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Pitched Roof, Low-sloped Roof, and Flat Roof- Commercial Roofing Guide

Commercial building owners and managers have at one time or another, had the need to deal with roof replacement, repairs, and maintenance. As a result their roofing assets will involve one of, or a multiple of, roof design configurations.

Flat roofs as an example represent a large portion of the roof coverings that relate to commercial warehousing and manufacturing facilities. Also included in this category are low rise and high rise apartment structures. In the past thirty years, flat roofs have increasingly become incorporated into the designs of residential housing.

In the context of initial construction, flat roofs can be less costly to construct due to their simple design; avoiding complex rafter configurations and the associated increased labour costs to construct such.

There are some things to understand about flat roofs. While they can be less expensive to construct, the cost of waterproofing flat roofs can be greater on average than that of a steep pitched roof. To qualify this statement, it is important to consider the type of substrate (deck) of a particular flat roof. For example a concrete deck can be costlier that steel pan decking or a wooden deck surface. In this context, a pitched roof may actual prove less expensive to build overall. In broad strokes however; the flat roof will generally be less expensive.

While flat roofs are termed such, factually they are constructed with a slope as per modern building code requirements. This is achieved by cambering the roof deck itself, or by using an engineered tapered insulation system. A two percent slope toward the drains is a typical requirement.

Low-sloped roofs are roof structures which are designed with a determined slope of between 1/12 pitch and 3.5/12 pitch. Such roofs are cambered most typically in one direction, two opposing directions, and in the context of residential building, sometimes four sloped “ hip roof” configurations.

Water-proofing of low-pitched roofs can include steel roof panels, and shingles using low-slope roof techniques; but most ideally, should be water-proofed using flat roofing membrane systems of one type or another. It is important to note that manufacturers of shingle products do not warrant their materials on pitches less than 3/12.

Also to consider with low- sloped roofs which have roof coverings other than flat roofing materials, is that such products are water-shedding, not water-proofing. This is a prime consideration for areas which may receive a lot of snow or prolonged rain seasons, such as in Western Canada and the western U.S.

Some considerations for low-sloped roofs, particularly where adjoining associated steep pitched roofs involves ventilation. A proper intake and exhaust ventilation path needs to be incorporated into the design of such roofs to avoid issues of condensation and super-heating of the roof deck.

Advantages of low-sloped roofs can be that they are easier and safer to walk on when working on such structures, (also like flat roofs). Insulating beneath low-pitched roofs is also an important design consideration. Many specifiers prefer the use of spray foam insulation beneath the decking to create a monolithic scenario where intake and exhaust of the particular attic space becomes less critical. (it is important to note on this point, that each building scenario can be different, and that one solution may not be the right solution for all scenarios).

As a result; low-pitched roofs need good design consideration well before construction and when retro-fitting such roofs. The services of a qualified engineer are highly suggested when specifying such roof assemblies that may be utilized on a low-pitched roof.

There can be much debate among people as to the longevity of a flat or low-pitched roof when compared to greater pitched roofs. Factually, there are cases to support either argument in that we have observed flat roofs that have lasted thirty plus years and we have seen sloped roofs which have begun to fail as early as fifteen years and vice-versa.

The formulae to consider when installing or replacing any roof goes like this:

  1. Poor materials and good workmanship can result in a bad long term result. (Usually decreased life-span of the roof assembly).
  2. Good materials and poor workmanship will almost assuredly result in a bad long term result.
  3. Good materials and superior workmanship will almost assuredly result in a roof that lasts for it’s intended life.

The morale of the afore-mentioned being: to give careful consideration to design elements of the roof assembly, taking into account associated building envelope requirements such as insulation, heating and cooling, and ventilation, as well as associated air-sealing requirements.

Next; selecting better quality materials that will last over time and finally;

Choosing a qualified and caring installation group that can execute the work well, and for whom the client can have assurance of long term service and warranty.

Pitched roofs which are steep in nature are designed to shed water. The steeper the pitch, the more quickly the roof sheds water. As a result, such roofs need to incorporate a guttering system that can adequately collect and disperse the increased volume and speed of water flow.

The most common water-shedding materials for decades has been shingles. Today fiberglass laminate shingles are the most popular choice having manufacturer warranties that suggest fifty years in some cases. Traditional and heritage materials include slate (the one hundred year roof option), cedar shingles and shakes which are a twenty to thirty year solution, and metal roofing panels which can last 30 to fifty years. Composite roofing products such as synthetic slate are also designed to be a premium solution.

Because pitched roofs are in fact water shedding, the installation of quality underlayment materials beneath the shedding materials are a key defense element against perils including some forms of ice dams and wind-driven rain.

Some water-shedding products tend to be superior in terms of animal intrusion resistance through the roof and are not subject to clawing, curling or cupping.

Steeper pitched roofs structurally speaking, have proven to be very sound for numerous decades, as witnessed in the one hundred year old plus housing stock found in the eastern U.S. and Canada.

Flat roofs and low-pitched roofs can be more vulnerable to deck deflection and perils associated with excessive snow load, and as such need to be designed with snow load in mind. Older roofs of this type should be cleared off from excess snow during winter months; and associated internal and external roof drains must be kept clean as a minimum degree of on-going maintenance.

When considering the selection of a water-proofing membrane for flat roofs and low-pitched roofs; there are a number of options and varied methods of installing certain membrane types. Such systems typically fall into two categories which are single ply membranes and multi-ply membranes.

In the single ply category there is TPO, PVC and EPDM. They offer solar reflectivity (not black EPDM),and they are light weight relative to other systems. They also can provide high tensile strength and elastomeric properties which can be important on very large expansive roof areas.

In the multi-ply category, are four ply Built up roofs (BUR), and two ply modified bitumen assemblies. These better known and understood materials have been proven choices for decades.

As a commercial property owner or facility manager, it is suggested to become familiar with the roof configuration types that one manages and to become educated on their condition and the water shedding or water-proofing materials covering those roofs.

To learn more about various commercial and industrial roof types and assemblies, or to reference previous articles on the subjects which can be found in our “articles” section at www.avenueroadroofing.com  You may also view us on YouTube™, or simply contact us for a consultation and or roof evaluation.

Certainteed
FIRESTONE
VELUX
RAIN PRO
IKO
Heritage Contractors
BBB
Cadar Shake & Shingle Bureau
Select Shingle Roofer
ACMO
WSIB
Canadian Condominium Institute