Chapter 3 - Underlayment and Flashing

Question 1

What is underlayment?

The process of installing underlayment is also referred to as:

Answer 1

Any roll-type waterproofing material installed under the finished roof.

Installing underlayment a.k.a. capping in, drying in or pre-felting

Question 2

What are the purposes of underlayment?

Answer 2

1. Protects roof sheathing before covering is applied
2. Acts as added weather protection
3. Protects asphalt shingles against resins in wood sheathing
4. Provides additional insulation (especially when nail strips create air layer between sheathing and roof covering)
5. Cushions heavy roofing units (tile or slate) to help prevent chipping when someone walks on roof
6. When low-slope roof mopped with hot asphalt, underlayment** prevents asphalt from leaking** through joints

Question 3

What is the difference in saturated organic felt and saturated fiberglass felt?

Answer 3

Saturated Organic: Primarily rag, wood, & cellulose saturated in either asphalt or tar.

Saturated fiberglass: Continuous/random glass fibers bonded with plastic, soaked in asphalt.

Question 4

What are pros/cons of saturated fiberglass felt over saturated felt?

Answer 4

Organic felts can deteriorate/wicken (absorb water)

Pros:
* Don’t rot or absorb water
* Last longer
* Fire resistance

Cons:
* Cost about 65% more
* Easier to tear
* Higher wind uplift risk

Question 5

What is synthetic felt?

What are its pros/cons (compared to other felts)?

Answer 5

Made of polypropylene fabric, more commonly used in modern roofing.

Pros:
* Tear-resistance many times greater
* Water-resistance
* No organic material, so no mold growth risk.
* Allows air to pass through

**Cons: **
* Subject to wrinkling and deterioration from volatile compounds.
* Provides no cushioning for brittle roofs (tile/slate)

Question 6

Is synthetic felt more expensive than other felts?

Answer 6

Yes, but ease of labor (lighter/rolls cover larger area) reduces labor costs, usually outweighing material costs.

Question 7

When is underlayment required?

Answer 7

For all shingle roofs
except:
* Under wood shingles
* Between shingle layers on a re-roof (if shingles not tore off)

Question 8

What materials can drip-edge be made from?

Answer 8

• 28 gauge (or thicker) galvanized metal
• Other non-corrozive, stain-resistant metals (copper/terne metal)

Question 9

What are the requirements for extending and nailing drip edge metal?

Answer 9

• Extend the drip edge back over the roof deck at least 3 inches.
• Nail on 8 to 10-inch centers in standard conditions.
• In high-wind areas, nail on 4-inch centers.

Question 10

What is the purpose of gravel stop on aggregate-surfaced roofs?

Answer 10

Gravel stop helps prevent loose gravel from falling off the edge of the roof

Question 11

How does gravel stop differ from a drip edge?

Answer 11

Gravel stop is similar to a drip edge but has a more pronounced lip to retain the gravel.

Question 12

What are the key steps for installing underlayment on a roof?

Answer 12

• Underlayment can be installed single or double-coverage.
• Install felts over the drip edge at the eaves and under the drip edge at the rakes.
• Roll out the felt, nail down one end, and stretch it.
• Roll out only as much felt as can be installed within an hour.

Question 13

What factors can cause underlayment felt to buckle during installation?

Answer 13

• Heat causes the felt to relax and buckle.
• The hotter the weather, the greater the buckling.
• Wind can also cause the loose felt to buckle.

Because of these factors, only unroll enough underlayment that can be applied within 1 hour.

Question 14

What is the process for installing underlayment in roof valleys?

Answer 14

• Install valley felt before installing felt on the main roof deck.
• Nail the valley felt in rows along the edges at 24-inch centers.

Question 15

What are the requirements for nails and tin caps when installing underlayment?

Answer 15

• Drive nails through tin caps (32-gauge, at least 1 5/8 inches in diameter) or use Simples nails.
• Use galvanized nails at least 3/4 inch long, or long enough to go all the way through the sheathing.

Question 16

What are the requirements for installing valley underlayment at the ridge?

Answer 16

• Extend the valley underlayment at least 6 inches beyond the ridge.
• Cut it to conform to the surface of the sheathing with no bulges.
• Nail it down securely.

Question 17

What is the correct method for installing felt parallel to the eaves?

Answer 17

• Install all felt parallel to the eaves, starting at the eaves.
• Allow the edge of the felt to overhang the drip edge at the eaves by at least 3/8 inch.

Question 18

What are the requirements for lapping and extending underlayment during installation?

Answer 18

• Roll and lap the felt at least 6 inches on both sides of hips
• Overlap 36-inch-wide felt in valleys by 6 inches on both sides.
• Extend the felt 3 to 4 inches up vertical surfaces, such as walls and chimneys.

Question 19

What are the top lap and exposure requirements for single and double underlayment coverage?

Answer 19

Single coverage: Use a 2-inch top lap with a 34-inch exposure
Double coverage: Use a 19-inch top lap with a 17-inch exposure
End laps: Vary based on underlayment type, roof slope, climate severity, and building codes

Question 20

What are the requirements for underlayment installation on a gable roof?

Answer 20

• Cut the underlayment 4-inches beyond the rake edge and trim it flush later
• For tile-covered gable roofs, wrap the underlayment down over the rake fascia and nail it to the barge board on 6-inch centers.

Question 21

What is the formula for calculating the overcut area of underlayment for gable roofs?

Answer 21

Overcut area (gables) = Linear feet of rake × 0.34 square foot per linear foot.

Question 22

What are the spacing requirements for interlayment felting on shake roofs?

Answer 22

• Use 30-pound, 36-inch-wide underlayment for the starter course at the eaves.
• Follow with 30-pound, 18-inch-wide interlayment courses.
• For the first interlayment, place the bottom edge of the felt twice the exposure distance above the butts of the starter course.
• For subsequent courses, the distance between the bottom edges of interlayment equals the weather exposure.

Question 23

For a shake roof with a 7 1/2-inch exposure and a 1 1/2-inch overhang, how do you calculate:

1) The distance from the eaves to the bottom edge of the first interlayment course?
2) The spacing for subsequent courses of interlayment?

Answer 23

1) First interlayment course:
Multiply the exposure by 2, then subtract the overhang.
Calculation: (2 x 7 1/2) - 1 1/2 = 13 1/2 inches

2) Subsequent courses: The distance between the bottom edges equals the exposure
(7 1/2 inches).