by sadia_badhon | November 27, 2018 2:55 pm
by Allen Lyte, B.Tech., C.E.T., RRO
Determining the timing of an appropriate roof management approach is one of the most critical decisions a property manager can make. The cost to fully replace a roof can strain budgets. Deferring the decision to properly repair or replace a roof when required can result in collateral costs associated with interior damages and concealed deterioration of building components. Regular roof maintenance is mandatory, but as a roof approaches the end of its serviceable life, additional repairs should be completed to address specific defects to achieve long-term performance. Implementing repairs to manage roofing assets can help defer full roof replacement and allow options to renew the existing roof. This article focuses on low-sloped roofs and deals with telltale signs when roof work is required and available options.
Know the material
Numerous types of roofing materials are employed to construct the roof assembly. These materials shed water and provide thermal resistance and air/vapour control. The order in which the roof membrane, insulation, and vapour retarder are installed determines the type of roof assembly. Conventional roof assemblies have an exposed roof membrane and are applied over the insulation and vapour retarder, supported by the structure. Protected assemblies, also known as inverted roof membrane assembly (IRMA) or protected membrane roof (PMR), have their roof membrane directly above the structure, concealed and protected by insulation and ballast to hold the insulation in place.
Roof insulation in low-sloped roofs falls into one of the following two groups:
In PMRs, only moisture-resistant insulations can be used since they will be exposed to moisture, and the most commonly used product is extruded polystyrene (XPS).
Vapour retarders can range from polyethylene (PE) sheets, asphalt-laminated kraft paper, modified bitumen (mod-bit) sheet, or simple layers of felt and asphalt. The type of structure and how the insulation is adhered or mechanically secured dictates the most appropriate vapour retarder. It is important to note vapour retarders with waterproofing properties can act as a secondary drainage plane and can defer and/or redirect water from where it breaches the roof membrane to where it leaks to the interior. The roof membrane in a PMR also performs the job of the vapour retarder.
Common roof membranes fall into the following four categories:
The most common single-ply roof assemblies are either thermoplastic based (e.g. polyvinyl chloride [PVC] or thermoplastic olefin [TPO]) or thermoset (e.g. ethylene propylene diene monomer [EPDM]). The light weight, flexibility, and large sheet size of single-ply membranes make them economical choices for large roofs. Single-ply membranes are installed by fully adhering, mechanically securing, or loose laying them and holding them in place with ballast (Note, ballasted single-ply membranes over insulation are not PRMs). While single-plies can come in a range of thickness, often the thinnest available is utilized making the roof less durable due to risk of damage to the only layer of roof membrane defense.
Built-up and mod-bit roofs
Built-up roofs (BURs) have the longest track record and yield a durable roof. However, they are labour intensive requiring special equipment to install the multiple plies of felt and asphalt. They cannot be installed on steep sloped surfaces and can be difficult to fit around highly detailed roof transitions.
Mod-bit membranes, developed to address asphalt shortages, come in premanufactured rolls of reinforcement coated with styrene-butyl-styrene (SBS) or atactic polypropylene- (APP) modified asphalt. Modified-bituminous membrane roofing is more flexible than traditional built-up and can be applied with hot asphalt, cold adhesive, or a mechanical process, or they may be self-adhered or thermally fused in place. The top surface of a mod-bit roof has a granular surfaced finish with multiple colours available, but they can be bare, embossed with metal, coated, or surfaced with gravel. The bare mod-bit cap sheet would only be used in protected roof assemblies where ultraviolet (UV) protection is not required. The thermally fused applications using a torch have been the largest concern with the application of mod-bit roofs, particularly on buildings in use due to the risk of fire.
Historically, liquid-applied roofs have been used as true waterproofing membranes, such as liquid-applied hot rubber over podium decks or liquid urethane in parking garages. Some of the early applications of liquid-applied membranes on roofs were sprayed polyurethane foam (SPF) using acrylic-, silicon-, or urethane-based materials. New products are also being introduced, such as poly methyl methacrylate (PMMA) and polyurethane methacrylate (PUMA). The flexibility of these materials and relatively easy application make them excellent choices for difficult-to-access areas and complex roof shapes.
Depending on compatibility of material, various roof membrane components can be used together to achieve performance requirements that may exceed a stock roof membrane. A single-ply membrane can be adhered with a liquid-applied waterproofing material or a mod-bit cap sheet can be installed over built-up plies of felt and asphalt to achieve a premanufactured exposed finish while attaining the redundant durability of the built-up plies. In contrast, an existing smooth roof can be coated with a liquid-applied membrane to restore or improve performance or a mod-bit roof can be surfaced with asphalt and gravel to obtain additional durability and drainage upgrades.
