Roof deck designs speaks to what are roof deck designs and why roof deck designs.
They are used for structural decks for low slope roofs may be wood panels over wood joists, solid wood decking over heavy timber framing, corrugated steel decking, panels of wood fiber bonded together with Portland cement, sitecast concrete slab and precast concrete slab.
For a durable low slope roof installation, it is important that the deck be adequately stiff under expected roof loadings and fully resistant to upwind lift forces. The deck must slope toward drainage points at an inclination sufficient to drain reliably despite the effects of structural deflections. A slope of at least ¼ inch per foot of run which is 1 in 50 or 2 percent which is normally required by the building code and by most manufactures of low slope roof membranes.
To create the slopes in a low slope roof, the beams that support the deck may be sloped by varying the height of the supporting columns. Or the deck may be constructed level and the slopes created by a layer of thermal insulation of varying thickness installed on the top of the deck. This layer may consist of lightweight insulating concrete or a system of tapered rigid insulation boards.
If the slope of a low slope roof is too shallow, ponding occurs and this is the formation of puddles of water that stand for extended periods of time. This leads to premature deterioration of the roofing materials in those areas. If water accumulates in low spots caused by structural deflections, progressive structural collapse becomes a possibility with deepening puddles attracting more and more water during rainstorms and becoming more heavy until the beam or joists become loaded to the point of failure.
The roof membrane must be laid over a smooth surface. A wood deck that is to receive a roof membrane should have no large gaps, knotholes or protruding fasteners. A sitecast concrete deck should be troweled smooth and a precast concrete plank deck, if not topped with a concrete fill, must be grouted at junctions between planks to fill the cracks and form a smooth surface. A corrugated steel deck must be covered with substrate board, thin panels of rigid insulation, wood or gypsum board that can bring the flutes in the deck and create a continuous smooth surface.
It is very important that the deck be dry at the start of roofing to avoid later problems with water vapour trapped under the roof membrane. A deck should not be roofed when rain, snow or frost is present in or on the deck material. Concrete decks and insulating fills must be fully cured and completely air dried.
If a deck is large, the roofing system should be provided with enough movement joints so that expansion and contraction or other movement within the deck does not excessively stress the overlying membrane. Where building separation joints occur within a building structure, these joints must carry through the roof membrane system as well.
Where such points do not occur or are too far apart to satisfy the separation requirements of the membrane, area dividers, which are much like building separation joints but do not extend below the surface of the roof deck may be installed.
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