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Underlayments, membranes and WRBs · Lesson 4 of 4

Underlayments, membranes and WRBs: defects, verification and case study

The common faults and the source each one misses, an inspection checklist, what to record, a fall from a rain-slick deck, and a roof-wall leak traced to its laps.

You will be able to: Diagnose reverse laps, missing turn-ups, short ice barriers and wrong or overexposed products, check a dry-in against its documents, and record corrective options with evidence.

12 min · 12 slides · 9 questions

All lessons in this module
Slide 1 of 12

Laps that run the wrong way

Reverse lap: a lower layer laid over an upper one, so water running down goes under the lap instead of over it.

At a roof-wall corner, house wrap tucked behind the step flashing sends water behind the flashing; PNNL puts the wrap over the kick-out and base flashing.

Correct and incorrect lapsFour pairs, correct on the left with a check mark, wrong on the right with a cross. Row 1: sheets on a slope, upper over lower, against lower over upper with water running under the lap. Row 2: WRB over the step flashing, against step flashing over the WRB with water behind it. Row 3: underlayment turned up the wall, against stopped at the corner. Row 4: ice barrier past the 24 in mark inside the wall line, against stopped at the wall line.correctwrongruns underwater behindthe flashing6 in minstops atthe corner24 in24 inABCDEFGH

Four pairs, correct on the left with a check mark and the word correct, wrong on the right with a cross and the word wrong. A: the upper sheet laps over the lower and water runs over the lap. B: the lower sheet laps over the upper and water runs under it. C: the WRB comes down over the step flashing. D: the WRB is tucked behind the step flashing and water runs behind it. E: the underlayment turns up the wall. F: it stops at the corner. G: the ice barrier passes the 24 in line inside the wall. H: it stops at the wall line.

A
Correct: upper over lower
B
Wrong: lower over upper (reverse lap)
C
Correct: WRB over the step flashing
D
Wrong: WRB tucked behind the step flashing
E
Correct: underlayment turned up the wall, 6 in minimum
F
Wrong: underlayment stopped at the corner
G
Correct: ice barrier to 24 in inside the wall line
H
Wrong: ice barrier stopped at the wall line
Sections, not to scale. A, C, E and G follow the code texts and the PNNL guide read for this module; B, D, F and H are the faults they prevent. Simplified: rows C to F leave out the house wrap already on the wall, which the turn-up laps over (see the exploded sections), and C and D leave out the roof underlayment.

Sources

Glossary

Reverse lap
A lower layer laid over an upper one, so water running down goes under the lap instead of over it. It breaks the shingle-fashion rule.
Exposure limit
How long a product may stay uncovered, set by its maker: Grace Ice & Water Shield HT 120 days, Malarkey Secure Start HT up to 6 months of UV exposure.
FACE report
A Fatality Assessment and Control Evaluation case report. Washington report 71-203-2021s describes a roofer who fell 30 ft from a rain-slick 5/12 roof deck.
Adhesive softening
Self-adhered underlayment adhesive going soft in an overheated roof; JLC warns it can drip onto the building exterior or find its way into gaps in the roof sheathing and into the building.