
Slope factor
Plain geometry, not a code value: the length along a roof slope per unit of level run, the square root of (12 squared + rise squared) divided by 12. At 6:12 it is 1.118. It converts a distance fixed in plan, such as the 24 in inside the wall line for an ice barrier in the 2018 IRC text Indiana adopts, into a length along the deck.
Turn-up
Roof underlayment carried up a sidewall before the step flashing goes on: at least 6 in over house wrap in the PNNL guide, and a 6 in leg in FEMA Fact Sheet 7.2.