Chapter 2 - Roof Sheathing, Decking & Loading Flashcards
What can cause eaves to sag on a roof?
Failure of roof framers to construct lookout rafters (perpindicular 2x4 rafters) between the eave (fly) rafter and next common rafter.
What can a sagging deck cause to asphalt shingles?
A sagging deck can stretch asphalt shingles, bending them over rafters so that granules pop off, discoloring the shingles.
* Lighter color over rafters (stretched)
* Darker color between rafters (sagged)
For what types of roofing is solid sheathing required?
- Asphalt, fiberglass, or metal shingles
- Mineral-surfaced roll roofing
- Built-up roofing
- Tile, slate, and metal roofing (except corrugated steel)
If installing shakes or wood shingles, solid sheathing should be used if:
- Average low temperature < 0 degrees fahrenheit
- Average january temperature of 25 degrees or less
- There’s a possibility of ice forming along eaves
Most solid roof sheathing is made of plywood. The American Plywood Association recommends a minimum plywood grade of:
Minimum grade of C-D Interior-and-Exterior Glue
* Most building codes also require this
What is deck deflection?
How do rafter spacings play into sheathing thickness?
Deck Deflection: Amount of decking material bends under load (due to concentrated weight)
Deck deflection should be limited to 1/240th of the total span of rafters.
* i.e. rafters 4 feet apart will allow deflection up to 4/240 ft or .20 inches before snapping
Roofing material manufacturers require minimum thickness of roof sheathing under a built-up roof:
1/2” minimum thickness
Unsanded Plywood sheathing must have a minimum index of:
32/16
(plywood can span a minimum of 32 inches between rafters, or 16 inches between 16 inches between floor joists)
What is dead load and live load of a roof?
Building codes require a minimum anticipated live load of:
Dead Load: Weight of the roof deck, underlayment, and roofing materials
Live Load: Permanent weights on roof + temporary weights (people/snow/water)
Building codes require a minimum anticipated live load of 30 psf
When is wood blocking used to support sheathing?
Where do you fasten wood blocking (relative to sheathing and rafters)?
When sheathing is NOT tongue and groove or edge clips are not used.
Install at unsupported edges of sheathing (between rafters), using 2x4 lumber.
How should sheathing panels be spaced relative to each other?
Space sheathing at least 1/16” between end joints (last piece along eave) and 1/8” between side joints (where 2 sheathing pieces join)
* Note: double this spacing in areas with high humidity
If using waferboard (OSB) instead of plywood sheathing, what is the minimum thickness required?
Minimum thickness: 15/32”
rafter span of 16”: 3/8” OSB
rafter span of 24”: 7/16” OSB
What is spaced (skip) sheathing and when is it used?
Square-edged sheathing of 1x4s and 1x6s that are spaced with gaps between the sheathing
* (gap = to center of shingle to center of next row of shingles)
- Used for wood shingles or shakes in milder climates (allows ventilation) or corrugated metal
What is the maximum amount of spacing for spaced sheathing?
2 1/2” (to prevent interlayment from sagging into attic)
What is the minimum roof slope recommended for wood shingles/shakes?
4 in 12 slope
However, can install on lower slopes if:
* Solid sheathing as bottom layer
* Built-up roof covering (hot mop) placed on solid sheathing
* 2x4” spacers installed vertically 24” O.C. across solid sheathing
* 1x4” or 1x6” nailing strips installed horizontally
