A roof is designated as flat if its slope is less than ten degrees.
Flat roofs have traditionally been used in hot climates where water accumulation is not a problem.
They were generally unkown in northern climated before the end of the last century
Slope less than 3:12 or 25 percent
Structural decks ordinary used
Wood panels over wood joist
Solid wood decking over heavy timber framing
Corrugated steel decking
Sitecast concrete slab
Precast concrete slab
Panels of wood fibre bonded together with portland cement
The decks be adequately stiff under roof loading and uplift forces
The decks must slope towards drainage points
A slope at least ¼ “ per foot run (1:50) is recommended
If the slope is too shallow, puddles of water will stand for extended periods
Leading to premature deterioration of the roofing materials in those areas
If water accumulates cause by structural deflection, progressive structural collapse becomes possibility
Deepening puddles attracting more and more water and becoming heavier
a) Water stands on the roof in puddles, causing slight deflections of the roof deck between supporting beams or joists b)If heavy rain continues, the puddles grow and joint. Accumulating weight of the water begins to cause substantial deflections. Deflection encourage water from a border area of the roof run into the puddle c) As structural deflection increase, the depth of the puddles increases more and more rapidly, until the overloaded structure collapses a b c
FLAT ROOF CONCRETE, TIMBER OR STEEL STRUCTURE Waterproof layer and insulation Cement screed Finishes
Below the structural deck
Between the deck and the membrane
Above the membrane
Below the deck
Between the deck
Above the membrane
ELEMENTS OF FLAT ROOF parapet Concrete gutter Steel decking parapet Lines indented for expansion caused by heat and easy to flatten the roof in a uniform manner to prevent any pool of water after rain. concrete
Water pool formed due to poor workmanship
PROBLEMS OF LEAKING ON FLAT ROOF
Weatherproofing the flat roof
The flat roof relied on some kind of membrane for keeping moisture out. In dry climates this is done with clay tiles, but in Canada asphalt, or rubber is necessary.
Flat roofs are never actually flat, a subtle slope directs standing moisture to drains at edges, or inside.
Advantages of flat roof design:
No space lost below roof, i.e. no dead space
Less material is used than in a sloped roof
The rooftop is potentially useful as a terrace, or sleeping porch
Potentially pleasing appearance
Easier to build than a sloped roof
Disadvantages of flat roof design:
Roof elements can not overlap, hence waterproofing must be more complex, and more thorough
Drainage is not automatic
Support of snow load must be insured
Flashing is special material used to make sure that no moisture leaks in those areas of a roof that are particularly vulnerable to penetration.
Areas where flashing is commonly used:
Wherever the roof is pierced by a chimney, skylight, air or plumbing vent
Where roofs meet walls
At the edges of flat roofs where they meet parapet walls (a wall that extends above the level of the roof)
Materials from which flashing is made of:
Sloped roofs: it is important to design flashing so that it throws the water downslope, and does not rely on mastic.
Flat roofs: it is important that flashing be extended above the level at which you expect snow or water to accumulate
3. Space frame A space frame or space structure is a truss -like, lightweight rigid structure constructed from interlocking struts in a geometric pattern . Space frames usually utilize a multidirectional span, and are often used to accomplish long spans with few supports. They derive their strength from the inherent rigidity of the triangular frame; flexing loads (bending moments ) are transmitted as tension and compression loads along the length of each strut.
4. ROOF SHELL
5. FOLDED PLATE ROOF
The most cost-effective
Increase occupant comfort by moderating the radiant temperature of ceilings and walls
Reduce heating and cooling energy
Cellulose insulation- most environmentally friendly thermal insulating material
Glass wool and mineral fibre
Polystyrene foam made from petroleum
Polyurethane foam insulation
New concepts of roof
PROFILED STEEL SHEET DRY BOARD ROOF SYSTEM (PSSDB)
The profiled steel sheet Dryboard panel is a composite structural system consisting of profiled steel sheeting/decking compositely connected to dryboards by simple mechanical connectors .
Structural components of PSSDB System
COMPONENTS OF PSSDB
AJIYA CLIP & LOCKING CL 660 --- A profiled steel sheet product of Ajiya Roofing products, Malaysia.
PRIMA FLEX--- A top grade cellulose fibre, Portland cement and finely ground sand produced by Hume Cemboard Berhad, Malaysia.
SCREWS --- Self-tapping, self-drilling screws.
Main Advantages of the System
Dry construction technique
Easily assembled by unskilled labour
Saving in foundation cost of the buildings
Less deep than the traditional timber joist flooring
Less loading to existing structure in renovation work
No propping/formwork required during construction
No waiting period for occupation after the completion of construction.
MAIN APPLICATIONS OF PSSDB PANEL
The profiled steel sheet dryboard system can be exploited for various structural purposes such as:
i) Flooring units.
ii) Walling units.
iii) Panels may be assembled to form Folded Plate Structures such as as roofs of buildings.
The normal position is not suitable to be used in real buildings.
Show slight softening in strength and bulging in volume under wet condition.
Even though a water proofing can be used from directly exposed to the hot and dry weather, but it is not a practical in practice
Based on the original concept of the system, the application has been extended to form a new concept of roofing system.
The new approach will eliminate the roof trusses normally required in traditional roof structure.
PSSDB Roof system
Erection of roof panels
Erection of roof panels
Completed classroom modules
A SITE ENGINEER SENT THIS TO THE DESIGNER:
S ub: High Priority Steel roof and column already constructed, kindly send us the foundation drawings. Site people can construct any thing, so please be careful with your drawings. Please send foundation drawings before superstructure drawings Regards, Site Engineer