Modern day roof systems are installed over high R value insulation courses. The addition of much insulation leaves the roof membrane exposed to higher temperatures and heat aging in the warm months and lower temperatures and freeze-thaw stresses in the colder months. The relatively stable interior temperatures of the building are no longer able to moderate the impact of ambient conditions on the roof membrane.

What roof system is the best option for a New Construction Building. Following is a discussion of Selection Criteria for roof systems that can be applied for the selection of a roof system. “It is essential to design a roof system, than to just pick one.”

The primary differences between or among roof systems is longevity of service life and performance during that service life. Roof systems must keep water out of the building interior all of the time – 24/7/365 – not some of the time. Other considerations include type and colour of the roof surface, ability to be installed well and of course, initial cost.

Initial cost of the roof system is important, but it is not the sole criteria by which to assess a new roof system. Low initial cost, short service life roof systems provide a high cost “life cycle cost” – defined as the total cost of the roof system over its service life including the initial cost, all maintenance costs and all other expenditures, including leak repairs, and repairs to the building after leak events – divided by the service life in years.

Roof systems are replaced when the building occupants can no longer tolerate the destruction, interruptions, aggravations, potential health and safety issues and multiple costs of repeated leaks. Leak repair, after leak repair is very seldom acceptable. Building functioning is paramount.

High life cycle cost roof systems are lower in initial cost, but extremely costly in the long term. Low initial price always means high, long term costs and building misery.

Low life cycle cost roof systems provide leak free, low cost performance over the long term – usually 35-40 years plus – but cost more initially. A New Building requires a low life cycle cost roof system that will prevent water incursion into the sensitive interior of the building for the very long term. The roof system at must perform and provide leak-free services on the building for a very long time. 

The very great concern of retention and storage of rainwater on the roof must be considered in depth. Large volumes of water in storage on the roof, present the potential for exceptional damage in the event of a breach in the roof system. A multi-ply roof membrane provides redundant waterproofing capability.

 There is no time at which the installation of a roof system will be less expensive, or less disruptive than during new construction – when the building site is a construction zone. When construction ends, the lowest cost period is gone.

Interior finishes, furnishings, sensitive equipment and important files are at great risk when a roof system leaks. Furniture is damaged, floor coverings ruined, equipment and computers devastated and data lost or compromised. It is the Owners and Occupants of the building that suffer the extensive losses when a roof system allows water to penetrate into the interior spaces of an important building. The construction forces and others who provided an inadequate roof system will feel no inconvenience, no economic losses, no damages. They will have moved on down the road. If questioned, fingers will always be pointed at others.

The one building system that can degrade – quickly and suddenly – the value and functioning of a very valuable building is an inadequately designed, poorly installed or otherwise, poor quality roof system. Roof leaks can destroy the interior and functioning of a building in very short order. Repeated roof leaks can interrupt the operations and quality of the environment of a building. Health and safety hazards can provide a real danger for the occupants.

Roof systems are extremely important – if the roof system on a building is of poor quality – there is, effectively, no building. The roof system deserves the ultimate attention on a building. Walls can be unsightly, floor treatments can wear out, exterior appearance can be undesirable – but a leaking roof will stop the 

functioning of a building. A roof system must be perfect – how much water falling on a desk, on a computer or an expensive piece of equipment is acceptable? The roof system must be respected and considered to be the absolutely critical system that it is – not just some bunch of materials on the top of the building that no one sees.

“It is essential to design a roof system, rather than to just pick one”.

In order to design a low life cycle cost roof system for a New Building, all relevant criteria must be considered – new synthetic and advanced materials must be included. Following is a process for consideration of criteria and evaluation of roof systems that satisfy the design criteria and length of service life.

It is simply not acceptable just to copy a roof system specification from another project, from a stray source or manufacturer’s sale sheet – that is not designing a roof system for a very important project – that is picking one.

It is very important to understand that once the building construction is completed – roof replacement is a very difficult activity. 

There are people who say it is no problem – roof replacement can be done. Anything can be done – but at what expense, at what cost in interruptions of building operations, at what consumption of time to facilitate roof replacement. Remember – roof systems are only replaced after leaks occur and re-occur.

These people not considering the extreme expense – many, many times the initial roof system installation cost. These people are not paying the bills. These people are not considering the Owner(s) – only their immediate needs. 

In recognition of that fact, the design of the roof system must incorporate materials and installation processes that will allow the roof system to serve and function for the very long term. To emphasize, the least expensive time to install a roof system on a building is when the building is a construction site.

To that end, the following roof design criteria specifically recognizes very long service life as a primary requirement.



All factors that can possibly be integrated into the design, implementation and final outcome for a very long term, very high quality roof system must be considered. 

Roof system design must be carefully and thoughtfully selected and carefully specified, installation must be carefully choreographed, watched and be extremely well coordinated by a very competent team. 