Maintenance to defer future repairs
While regular maintenance is generally understood to be a critical requirement to prolong the life of a roof, it typically does not occur until a leak event occurs and repairs are required to address this deferred maintenance.
Basic housekeeping related to cleaning helps prevent damage to the roof membrane. The roof membrane can be damaged by unwanted vegetation growth, chemical or oil spills, and dead loads directly on the roof membrane. Any activity directly over the roof membrane, such as construction activities or pedestrian access, needs to be avoided to prevent damage. Depending on the duration of the roof activities, protection in the form of catwalks or plywood over insulation should be provided when increased foot traffic is unavoidable.
The majority of roof leaks do not occur through the actual roof membrane, but via other rooftop components such as mechanical equipment sitting on or penetrating through the roof. Often replacing inexpensive sealant around exhaust stacks/vents will prevent a leak. This condition can be easily evaluated by building maintenance staff when the sealant is visibly split or debonded. More complex rooftop mechanical equipment often become the source of leakage when condensation trays become clogged and backup, or when failed duct waterproofing allows water to seep in.
Since stress on the roof membrane is greatest at transitions at the roof’s edge it is critical to have durable roof membrane flashings and then protect them with sheet metal flashings. While the sheet metal flashings should be designed as largely esthetic, in addition to protecting the membrane flashings from abuse the drip edges do provide a certain amount of water shedding away from the building. If allowed to deteriorate too far the sheet metal can create a safety concern if it becomes loose and blows off.
PMRs require the least maintenance due to the roof membrane being protected by the overlaying insulation. Clear drainage is critical for this type of roof assembly to avoid displacement of the insulation which could then float on ponding water. Trapped stone ballast between displaced insulation and the roof membrane can cause abrasion damage over time.
A visible roof membrane can be prone to various modes of failure in an exposed conventional assembly. Bituminous-based roof membranes can develop blisters from entrapped air/moisture and ridges lead to split roofs over time from differential movement. As the protective surface of the bituminous roof wears due to foot traffic, wind scouring, and asphalt degradation, the primary waterproofing layer can be damaged and expose the roof membranes’ reinforcement.
All bonded roof membranes have the risk of de-bonding from their substrate or at their own seams that could lead to eventual failure of the system. Lighter single-ply membranes are prone to punctures, tears, and tenting at areas where the membrane pulls away from upturns (e.g. parapets), thus causing stress on the roof system. When the insulation or the single-ply membrane is mechanically secured the fastener can puncture the membrane from below. Fasteners can “back out” over time as a result of dynamic wind loads. Crushed insulation around fasteners leaves the membrane unsupported and the head of the fastener can then penetrate the membrane.
Liquid-applied membranes can suffer from similar wearing concerns. Since liquid-applied membranes tend to be smooth and lighter in colour, they run the risk of discolouration due to dirt pickup similar to single-ply membranes. Improper installation or excessive wear becomes an issue when the reinforcement of a liquid-applied membrane becomes visible or exposed. Water can wick through this reinforcement layer just like any reinforced roof membrane. Depending on the vapour permeance of the liquid-applied membrane, water blisters can form or the membrane can simply de-bond if substrate preparation is not done correctly.
When repairing is no longer viable
At some point, when the leakage occurrences surpass tolerance levels, continued repairs will need to be evaluated. This varies for every roof depending on the sensitivity of the space and occupants below the roof. When the failures move from a localized incidence to widespread leaks the costs to repair begin to exceed maintenance budgets and are no longer economically viable. Fortunately, once repairs are realized to be no longer feasible there may be options to renew the roof instead of full replacement.
Renew or replace?
Benefits of renewing a roof include reduced costs, condensed construction schedule, less impact onsite, and deferral of roof disposal to landfill sites. However, a roof renewal is only an option if the existing roof has not totally failed and contaminated the entire assembly with water. This comes down to the extent of leakage, type of insulation, and ability to dry. Waterproof vapour retarders or solid concrete structures, for example, prevent moisture escaping the roof assembly.
The service life for the renewal project depends on the building’s long-term ownership plans and available budget.
Depending on the condition of the roof, service life/performance, and available budget, there are a range of renewal options based on the type of roof membrane. Asphaltic BURs begin to lose performance with the aging/oxidation process of the asphalt waterproofing component. When the field of the roof begins to show signs of inflexibility and the upturn/penetration flashings are still in good condition, a BUR can be re-saturated. The re-saturation process can help revive the asphalt in the top pour of asphalt forming the flood coat, the primary waterproofing component in a BUR.
