Carpentry 2: Roof Coverings
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What are the primary functions of roof sheathing?
- Strengthen the roof system by providing rigidity and
- Preventing rocking of the rafters and
- Provide a nailing base for the roof finish
What is the most commonly used sheathing material?
In Canada, is it permissible to use roof sheathing that is identified by US markings only?
What grade and minimum thickness of plywood and OSB could be applied to trusses 600mm on centre?
- 3/8" (9.5 mm) plywood or OSB 0-2 graded can be used if H clips or horizontal blocking is installed.
- If neither measure is employed, the minimum sheathing thickness allowed is 1/2" (12.5 mm)
Describe what each number and letter in the following grade marking means: 2R24
- 2 - horizontal edges must be supported
- R - suitable for roofs
- 24 - max span
Do all of the edges of 18 mm fibreboard roof sheathing required support?
Describe a vented roof panel.
- Rigid insulation
- 12-19 mm space for ventilation
Describe what conditions must exist in order for ice damming to occur.
- Hot roof melts snow
- Cold overhang refreezes causes damming
- Water can creep up shingles and into house
What materials are used for asphalt shingles?
- Heavy felt or fibreglass impregnated with hot ashpalt
- The upper surface is covered with a finely crushed slate
What materials are used for roll roofing?
The same but in a roll.
Describe the appearance of a wood hand-split shake.
A rough slit face and sawn back and varying thickness
Describe the appearance of a wood shingle.
Smooth sawn on both surfaces and uniform.
Does a standing seam metal roof have exposed fasteners?
No fasteners are exposed, resulting in a weather tight roof.
What types of screws or nails are used to fasten corrugated galvanized roofing?
- Resilient washer screws or nails.
- The fastener heads are exposed and special care must be taken in locating and tightening the fasteners to prevent water penetration.
What is the most important factor to consider when applying tile to an existing roof surface?
That the roof has adequate structural support.
List 2 reasons to use PVC roofing materials.
- 1. excellent durability
- 2. corrosion resistance
What is the primary application of plastic roofing materials?
List 3 applications of fibreglass roofing?
- Agricultural barns
Describe the traditional built-up roofing system.
- Several layers of hot application and asphalt bitumen and alternate plies of asphalt saturated roofing felt.
- The built-up roofing membrane is applied over rigid insulation board and followed with a flood coat of bitumen and a protective layer of light-coloured gravel.
Describe the purpose of each component of the roof membranes: asphalt bitumen, roofing felts, gravel cover
- Asphalt bitumen: main waterproofing
- Roofing felts: reinforce the membrane
- Gravel cover: protects the asphalt from the sun's harmful effects
What must be done to open valleys to protect them from moisture penetration?
Application of valley flashing with a minimum of 600 mm wide sheet metal or 2 layers of roll roofing, as per code, centred on the valley.
How far should the step flashing extend up the wall behind the sheathing membrane?
What is the maximum size a chimney can be before a cricket or saddle is required?
How far must eave protection extend up the roof slope?
A minimum of 900 mm not less than 300 mm up the inner face of the exterior wall.
What are the lowest slopes to which normal and low slope asphalt shingles can be applied
Where is flashing required on a roof surface?
All roof penetrations and intersections
When underlayment is used, how should it be laid and how much should it overlap?
- Parallel to eaves and
- should overlap the eave protection by 100 mm
- Each row should be top or end lapped a minimum of 50 mm
What is the purpose of the starter strip used when asphalt roofing?
Prevent water from getting through to roof at gaps between shingles
Where should asphalt shingles be nailed?
- 25-40 mm from the sides
- not less than 12 mm over each cut-out
How do you determine the proper exposure when applying asphalt shingles?
Manufacturers recommendations, usually 140mm maximum
List 3 options for securing asphalt shingle tabs.
- 1. Self-sealing strips
- 2. Interlocking features
- 3. A dab of roofing cement no larger than 25 mm in diameter under each tab
What is the benefit of applying spaced decking beneath wood shingles or shakes?
- Allows the underside to breath dissipating moisture.
- Wood shingles decay begins on the underside when moisture is trapped.
What type of underlayment must be used beneath wood shakes?
- Breathable type underlayment not less than 450 mm (18") wide interlaid between each course of shakes.
- The bottom edge is positioned above the butt line (bottom edge of shingle), equal to a distance of double the length of the exposure.
How far apart should individual wood shingles and shakes be applied?
- Individual shingles: 6 mm
- Shakes: 6-9 mm (to allow for expansion of the wood)
Roll roofing is typically applied to what type of roof surface?
Lower sloped roofs
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