Roof: Encyclopedia - Roof

Roof

A roof is the top covering of a building that sheds rain or snow, keeping the building interior dry. Roofs may be 'pitched', 'domed', 'low slope' or 'flat' in form; however, roofs are rarely truly flat. Flat roofs are commonly found on industrial/commercial type structures while low slope is found on pre-fabricated or steel structures such as arenas. Pitched roofs are the primary design found on residential homes. The most common type of flat roof for industrial/commercial structures are the conventional built-up-roof (BUR) assembly and the protected roof membrane assembly (PMRA)The most popular roof membranes are the 4ply (felt and asphalt), the 2 ply modified bitumen and the single plies such as the elastomeric (EPDM) and thermal plastic (PVC), drains provide run off for rain and snow.

Roof shapes and pitches are often the product of design, material constraints and/or climatic considerations. Roof designs in mild climates or where there is little precipitation tend to exhibit flat roofs and drainage at the exterior wall; an example of residential flat roof is that of the adobe construction in the American Southwest. In northern climates where temperatures drop below freezing we see major changes in both flat roof and pitch roof design. The need for greater thermal resistance will dictate the type of roof design as well as the waterproofing membrane specification. In areas that have a high accumulation of snow such as; Buffalo USA or Montreal Canada a minimum slope of 4 inches in 12 inches (4/12) of roof surface is required. We often see roof pitches that exceed 45 or 50 degrees. Drainage on cold climate roofs is typicaly installed at interior locations rather than on the exterior wall as we see in the southern climates. By placing the drains toward the interior freezing of the rainwater leader is avoided. Weight of accumulated snow has been known to cause major damage to structures, freeze thaw cycling plays havoc on the flat roof assembly as well as the pitch roof design. Migrating water during thaw periods enters voids then freezes and expands. Daily repetitions of this cycling effect eventually causes damage to the shingle and/or membrane.

Some building styles, for example, geodesic and A-frame, blur the distinction between wall and roofs. Pitched roofs are often covered with asphalt shingles (in the US) although thatch, wood shake, steel, corrugated galvanised iron, slate and tile roofs are used elsewhere.

Roof - Weather proofing materials

The weather proofing material is the topmost or outermost layer, exposed to the weather. Many different kinds of materials have been used as weather proofing material:

• Shingles & Faux Shingles
  • Slate. Long life, High cost
  • Ceramic. Long life, High cost
  • Metal. Long life. High cost, should be on roofs of 6/12 pitch or greater.
  • Cement (usually reinforced with fibres of some sort)
  • Asphalt shingle (bitumen embedded in a paper or fibre mat) Cost effective. Various life span expectancies.
  • Cedar Long life. High cost. Should be allowed to breath.
• Membrane
  • Thermoset (e.g. EPDM synthetic rubber). Synthetic rubber strips chemically welded together with solvent or tape. Primary application is big box store with large open areas and little verticle protrusions. Relies heavily on the skill of the applicator. Detail critical.
  • Thermoplastic plastic (e.g. PVC). Plastic strips welded together with hot air. Can be rewelded. Lends itself well to bothe big box and small roof application because of its hot air weldability. Relies heavily on the skill of the applicator. Detail critical.
  • Copolymer (e.g. TPO) polypropylene and ethylene propylene rubber. Welded together with hot air. Combines the best features of EPDM and PVC. Relies heavily on the skill of the applicator. Detail critical.
  • Modified bitumen ("torch on or asplalt bonded"). Asphalt embedded in a fiber mat, seams sealed by locally melting the asphalt with a torch or a hot mopping of asphalt. Lends itself well to all applications. Requires some training. Virtually foolproof.
  • 4 ply ("Felts" of either asphalt impregnated paper or fiberglass reinfoced synthetic mat laid out in strips and bonded togeter in plys using hot asphalt and then surfaced with a layer of pea gravel. Lends itself well to all flat roofs. Requires some training. Virtually foolproof.
  • Spray-on foam (isocyanurate foam with an elastic rubber top coat)
• Panel
  • Corrugated metal
  • Standing-seam metal
  • Flat-seam metal
• Fabric
  • Dacron/polyester
  • Teflon embedded in fibreglass
• Straw thatch

Roof - Durability and Cost

Typical composition shingle roofs can last anywhere from a thin 20-year shingle to the thickest of 50-year shingle. They are relatively inexpensive and you can put on up to 2 - 3 layers of shingles before weight becomes a problem and if you under stand that the new (over-laid) roofs will be installed right on top without getting a good look at the structure. Once the 3rd layer is done, the roof has to be stripped off sometimes down to the rafters and new plywood installed for a good nailing surface. Even though older houses may have lap-board installed, roofing companies will take this off too because they will not guarantee that the shingles will become uneven and start leaking. Sometimes new plywood can be installed right over the existing roof boards to help keep the labour cost down.

Slate roofs, when properly installed, are considered the best there is. They last around 75 - 150 years or longer depending on the slate used and the careful attention of the roofer, but are extremely expensive to install (sometimes the same price as the rest of the house) and it should be installed by someone that knows what they're doing, or at least someone with the skills of a roofer and the understanding of basic fluid dynamics. Keeping the nail heads covered and protected from the weather properly is the key to a long lasting slate roof. Often, the problem with slate shingles is that the nails used to hold the slate shingles in place rust out before the slate starts to wear out.