Given the upturn/penetration flashings, including upturns at roof edges, are the most common failure points it makes sense to replace these at the time of restoring the flood coat. The change in direction of the roof membrane at these locations increases stresses that can cause localized failure (e.g. splitting membrane seams and de-bonded membrane terminations). When doing a full roof restoration, the re-saturate material is often upgraded to a more robust material to augment the existing flood coat waterproofing. Typically, this involves removing the existing gravel surfacing and fixing defects in the BUR, such as blisters or ridges, prior to roof membrane renewal.
The advancement of liquid-applied products provides the opportunity to fully coat existing smooth-surfaced roofs. Single-ply, mod-bit, and existing liquid-applied are best suited for this renewal option. BUR would need the gravel surfacing removed. The fully bonded nature of the liquid-applied overlay helps prevent inter-ply moisture transfer regardless of the moisture being trapped in the existing insulation.
Roof overlays generally refer to the application of a new roof membrane over the existing roof. Depending on the roof assembly and membrane being used as the overlay, it may be applied directly over the existing roof membrane. However, if the existing membrane is too deteriorated and the added weight is a concern, the existing roof membrane can be peeled to expose the underlying insulation. This allows any wet insulation to be more easily identified and replaced.
Typical rigid insulations used under conventional roof membranes include foamed plastics, wood, or mineral fibre, and semi-rigid fibre. Polyisocyanurate (polyiso) insulation has the largest market share of the base insulation foams due to its cost versus thermal resistance. However, some feel it does not provide the best substrate for a roof membrane given its low density and dimensional instability. Semi-rigid mineral wool or fibreglass insulation have fair thermal resistance, and while not dense, they are dimensionally stable and have the ability to dry if exposed to moisture. Wood fibreboard, commonly used as cap insulation over the base, is used for its density and dimensional stability instead of its thermal resistance. Wood fibreboard insulation is the most moisture absorbing of these materials and moisture will lower the fibreboard’s already low thermal resistance rating. The greatest cost impact of wet insulation is not the reduction in energy performance, but the effect on the roof membrane’s performance. The roof membrane can develop blisters from the entrapped moisture, creating increased stresses on the roof membrane. Moisture can affect the adhesion of the roof membrane and reduce the wind uplift resistance. Long-term entrapped moisture will reduce the service life of the roof and limit the effectiveness of roof repairs as well as limit options to renew the roof.
In new construction, the amount of insulation required depends on the type of building occupancy and the geographical location as dictated by building codes. Designing to meet or exceed the minimum current code requirements is the best practice; however, code requirements do not need to be followed when renovating an existing roof. The existing insulation values should be met as a minimum during a roof renovation. Upgrading the base insulation for increased thermal resistance should be attempted where existing upturn clearances under doors, mechanical equipment, etc., permit. Tapered insulation should be included to improve drainage to prevent standing water. Lastly, a durable, dense, dimensionally stable cap insulation should be applied overtop.
Deciding to renew a roof instead of performing full replacement may appear to be a simple choice, but, in fact, involves numerous considerations critical to the success of the building. For best results, building owners should obtain the advice of experienced professionals when determining the best solution. A structural engineer must be engaged if the roof load is going to be increased (such as installation of a new roof top mechanical unit) or the building structural system is being modified.
Finally, replacing a roof can present a great opportunity to protect and enhance an investment. This is the time for owners to evaluate their expectations of their building’s service life. While some building owners simply choose to replace like-for-like, many take this opportunity to change aspects of the roofing system to improve durability and longevity, increase energy performance, and control costs. Long-term investors will benefit from new robust roof assemblies with increased service lives and which require minimal maintenance to achieve the best life-cycle cost. However, if ownership goals are limited to the short-term, renewing the roof in lieu of full replacement reduces capital costs. Increasing the insulation in either scenario will reduce energy costs and should be completed when it meets the owner’s financial model. Again, an experienced professional can help navigate the structural, serviceability, and durability concerns to ensure the most appropriate system is specified for the building and budget.
Allen Lyte, B.Tech, C.E.T., RRO, is a principal partner in the consulting firm W. Allen Partners Inc. specializing in roofing materials, their application, and building science. Lyte is an active director on the executive board, and past president of the Ontario chapter of the Roof Consultants Institute (RCI). He can be contacted at firstname.lastname@example.org.
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