The demise or failure of a roof system on a very important commercial building, can mean the demise of functionality for that building.

A comprehensive listing of “Roof System Design Criteria” must be established to aid in roof system selection. A specific custom designed roof system is necessary – “it is essential to design a roof system, than to just pick one”.

Very sensitive commercial buildings present a number of challenging variables for roof system design. These variables include – but are not limited to:

  • The sensitive nature of the building – this is the vehicle by which people earn their livelihood
  • The potential for the accumulation, retention and storage of large amounts of water on the roof area
  • The significant, potential, negative impact of wind and storm events and exposure of the roof to debris and significant leakage events
  • Traffic on the roof to service units, repair systems and perform maintenance activities
  • The physical orientation of the roof areas, i.e. some levels can be relatively inaccessible to the work site/set-up area 
  • A very large dynamic roof area
  • The PRIMARY requirement for a very long service life roof system 

Roof System Design Considerations and Information Page 5 of 7

  • The relative inability to perform significant maintenance operations on the surface of the roof systems – i.e. renew surfacing of the roof membrane
  • The sensitivity to noise, confusion and odours of the interior environment
  • Exceptional necessity of a redundant, robust waterproofing membrane
  • The very significant exposure of the roof areas to winds, precipitation and potential for highly complex details and features to allow water entry
  • Drainage requirements – positive drainage is always preferable – but water retention on the roof is a fact of life – the structure and drainage design must accommodate retained water to the degree possible
  • Proper and extremely long term base flashing details at the perimeters, smaller sub roofs, skylights, projections and penetrations 
  • Piping, electrical conduits and other rooftop accessories that must be well detailed and flashed to provide very long term protection for the roof system 
  • Detailing of all pipes and projections through the roof deck and roof system
  • Insurance and code provisions – FM requirements and/or other specific requirements
  • Extreme Puncture resistance requirements – water retention of the roof dictates a robust, puncture resistant roof membrane
  • Lowtemperature flexibility, elongation, water immersion and movement tolerance
  • Fire resistance and resistance to spread of fire
  • Effective and proper surfacing for the roof system – a permanent surfacing system is highly preferable to surfacing systems that will require renewal or significant maintenance 
  • Energy efficient and environmentally responsible design – an very long service life is environmentally responsible and protective of the building interior
  • Flashing of fans, ventilator and other smaller rooftop units and projections 
  • Low maintenance requirements

Very low life cycle costs – initial costs are not the only value consideration – the cost of the roof system over the long term must be considered

Additional Roof System and Specification Considerations

  1. Wind Uplift Calculations should be made. All applicable factors including exposure, exposure type, height of roof area, structural components, perimeter type and configuration, wind speeds, precipitation, surrounding topography must be considered and calculations made according to Factory Mutual standards – Factory Mutual/National Research Council standards are generally recognized to be the most stringent in the industry.
  1. Moisture Measurements and Interior Conditions should be assessed. Roof system type and amount of insulation, vapour barrier type and configuration and other roof system factors must act in concert with the interior conditions. 
  1. Insurance Coverage – the requirements of all concerned insurance carriers must be addressed. Factory Mutual requirements, or others, as required, must be tabulated in written form and the roof system must satisfy those requirements – non-specific mention of the requirements is not enough.
  1. Perimeter Details must be constructed to resist wind forces, provide very long service life and protection of the roof system flashing system. Full metal coverage and proper securement including continuous starter strips, stainless steel fasteners and other fastening techniques are required. 
  1. Roof Membrane Construction must be mechanic and building friendly and provide redundant waterproofing. The roof system must stay attached to 

the building and all design criteria must be satisfied before the specification is written. 

  1. Roof System Specifications must be all inclusive. Procedures and execution processes must be clearly spelled out. Materials must be compared to a standard and the manufacturer(s) and type and reference standards must be listed. 
  1. Building Requirements, limitations, procedures and constraints must be clearly and prominently listed in the specification. 

Roof System Design Considerations and Information Page 7 of 7

  1. Quality Control and Resolution of Details – the roofing professional on the project must have the knowledge and decision making skills to address quality issues, difficult details and other situations that require resolution – merely having a “body” on the roof who must “call the office” for resolution is of little practical use – the professional on the roof must be just that – experienced, knowledgeable and willing and able to make decisions and resolve issues. The expertise must be on the roof.
  1. Contractor Responsibilities must be clearly delineated in all project documents. The contractor is engaged to install the roof system, not to be a roof consultant or adviser. The contractor must provide the labour, materials and equipment necessary to complete the project successfully and to the highest quality possible. The roofing contractor must be of the highest quality – with best labour, the best equipment and the most competent management.
  1. Manufacturers must be pre-screened for appropriate roof system components of appropriate quality. All components must be compatible and the manufacturer must provide warranty coverage that means something. If a warranty claim arises, they should be prepared to write a cheque without question or hesitation. This type of manufacturer exists and will also provide good technical support and other services as required.