Steel roofs last about 50 years or more depending on the moisture barrier (underlayment) used and is in between the shingle roofs and slate roofs when it comes to price. The main attraction is probably the efficiency of which they withstand the elements. Snow load areas benefit from a steel roof by that way they will shed the weight of snow easily and resist the force of wind better than a wood shingle or a concrete tile roof.

Roof - Common Roof Questions and Answers

For home owners considering a new roof or for those purchasing an existing home one should ask the following questions;

Q. Will this new roof increase the value of my home?

A. No. It will however, make it easier to sell.

Q. When they say 20 year shingle does that mean it is warranted for 20 years?

A. No. It is similar to tyres. The manufacturer, if he accepts fault, will pro-rate the value of the shingle and give you a substantially lesser amount. Purchased or paid for warranties are better.

Q. Do I really need a 50 year roof?

A. You must calculate cost versus time for that 50 year period and decide whether or not you will realize a cost saving. Will a potential buyer even care if you have a 50 year roof and, better still, will you be alive 50 years from now.

Q. The house I am puchasing has 3 layers of shingles is this allowed?

A. Yes. However the previous owner(s) delayed the dumping and labour charges by putting shingle over shingle. If you have to replace the shingle now you must remove all 3 layers and bear the dumping cost, which will be higher as it goes by weight, and the additional labour.

Q. I am considering Architectural shingles to enhance the look of my home. What should I look for?

A. First stand on the street and look at your roof. Are the shingles clearly visible. Is the slope 6/12 (6 inches in 12 inches) or greater? If the answers are yes, then spend the extra on Architectural Shingles you will enjoy your home more.

Q. Should I pay the extra for ice and water shield?

A. By all means do so. Especially for those in northern climates. In fact it is a code requirement in most areas.

Q. Can I put a slate roof on my home?

A. Again this depends on your line of sight first of all and then you must determine if the structure was designed to accept the load and finally, does your budget cover the cost of a top of the line roof.

Commercial/industrial roofing considerations

Q. We are considering the replacement of an existing roof assembly. I contacted 3 professional roofers received 3 different opinions and 3 different prices. How do I know which one to hire?

A. There are people in the industry such as, Roof Consultants or Architects, who are independent of the roofer and can give you an accurate analysis and budget for your roofing needs.

Q. My contractor insists on installing a 4 ply BUR roof. How long do they last?

A. Today's 4 ply felt and asphalt roof is only lasting 15 years and that is if you maintain them well. The asphalt no longer has the physical characteristics it had 30 years ago. As a result the industry standard has gone from Type I asphalt in the field area of the roof to Type II to enhance viscosity and flow characteristics that are critical to the roof assembly. It is still one of the better roofs. It can be made better by adding modifiers to the asphalt however these will add to the cost per square foot of installed roof.

Q. Do I require a vapour retardant membrane?

A. Climate conditions will dictate whether a vapour retardant membrane is necessary. Local building codes will prevail.

Q. Where does the vapour retardant membrane go?

A. It is always applied on the warm side. The purpose of the membrane is to limit flow of moisture-laden air through the roof or wall assembly and prevent dewpoint from occurring and wetting the insulation. Moisture laden air moves from the warm side to the cold side of a structure. In the southern (Equatorial)climates it has a tendency to move from the outside to the inside while in the Polar climates it is reversed. In Polar climates such as that found in Canada or Sweden it is not unique or unusual to find ice forming in a roof or wall insulation due to the lack of, or damage to, the vapour retardant membrane.

Roof - Solar roofs

Newer systems include solar shingles which generate electricity as well as cover the roof. There are also solar systems available that generate hot water or hot air and which can also act as a roofcover.

There are different types of integrations of solarsystems in roofs.

• integrated in the covering of pitch roofs b.e. solar shingle.
• mounted on an existing roof b.e. solar panel on a tile roof.
• integrated in a flat roof b.e. PVC.
• mounted on a flat roof with a construction and additional weight to prevent uplift from wind.

Several systems of construction transmit the weight of the roof to the walls of the building and tie the roof into the structure. These include: ashlar-piece, brace (can be arched or wind), collar-beam, crown-post, hammer-beam and -post, king (or queen) post, purlin, rafter (common or principal), ridge beam, ridge-board, strut, tie-beam (Tie rod), truss, and wall-plate.

A roof has different areas. For example, the eaves are the area where the rafter ends form the edges of the roof that run horizontally across the façade.

By extension one can speak of the roof of a tent, automobile, etc. A convertible is an automobile built with a folding, retracting, or removable roof. In a car, a moon roof is a see-through opening in the roof of the car, whereas a sun roof typically opens up.

Roof - Terminology

• An attic is the story under a non-flat roof.
• Galvanised roofs are normally used on sheds.
• "Pavillion" is a term used by architects to refer to any autonomous, self-contained structure, regardless of size or roof configuration.

Roof - Roof shapes

Roofs can be shaped to shed water easily. These include:

• lean-to
• single-sloped or shed roof
• ridged
  • pitched or gabled
    • shaped gable
    • Dutch gable - a hybrid of hipped and gambrel
    • crow-step gable
    • corbie-step gable
    • salt-box
  • saddleback
  • hip roof
  • half-hipped
  • mansard
  • gambrel
  • pavilion
• conical
• domical
• pyramidal

See also

• Roof garden
• Tented roof
• Building construction
• Green roof
• Roofing felt

Adapted from the Wikipedia article "Roof", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki