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7 Cards in this Set

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1. What are the major differences between a low-slope roof and a steep roof?
A low-slope roof is one whose slope is less than 3:12, or 25%. A low-slope roof is made up of the deck which supports the roof, thermal insulation, a vapor retarder, the roof membrane, drainage components and flashings.

A steep slope roof is a roof with a pitch of 25% or greater. Roof coverings for steep roofs fall into 3 categories: thatch, shingles, and architectural sheet metal.
Advantages of steep slope roof.
• Drains itself quickly of water, giving wind and gravity little opportunity to push or pull water through the roofing material.
• Steep roofs can be covered with roofing materials that are fabricated and applied in small, overlapping units (shingles of wood, slate or artificial composition; tiles of fired clay or concrete; or even tightly wrapped bundles of reeds, leaves, or grasses.
Advantages of low-lope roof:
i. A low-slope roof can cover a building of any horizontal dimension, whereas a steep roof becomes uneconomically tall when used on a very broad building.
ii. A building with a low-slope roof has a much simpler geometry that is often much less expensive to construct.
iii. Low-slope roofs, when appropriately detailed, can serve as balconies, decks, patios, and even landscaped parks.
Disadvantages of a steep-slope roof:
Cannot cover a building of any horizontal dimension because it becomes too tall on a broad building
Disadvantages of a low-slope roof:
i. Water drains relatively slowly from the surfaces
ii. Small errors in design or construction can cause them to trap puddles of standing water
iii. Slight structural movements can tear the membrane that keeps the water out of the building.
iv. Water vapor pressure from within the building can blister and rupture the membrane.
3 positions in which thermal insulation may be installed in a low-slope roof and adv. and disadv. of each.
i. Insulation below the structural deck. Adv: Relatively economical and trouble-free; Disadv: It leaves the deck and the membrane exposed to the full range of outdoor temperature fluctuations.
ii. Insulation between the deck and the membrane. Adv: The insulation protects the deck from temperature extremes and is itself protected from the weather by the membrane. Disadv: roof membrane in this type of installation is exposed to extreme temperature variations. Any water or water vapor that may accumulate in the insulation is trapped beneath the membrane, which can lead to decay of the insulation and roof deck, and blistering and eventual rupture of the membrane from vapor pressure.
iii. Insulation above the membrane is a relatively new concept. Adv: 1. Advantages: The membrane is protected from extremes of heat and cold, and the membrane is on the warm side of the insulation, where it is immune to vapor blistering problems. Disadv: Because the insulation itself is exposed to water when placed above the membrane, it must be made of a material that retains its insulating value when wet and does not decay or disintegrate. Extruded polystyrene foam is the one material that has all these qualities. However, polystyrene foam cannot be exposed to sunlight (which disintegrates it). It is protected with a layer of ballast consisting of crushed stone, a thin concrete layer laminated at the factory to the upper surface of the insulating board, or interlocking concrete blocks.
4. Compare a built-up roof membrane to a single-ply roof membrane
The membranes used for low-slope roofing fall into 3 general categories: built-up roof membranes (BURs), single-ply roof membranes, and fluid-applied roof membranes.

• A built-up roof membrane (BUR) is assembled in place from multiple layers of asphalt-impregnated felt bedded in bitumen. The felt fibers may consist of cellulose, glass or a synthetic. The felt is saturated with asphalt at the factory and delivered to the site in rolls. The bitumen is usually asphalt derived from the distillation of petroleum, but for dead-level or very low slope roofs, coal tar pitch is used instead, because of its greater resistance to standing water.

• Single-ply membranes are a diverse and rapidly growing group of sheet materials that are applied to the roof in a single layer. As compared to built-up membranes, they require less on-site labor, and they are usually more elastic and therefore less prone to cracking and tearing. They are affixed to the roof deck by any of several means: by fasteners concealed in the seams between sheets; or if they are sufficiently flexible, by ingenious mechanical fasteners that do not penetrate the membrane.