LOW SLOPE ROOFS BASIC INFORMATION AND TUTORIALS

COMPONENTS OF LOW SLOPE ROOFS


Low-slope roofs can have slopes as minor as 1⁄8 inch per 12 inches. These roofs employ a waterproof roofing system and are found primarily on commercial structures.

A low-slope roof system generally consists of a roof membrane, insulation, and one of a number of surfacing options. To control the application and improve the quality of low-slope roofing, a variety of specifications and procedures apply to the assembly of the roofing components.

These specifications and procedures are generally accepted and used throughout the United States. Roofing systems that meet these specifications normally can be expected to give satisfactory service for many years.


Climatic conditions and available materials dictate regional low-slope procedures, which can vary greatly in different parts of the country. Low slope roofs are essentially a custom product. They are designed for a specific building, at a specific location, and manufactured on the jobsite.


Membrane Components
Low-slope membranes are composed of at least three elements: waterproofing, reinforcement, and surfacing. Some materials within the membrane might perform more than one function. The waterproofing agent is the most important element within the roof membrane.

In BUR and modified bitumen roofing (MBR), the waterproofing agent is bitumen. In single-ply roofing, the waterproofing agent is synthetic rubber or plastic.

The reinforcement element provides stability to the roof membrane; it holds the waterproofing agent in place and provides tensile strength. In BUR, reinforcement is typically provided by organic or glass-fiber roofing felts. In MBR, the reinforcement is generally glass-fiber felt or polyester scrim, which is fabricated into the finished sheet by the manufacturer.

Polyester and other woven fabrics are used as reinforcements for elastomeric and plastomeric, single-ply membranes. Some singleply membranes do not require reinforcement because the waterproofing material is inherently stable.

The surfacing materials protect the waterproofing and reinforcement elements from the direct effects of sunlight and weather exposure. They also provide other properties, such as fire resistance, traffic and hail protection, and reflectivity.

Some single-ply membranes are self- or factory-surfaced. Aggregate, which is field-applied, and mineral granules, which are usually factory-applied, are the most common types of surfacing materials. Smooth-surfaced coatings, however, are increasing in popularity.


Membrane Classifications
Low-slope roof membranes can usually be grouped, or classified, into the general categories reviewed below. There are, however, hybrid systems that might not fit into a category, or that might be appropriate in several categories.

BUILT-UP ROOFING (BUR)
BUR, which uses asphalt or coal tar products, is by far the oldest of the modern commercial roofing methods. Many commercial buildings in this country have BUR roofs. The large number of 20-, 30-, and even 40-year-old BUR roofs that are still sound attests to the system’s durability and popularity.

Roofing materials continue to evolve, however, and improvements are continually being made to asphalt and coal tar pitch, the basic bitumen components of BUR. Asphalt tends to be more popular with most roofers than coal tar.

MODIFIED BITUMEN ROOFING (MBR)
Since the first MBR membranes were manufactured in the United States in the late 1970s, they have become one of the roofing industry’s fastest-growing materials. The popularity and specification of MBR membranes has increased steadily for more than two decades. Contractors have found the materials easy to use and easily inspected. MBR systems provide a time-tested, high-performance, reliable roof.

SINGLE-PLY SYSTEMS
Since they first appeared in the 1950s, single-ply materials have become increasingly popular in the United States. Whether imported from Europe or produced domestically, these high-tech products have proven themselves in a wide variety of climates during more than three decades of use.

STEEP SLOPE ROOF STYLES BASIC INFORMATION

While low-slope roofs are generally limited to flat-roof styles and are seldom found on residential structures, steep-roof styles vary greatly.

Of the steep-roof styles, the gable roof is the most common. It has a high point, or ridge, at or near the center of the house or wing that extends from one end wall to the other.

The roof slopes downward from the ridge in both directions. This roof style gets its name from the gable, which is the triangular section of end wall between the rafter plate and the roof ridge.

The roof on one side of the ridge is usually the same size and slope as the roof on the other side. The gable roof of the saltbox house is an exception.

An architecture common in New England, the saltbox has different slopes and slopes of different lengths. A hip roof also has a ridge, but the ridge does not extend from one end of the roof to the other.

The lower edge of the roof, or eave, is at a constant height and the roof slopes downward to the eaves on all sides. The point where two roof surfaces meet at an outside corner is called a hip. The junction where two roof surfaces meet at an inside corner is called a valley.

