FLAT ROOFS AND THEIR MAINTENANCE
The roof is the most critical component of the building enclosure. However, roofs, flat roofs in particular, are often ignored until a water leak occurs. Unfortunately, extensive damage to the roof system may have already occurred by the time a leak is recognized.
Southern Ontario provides some of the toughest environmental conditions for roofing. Frequent and rapid fluctuations in the temperature cause expansion/contraction forcers and freeze/thaw problems that deteriorate the roof system.
Generally, there are two types of flat roofing systems available. The conventional system, the system most widely used in this area, has the insulation placed below a water proofing membrane.
The membrane is usually protected on top by a gravel layer. The second type of roofing system is the protected membrane or inverted roofing system which has the insulating material placed on top of the water proofing membrane. The insulation protects the membrane from external thermal effects. Gravel, paving stones, earth or a variety of other materials are placed above the insulation as ballast to hold the insulation in place.
In a roofing system, the purpose of the membrane is to stop the passage of water through the roof. There is a variety of membrane types that can be used with both the conventional system and the inverted system. The majority of roofs use a membrane which is made up of layers of felts saturated in bituminous type material. The future will bring the use of more single ply and modified bitumin membranes.
Ideally, inspection and maintenance is initiated at the time of original building construction. A detailed layout of the roof locating each, drain, stack, heater/air conditioner, expansion joint, etc., should be drawn. The use of a grid patter on the layout will assist in the location of each item. Marks, such as paint lines, may be used to physically indicate the grid on the roof.
The location of test results and repairs should be marked on the layout and records kept for each inspection made. Roof history is required to evaluate performance and to facilitate repair cost analysis.
An inspection of a roof is generally not started on the roof itself. A walk around the building to inspect the upper portion of walls near the roof junction should be one of the first steps. Damp marks, efflorescence or spalling may be an indication that roof water is entering the exterior walls. Also, damage to edge flashing can be seen during the walk around.
The second step should be examination of the underside of the roof deck to reveal evidence of roof leaks. Stained acoustic ceiling tiles often indicate problem areas. Moisture or corrosion on the underside of the roof deck may indicate a roof leak or a condensation problem caused by localized poor insulation.
A visual inspection, by walking over the roof surface, can uncover major problems such as the ponding of water, wind damaged flashing, and punctures due to impact. Ponding occurs when roof drains become plugged or when sagging of the roof structure occurs. With minimum knowledge of roofing, problems such as, blisters, ridging, alligatoring and gravel scouring can also be located by visual inspection.
Unfortunately, membrane problems are much more difficult to locate with inverted roofing systems because the membrane is covered. During visual inspections it is important that the membrane around each protrusion (vent cap, air conditioner, etc.) be carefully investigated. The flashing in these areas can be dislodged by high winds exposing the membrane to ultra violet light and other environmental effects which case deterioration. Also, splitting of the membrane around protrusions can be caused by differential movement.
Roofing problems can be very costly if not recognized in their earliest stages. When a leak occurs in the waterproofing membrane of the conventional roof, water can travel along the corrugations of the steel roof deck and saturate large areas of insulation before the leak is detected. Saturated insulation looses its thermal properties and should be removed. Untreated problems can lead to extensive repairs. Large repairs are difficult to seal and may lead to future problems where repair areas meet existing roof areas. The interface of repairs and existing roof areas is critical and this should be considered before repair procedures are selected.
A number of inspection tools are available to the engineer or roofing expert. Electrical resistance moisture meters, nuclear moisture meters and infra red thermograms are relatively complex devices for measuring moisture in a roof. However, the results from these testing devices must be carefully analyzed and interpreted as they can be misleading. Test results should be confirmed with roof cores or samples, to ensure that the analysis is correct.
Flat roofs, both inverted and conventional, are fragile materials that require constant monitoring and maintenance. Recognition of a roofing problem in its early form can save a costly repair or replacement. We recommend that monthly visual inspections be conducted by knowledgeable building maintenance personnel and that a detailed inspection be conducted by a qualified engineer with increasing frequency as the roof ages. Drains can be cleaned by building maintenance crews and qualified roofing contractors should be retained to correct minor leaks, blisters and wind damage. If the roof is suffering from large areas of damp insulation, failed expansion joints or extensive ridging etc., an engineer should be contacted.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2