The Roof Structure

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SHAPES OF ROOFS:

Roofs can be constructed in many shapes. The flat roof is a single plane, almost—but not quite—level. A shed roof is a flat roof, but one which has definite pitch or slope. A gable roof consists of two inclined planes which meet at a peak over the center line of the house. The peak of the gable is called the ridge. A hip roof is one which slopes down in four inclined planes. A gable roof may have hip ends. A mansard roof is one which has sloped planes around its exterior perimeter, but is flat in the upper center portions. See (ill. 45).

(Caution) All roofs should have some pitch, so as to shed water. Roofs must not be constructed dead level, or so as to hold or pond water. Many years of sad experience have demonstrated that there is no absolutely watertight roofing material or system that will be able to remain leak free with standing water on it. Manufacturers of roofing materials will not guarantee nor stand behind their materials or systems if the roof does not have at least minimal pitch. The actual amount of pitch required varies for different roofing materials; some requiring as little as 1/4 inch per foot, others requiring significantly more to be trouble- free. See ROOFING, for more discussion on this.

PARAPETS:

A parapet is a configuration in which the roof terminates into an upward extension of the exterior wall. Parapets are built at locations where it's desirable to not have roof run-off water, or for other reasons related to the design of the building. See (ill. 46).

(CAUTION) Parapets require special flashing treatment to make the intersection of the roof plane and the vertical wall watertight. See (ill. 47). Also, roofs which pitch water towards a parapet should be avoided because this traps the water in a valley, tending to cause standing water conditions with Subsequent leaks. Parapets also require special waterproof materials to cap their top. Parapets, therefore, are not altogether desirable, and if used, require very special care in the initial installation of their flashings as well as frequent maintenance attention.

ill. 45: ROOF SHAPES: FLAT; SHED; MANSARD; GAMBREL

ill. 46: VIEW OF ROOF MEETING PARAPET

ill. 47: DETAIL OF PARAPET-TO-ROOF FLASHING

Wherever possible, roof planes should extend beyond the exterior faces of the building. This directs water away from the building, as well as provides shade from the affects of the sun. The amount of overhang of the roof is determined by design considerations, building orientation, climate, and the need for exposure to, or protection from, solar radiation.

All pitched roof structures inherently create structural forces which are constantly at work trying to cause the plane of the roof to move outward and downward, towards the direction of the pitch. These forces must be resisted by bracing, ties, collar beams, and other supports. See (ill. 48).

TRUSSES:

Trusses are a structural system quite commonly used in roofs of all types of buildings, including residences. A truss is a geometric configuration of structural members arranged in triangular patterns, wherein the intersection of each member requires a special connection. Trusses can be made to span long distances free of intermediate support other than at their ends; and , utilize members which are relatively small in size. Trusses made of wood are relatively inexpensive. They are a viable alternative to span longer distances which would be excessive for normal solid members, or where interior intermediate supports are not possible nor desirable. They also form large voids for the easy passage of ducts, pipes, etc. See (ill. 49) for some truss configurations.

(CAUTION) The chief concern regarding the use of trusses, is that their design must be done by qualified professionals, and their construction—especially the joint connections—be executed exactly as per the design. If there is any doubt about the design or fabrication of a truss proposed for your building, insist on the calculations, have them checked by another professional, and require a sample truss to be load-tested.

ill. 48: STRUCTURAL FORCES IN PITCHED ROOFS: without restraint roof collapses down and outward; ceiling joists act as horizontal ties; collar beams act as horizontal ties

ill. 49: TYPES OF TRUSS JOINT CONNECTIONS: TRUSSED RAFTER; SCISSORS TRUSSED RAFTER; PITCHED PRATT TRUSS; special metal connectors both sides; bolts and split ring connectors

VENTILATION:

Any parts of a roof system which will result in trapped or closed-in dead air spaces, such as attics, must be ventilated. The reasons for ventilation are two-fold:

1) To allow hot air which builds up in trapped spaces to escape to the outside, thereby diminishing the effects of that heat on the interior of the building; and

2) To relieve the build-up of moisture which occurs in trapped air spaces due to the migration of humidity (water vapor present in heated air) into those spaces, and then condensing out of the air as liquid moisture. This moisture can wet and soak insulation, thereby significantly diminishing its effectiveness. It can also cause deterioration of framing members, rusting of nails and fasteners, and stain interior surfaces such as ceilings and walls.

(CAUTION) Ventilation should be provided around the perimeter of dead air spaces at both low points and high points. This allows natural convection currents as well as wind to move air thru the dead space. Vents can be fixed openings provided in the bottoms—or soffits—of roof overhangs, be special continuous vents at ridges, louvers high in side walls, and wind or electric- powered moving ventilators. There are standards and formulas provided by codes, and by the FHA for the amount of open ventilation area required for various kinds of spaces and conditions. See (ill. 50) for various ventilation schemes.

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