Functions of Roof Q3

Q.
a) Explain the function of roof in the following context:-

i) Weather resistant
ii) Thermal insulation
iii) Strength and stability (10 marks)

b) Using diagram, explain the differences between the following types of pitched roof:-

i) Single Roof
ii) Double Roof
iii) Trussed or Framed Roof (15 marks)

(25 marks, 2012 Q3)

A.
Functions of roof - please go here.

i) Weather resistant - roof should be able to withstand weather.

Weather resistance includes protection from rain, sun, wind, heat and cold. Its integrity is an important part of the building structure, and has to withstand the dead load imposed by the roofing and framing, and being leakproof and durable. In certain climate, it has to withstand the stress of typhoon, snow and sand storm. Thus, function of roof in weather resistance includes the effect of physical (like forces of wind) as well as geothermal influences (heat, cold and sunlight).

Ref:
Mrema, Gumbe, Chepete & Agullo. (2011) Rural Structures in the Tropics. Design and Development. Food and Agriculture Organisation of the United Nations. Page 173.

ii) Thermal insulation - Cool Roof and Green Roof.

Cool roofs fall into one of three categories: roofs made from cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green roofs.

Cool roofs

White thermoplastic membrane roofs, are inherently reflective, achieving some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of white thermoplastic, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the roof absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation.

Coated roofs

An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 500 reflective coatings can be found in the Cool Roofs Rating Council.

Green roofs
Main article: Green Roof

Green roofs provide a thermal mass layer which helps reduce the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5). Green roofs may not reflect as much as a cool roof but do have other benefits such as evapotranspiration which cools the plants and the immediate area around the plants, aiding in lowering rooftop temperatures, naturally.

Ref:
Wikipedia on "Reflective surfaces (geoengineering)"
http://en.wikipedia.org/wiki/Reflective_surfaces_(geoengineering)#Types_of_cool_roofs

iii) Strength and Stability - The roof structure or framework must be of adequate strength to carry its own weight together with the loads of wind, snow and foot traffic.

1. The strength of a roof depends on the characteristic of the materials from which it constructed and the way they put together in the form of a flat platform or some form of triangulated frame.
2. Pitch roof depends for strength on the triangulated form of the roof structure, in which the considerable depth of the roof at mid span gives it strength and for which comparatively slender sections can safely be used.
3. A flat roof acts in much the same way as a floor, a s a level platform designed to support loads. The loads that a roof supports, wind pressure or uplift and snow, are generally less than those supported by a floor.
4. A roof is constructed to support the dead load of the roof structure and its covering, insulation and internal finishes, snow loads and pressure or suction due to wind without undue deflection or distortion.
5. The stability of a pitched roof depends on the depth of the triangular framing at mid span and it is this depth that gives a pitched roof its stability across the span of the roof.
6. The stability of a flat roof depends on adequate support from walls or beams and sufficient depth or thickness of timber joists or concrete relative to spans and assumed loads to avoid gross deflection under load.