Chapter 3 - Underlayment and Flashing Flashcards
What is underlayment?
The process of installing underlayment is also referred to as:
Any roll-type waterproofing material installed under the finished roof.
Installing underlayment a.k.a. capping in, drying in or pre-felting
What are the purposes of underlayment?
- Protects roof sheathing before covering is applied
- Acts as added weather protection
- Protects asphalt shingles against resins in wood sheathing
- Provides additional insulation (especially when nail strips create air layer between sheathing and roof covering)
- Cushions heavy roofing units (tile or slate) to help prevent chipping when someone walks on roof
- When low-slope roof mopped with hot asphalt, underlayment** prevents asphalt from leaking** through joints
What is the difference in saturated organic felt and saturated fiberglass felt?
Saturated Organic: Primarily rag, wood, & cellulose saturated in either asphalt or tar.
Saturated fiberglass: Continuous/random glass fibers bonded with plastic, soaked in asphalt.
What are pros/cons of saturated fiberglass felt over saturated felt?
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
What is synthetic felt?
What are its pros/cons (compared to other felts)?
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)
Is synthetic felt more expensive than other felts?
Yes, but ease of labor (lighter/rolls cover larger area) reduces labor costs, usually outweighing material costs.
When is underlayment required?
For all shingle roofs
except:
* Under wood shingles
* Between shingle layers on a re-roof (if shingles not tore off)
What materials can drip-edge be made from?
- 28 gauge (or thicker) galvanized metal
- Other non-corrozive, stain-resistant metals (copper/terne metal)
What are the requirements for extending and nailing drip edge metal?
- 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.
What is the purpose of gravel stop on aggregate-surfaced roofs?
Gravel stop helps prevent loose gravel from falling off the edge of the roof
How does gravel stop differ from a drip edge?
Gravel stop is similar to a drip edge but has a more pronounced lip to retain the gravel.
What are the key steps for installing underlayment on a roof?
- 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.
What factors can cause underlayment felt to buckle during installation?
- 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.
What is the process for installing underlayment in roof valleys?
- Install valley felt before installing felt on the main roof deck.
- Nail the valley felt in rows along the edges at 24-inch centers.
What are the requirements for nails and tin caps when installing underlayment?
- 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.
What are the requirements for installing valley underlayment at the ridge?
- 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.
What is the correct method for installing felt parallel to the eaves?
- 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.
What are the requirements for lapping and extending underlayment during installation?
- 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.
What are the top lap and exposure requirements for single and double underlayment coverage?
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
What are the requirements for underlayment installation on a gable roof?
- 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.
What is the formula for calculating the overcut area of underlayment for gable roofs?
Overcut area (gables) = Linear feet of rake × 0.34 square foot per linear foot.
What are the spacing requirements for interlayment felting on shake roofs?
- 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.
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?
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).