A shed roof slopes in only one direction, like half a gable roof. The roof has no ridge and the walls that support the rafters are different heights. The shed roof has several variations. One is the butterfly roof, where two shed roofs slope toward a low point over the middle of the house.

In another variation, two shed roofs slope upward from the eaves, but do not meet at a ridge. The wall between the two roofs is called a clerestory, and is often filled with windows to let light into the interior
of the house.

A gambrel, or barn roof, has double slopes: one pair of gentle slopes and one pair of steep slopes. Like a gable roof, the gambrel roof slopes in both directions from a center ridge. At a point about halfway between ridge and eave, however, the roof slope becomes much steeper.

In effect, the lower slope replaces the upper exterior walls of a two-story house. It is common to add projections through the roof, called dormers, for light and ventilation.

Just as a gambrel roof is like a gable roof with two different slopes, a mansard roof is like a hip roof. From a shorter ridge, the roof drops in two distinct slopes to eaves that are the same height all the way around the structure.

Up to 40 percent of the building is roof with the mansard roof design. In addition to typical residential applications, mansard roofs are often used for apartment complexes, commercial buildings, and even institutions such as schools.

INSTALLING METAL GUTTERS OF ROOFS CIVIL ENGINEERING TUTORIALS

INSTALLING METAL GUTTERS OF ROOFS CIVIL ENGINEERING BASICS
How To Install Metal Gutter Of Roof?

Installing Metal Gutters

Most metal gutters in residential drainage systems are installed with large spikes that pass through the sides and into the fascia board (Fig. 14-8).

A metal tube or sleeve, called a ferrule, mounts around the spike and maintains the spacing of the gutter sides. Locate these spikes 24 to 30 inches apart, depending on snow conditions.

Two other hanging methods can be used for metal gutters. Sickle-shaped hangers can be fastened to the fascia boards about every 30 inches. The gutters are laid on top of the hanger (Fig. 14- 9).

This method eliminates the need to drill holes in the gutter and thus is easier than spiking the gutters in place. The sickleshaped hangers are more expensive than spikes.

The second way to hang metal gutters is to use strap hangers. These hangers have flanges that are nailed in place under the roofing material (Fig. 14-10).

This type of hanger is best used on new work, since there is always a chance of damaging the existing roofing when it is pried up to install the hanger. Elbows usually connect outlet tubes and collector spouts.

Squeeze the end of the downspout and insert it into the large end of the elbow. Downspouts are fastened to the wall with downspout straps. Be sure the straps are long enough to anchor the downspout securely to the wall.

If the gap between the wall and the downspout is too wide, shim the downspout strap 1 inch away from the wall with a product that is weather-resistant or waterproof. Use two straps on each 10-foot length of downspout and three straps on two joined 10-foot sections.

To enable downspouts to perform their maximum service, install them so that they carry water as far away from the building’s foundation line as possible. This is very important—foundation problems are common triggers of very complex and expensive litigation.

There are several ways to achieve this, but the most popular are to use a concrete splash block directly under the downspout, which runs the water toward the driveway or a similar draining surface, or to connect the downspout directly to an underground line that leads to the storm sewer system.

ROOF DRAINAGE BASICS AND TUTORIALS

ROOF DRAINAGE BASIC INFORMATION
How To Design Basic Roof Drainage?


Many roof failures have been caused by excessive water accumulation. In most cases, the overload that caused failure was not anticipated in design of those roofs, because the designers expected rainwater to run off the roof.

But because of inadequate drainage, the water ponded instead.

On flat roofs, ponding of rainwater causes structural members to deflect. The resulting bowing of the roof surface permits more rainwater to accumulate, and the additional weight of this water causes additional bowing and collection of even more water.

This process can lead to roof collapse. Similar conditions also can occur in the valleys of sloping roofs.

To avoid water accumulation, roofs should be sloped toward drains and pipes that have adequate capacity to conduct water away from the roofs, in accordance with local plumbing codes.

 Minimum roof slope for drainage should be at least 1⁄4 in / ft, but larger slopes are advisable.

The primary drainage system should be supplemented by a secondary drainage system at a higher level to prevent ponding on the roof above that level.

The overflow drains should be at least as large as the primary drains and should be connected to drain pipes independent of the primary system or scuppers through the parapets.

The roof and its structural members should be capable of sustaining the weight of all rainwater that could accumulate on the roof if part or all of the primary drainage system should become blocked.

