Roof Construction

A roofing and solar forum would be incomplete without at least one article on the fundamentals of roof construction. Although the build of our subjects cover re-roofing topics, we do cover new roofing or roofing for new construction. With this in mind it was expedient for us to include at least on article on general roof construction.

Most existing roofs are described by this definition: a system of beams and an outer skin or layer (to repel water), made in varying shapes and styles. Below, view an illustrated example of a typical US or Australian roof (sloped or low pitch):

Pitched Roof Construction

A standard ridged roof includes rafters, set an an incline, that span from a central ridge beam (or plate) over the walls creating an overhang. Horizontal roof structures (called Purlins) are sometimes used to further reinforce the roof and allow support for the roof covering. Heavy duty Purlins are especially used with extremely long rafters which require additional structural support and stability. Purlins are usually made of steel or wood, depending on the type and style of construction. Joists (tie beams) connect the lower ends of the rafters to increase stability and prevent the walls from moving. As needed, collar beams and collar ties are attached above the joists (as needed) towards the ridge plate (or beam) for added strength and stability.

In high tornado or hurricane risk areas, roofs systems are designed to be held down as much as they are designed to distribute transfer weight of the roof structure to the walls (via the joists, wall plates, trusses, etc). In areas where tornadoes or hurricanes are common, roofs are usually designed with additional reinforcement. In places, such as Northern Australia (where cyclones are inevitable), additional steel brackets are used to attach the roof to a reinforced concrete block wall.

A Hipped Roof with blue metal purlins under construction in cyclone prone Northern Australia:

Flat Roof Construction

Flat roof construction is similar; thick central beams support rafters, which in turn support purlins or horizontal roof structures, to which a roof membrane is attached. Some typical flat roof membranes include Built up Roofs (BUR), EPDM, Polyurethane Sprayed Foam Roofing, and Modified Bitumen. Sprayed Polyurethane Foam Roofing (Foam Roof) is a newer type of roofing material, which has risen in popularity during the last few decades due to high R-values and excellent insulation properties, amazing water repulsion due to the seamless monolithic installation, reflective coatings, and light sturdy self flashing qualities, which all synergisticly improve the energy efficiency of homes where it is installed. Depending on other factors, such as existing insulation and double paned glass, a foam roof may cut heating and cooling costs by as much as 50-80%. As the general popularity of green construction has risen, so has the popularity of green foam roof systems. Nothing but natural rooftop gardens are more 'green' than a foam roof system. While only specially designed homes can support a rooftop garden, a foam roof system can replace any existing flat or low slope roof system.

A commercial polyurethane foam roof system is pictured below:

Roof Construction Materials

All roof systems have a final membrane, or topmost outer layer which is built to withstand outside weather. Weather proofing varies widely according to worldwide weather differences, styles, local traditions and preferences, as well as the geographic availability of materials. The following list is provided in alphabetical order from Asbestos to Thatch.

Asbestos shingles: Durable, fireproof, but rarely used due to the the discovered health risks which included cancer. Now most building regulations in most countries have disallowed new construction with any asbestos.

Asphalt shingle: Cheaper than slate or tile and usually a comparably shorter life-span as well. A fiberglass or organic material is covered with bitumen and surfaced with man made ceramic specs.

Concrete: Concrete tiles have higher weight requirements for roof structure load capacity due to higher weight than slate. Normally the tiles are reinforced with a fibrous material.

Ceramic tile: Expensive, but also high quality, lasts the life of the building (over one hundred years).
• Imbrex and tegula: an ancient style of tile dating back to greco-roman times.

Mechanically Seamed Metal: Expensive, but long life span roofing material, only suitable for low pitch or flat roofs.

Membrane: usually a roof membrane comes in small sheets which are attached to the roof surface and sealed to create a weatherproof barrier.
• Thermoset membrane: a synthetic rubberized material, such as EPDM, adhered together.
  
• Thermoplastic: Plastic membrane sheets (PVC, TPO, CSPE), melded together with heat to create a monolithic membrane.
  
• Modified Bitumen: Asphalt is mixed with APP or SBS polymers then applied to a polyester or fiberglass material. Seams are joined by melting the hot asphalt with hot mops or by joining with adhesive. Generally used on flat or low slope roofs.
  
• Built-Up Roof (BUR): Multiple layers of asphalt saturated felt or fiberglass are adhered with hot asphalt, coal, or tar pitch adhesive. Normally covered with a thin layer of rocks. Also referred to commonly as "tar and gravel". Mostly used in flat or low slope roof applications.
  
• Sprayed-in-Place Polyurethane Foam (SPUF)(SPF): Lasts the lifetime of a building when properly installed, usually installed on low pitch or flat roofs. Sprayed polyurethane foam (foam roofing) creates a seamless, monolithic barrier and is walkable within one minute after spraying. Generally, acrylic cool roof elastomeric coatings are applied.

Metal roofing: comparatively less expensive roofing material, except in the case of copper.
• Galvanized Steel: Cheap, inexpensive (and shorter lifespan) wavy corrugated metal, designed to resist lateral tension, set in place with outer fasteners. Most typically this is used for warehouses and sheds. Galvanzied steel, also referred to as "Corro" or "Gal Iron", has mostly been replaced by steel zinc or aluminum coated roofing. The zinc or aluminum coated steel lasts four times longer than corrugated galvanized steel.
  