WIND LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC AND TUTORIALS

WIND LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC INFORMATION
Wind Load Calculations On Roofs For Design


Wind Loads Calculation
Wind loads are randomly applied dynamic loads. The intensity of the wind pressure on the surface of a structure depends on wind velocity, air density, orientation of the structure, area of contact surface, and shape of the structure.

Because of the complexity involved in defining both the dynamic wind load and the behavior of an indeterminate steel structure when subjected to wind loads, the design criteria adopted by building codes and standards have been based on the application of an equivalent static wind pressure.

This equivalent static design wind pressure p (psf) is defined in a general sense by
p = qGCp (9.136)

where q velocity pressure, psf
G gust response factor to account for fluctuations in wind speed
Cp pressure coefficient or shape factor that reflects the influence of the wind on the various parts of a structure

Velocity pressure is computed from
qz = 0.00256 KzKztKdV^2I (9.137)


where Kz velocity exposure coefficient evaluated at height z
Kzt topographic factor
Kd wind directionality factor
I importance factor
V basic wind speed corresponding to a 3-s gust speed at 33 ft above
the ground in exposure C


Velocity pressures due to wind to be used in building design vary with type of terrain, distance above ground level, importance of building, likelihood of hurricanes, and basic wind speed recorded near the building site.

The wind pressures are assumed to act horizontally on the building area projected on a vertical plane normal to the wind direction.

ASCE 7 permits the use of either Method I or Method II to define the design wind loads. Method I is a simplified procedure and may be used for enclosed or partially enclosed buildings.

ASCE 7 Method II is a rigorous computation procedure that accounts for the external, and internal pressure variation as well as gust effects. The following is the general equation for computing the design wind pressure, p:
p = qGCp qi(GCpt) (9.138)

where q and qi velocity pressure as given by ASCE 7
G gust effect factor as given by ASCE 7
Cp external pressure coefficient as given by ASCE 7
GCpt internal pressure coefficient as given by ASCE 7

Codes and standards may present the gust factors and pressure coefficients in different formats. Coefficients from different codes and standards should not be mixed.

SNOW LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC AND TUTORIALS

SNOW LOADS ON ROOFS DESIGN AND CALCULATIONS BASIC INFORMATION
What Are Snow Loads On Roofs Design And Calculations


Snow Loads Calculation
Determination of designing snow loads for roofs is often based on the maximum ground snow load in 50-year mean recurrence period (2% probability of being exceeded in any year).

This load or data for computing it from an extreme-value statistical analysis of weather records of snow on the ground may be obtained from the local building code or the National Weather Service.

Photo courtesy of Ask The Builder

Some building codes and ASCE 7-95 specify an equation that takes into account the consequences of a structural failure in view of the end use of the building to be constructed and the wind exposure of the roof:

pf = 0.7CeCtIpg (9.134)

where Ce wind exposure factor (range 0.8 to 1.3)
Ct thermal effects factor (range 1.0 to 1.2)
I importance factor for end use (range 0.8 to 1.2)
pf roof snow load, lb per ft2
pg ground snow load for 50-year recurrence period, lb per ft2

The “Low-Rise Building systems Manual,” Metal Building Manufacturers Association, Cleveland, Ohio, based on a modified form of ASCE 7, recommends that the design of roof snow load be determined from

pf = IsCpg(9.135)
where Is is an importance factor and C reflects the roof type.

What are the components of a waterproofing system in the roof of a typical pumping station?

WATERPROOFING SYSTEM ROOFING TUTORIALS
Civil Engineering Tutorials


In the design of a waterproofing system at the roof of a pumping station, normally the following components are:

(i) Above the structural finish level of the concrete roof, a screed of uniform thickness is applied to provide a smooth surface for the application of waterproofing membrane. (Screed of varying thickness can also be designed on the roof to create a slope for drainage.)

The screed used for providing a surface for membrane should be thin and possess good adhesion to the substrate. Moreover, the screed aids in the thermal insulation of the roof.

(ii) Above the screed, waterproofing membrane is provided to ensure watertightness of the roof.

(iii) An insulation board may be placed on top of waterproof membrane for thermal insulation. In cold weather condition where the loss of heat at the roof is significant, the insulation board helps to reduce these losses.

On the contrary, in summer the roof is heated up by direct sunlight and the insulation layer reduces the temperature rise inside the pumping station.