• Standing-Seam: another metal type roof, but with hidden fasteners.
  
• Mechanically Seamed metal roofing with hidden fasteners further reinforced and sealed with various sealant types, mostly for flat or low pitch applications.
  
• Flat-Seam metal seams sealed with solder.

Metal shakes or shingles: High durability, high cost, and long life material, suitable for medium or greater pitched roofs (3/12 & up).

Shingles: refers to many small (usually rectangular) overlapping materials, only suitable for medium or high slope roofs. Shingles are most commonly made of asphalt or wood. In colder climes an underlying ice dam membrane is installed to prevent water seepage. Larger wooden shingles are called "shake" in North America.
• Red Cedar: Expensive material, may last up to 30 years unless it comes from a young tree. Must be allowed to breathe to prevent dry rot or other issues.

Slate: Sometimes referred to as shale, this expensive, but long lasting material lasts 100-200 years, depending where it was quarried. Slate is attached in thin sheets making it lighter than tiles made of concreate, but heavier than most other roof coverings.
• Sea Green Shale: This type of slate roof material lasts up to 200 years when properly installed.

Stone Slab: thick one to two inch slabs were used as roof tiles in some areas than england (much thicker than shale or slate). Stone slabs require heavy load bearing architectural designs. This weatherproof roofing material has become obsolete due to high cost and high structural requirements.

Thatch many layers of overlapping plant stalks, this type of roofing has been used since ancient times.
• Wheat Straw: common in areas of Europe such as England and France.
  
• Sea Grass: lasts sixty years or more (much longer than straw) and commonly used in areas bordering an ocean or sea, such as Scottland.

Roof Construction Styles

The following is a list of some various roof construction styles, pictured by letter in the illustration above
1. Lean-To or Shed Roof: This is also a "pitched" or "pitch" roof (synonymous with "sloped").
   
2. Hip Roof: refers to all downward sloping sides, generally with a gentle slope.
3. Square Hip Roof: a square hipped roof has four symmetrical sides that resemble a pyramid. Hipped roofs are more complex to construct than gable roofs due to the increased number of trusses required.
4. Gambrel Roof: with roots in norman english meaning "curved stick" or "notched stick" this is a classic style of sloped roof most commonly seen on barns. Such roofs are generally characterized by two or three symmetrical sloping sections where each section has a slightly increased slope. More contemporary roof designs sometimes use this design (see image below).
5. Mansard roof: (Mansard, Victorian, Second Empire) also referred to as Mansard (named after a famous french architect who revived the style), this stlye roof was later revived yet again by Napolen III during the rebuilding of France, during his Second Empire (Mansard style sections were at one time used on the Louvre Museum in Paris). Second Empire, or Mansard roofs were popular in early American times, and are hence referred to as Victorian as well. The style is seen as advantageous because it allowed near full use of attic space, and is often used during the restoration of older buildings.
6. Victorian, Second Empire, or French Roof: all these terms are synonymous with a Mansard roof. See the definition above (e. Mansard).
7. Ogee Roof: Characterized by a downward sloping double curve. The picture above displays a four sided ogee (at G), but an ogee may also refer to a "hip roof" style, which is altered by a double curve (see illustration below).
8. Curb Roofs: In architecture, a curb roof is a roof in which the rafters, instead of continuing straight down from the ridge to the walls, are at a given height received on curb plates, which in their turn (and at a lower point) are supported by rafters more inclined toward the horizon, so that this kind of roof presents a bent appearance. It is also called also a Mansard Roof, from the name of its French inventor. Some examples of curb roofs are Gambrel, Mansard, Victorian and Second Empire roofs (see D, E, and F). Each of the three "k"s and corresponding arrows points to where the curb plate is installed (the point at which the roof 'bends'.

Gambrel roofs are not just installed on barns; below is an example of a Gambrel Roof on a more contemporary home:


An Ogee Roof is a downward pitch roof characterized by a double curve roof, as pictured below, or above at G.

Ogee Roofs

Thank you for this informative and well structured post. I had heard of ogee roofs before, but I found it was difficult to find a clear definition, even on the web. I found both pictures helping--the ones displaying the curve that characterizes an ogee roof.

for roof construction first we decide that which type of home. decide the flow of of water on Terrence in which side. what about slope for long life roof. Decide angle of it . all the things are easy with a Roofing company.

Roofing wrote:for roof construction first we decide that which type of home. decide the flow of of water on Terrence in which side. what about slope for long life roof. Decide angle of it . all the things are easy with a Roofing company.

I am having trouble understanding your grammar--although if I read into this (a lot), I think it makes sense.

Hello everyone,

I am new to this forum. I've seen this post on roofing construction. Nice article has been written on fundamentals of roof construction. I think Shed Roof constructions are the most common types of roofing as I can see the most of the houses here in my residing area. What you guys prefer for robust roofing construction ?

robsam wrote:Hello everyone,

I am new to this forum. I've seen this post on roofing construction. Nice article has been written on fundamentals of roof construction. I think Shed Roof constructions are the most common types of roofing as I can see the most of the houses here in my residing area. What you guys prefer for robust roofing construction ?

To some extent I think it depends on the area of the country that you're in. The best roof in southern Texas might be a little different from the ideal roof in Alaska. What type of roof do you think is best?