Read Our Blog

What are the Common Causes of Flat Roof Leaks?

Some commercial and residential building owners become concerned because they have flat roofs on their properties.

It would seem that people are less concerned generally about sloping roofs, likely because people are more familiar with the concept of shingles or metal, acting as their water barrier, and sloped roofs are abundant in the realm of residential real estate.

The fact is; flat roofs do require a particular type of expertise to install such waterproofing systems well and to repair such assemblies.

To complicate matters; there are many different types of flat roof waterproofing systems, and there are even more variations on how those systems may be applied. Some products such as modified bitumen for example; can be adhered with hot asphalt, torch applied, or cold asphalt applied.

When it comes to the subject of the cause of flat roof leaks, factually there are a finite number of reasons which can be easily understood. Even a layperson can learn the basics of flat roof leaks and have some confidence in selecting an appropriate solution when engaging a qualified flat roofing contractor to correct the problem.

Related Content: Determining When to Repair, Recover, or Replace a Damaged Commercial Flat Roof.

First Concept: Sloped roofs are covered by water-shedding materials. Flat roofs are covered by waterproofing materials.

Second Concept: The substrate (structural decking); underneath the water-proofing membrane is constructed of one of the following: wood (boards or plywood), corrugated steel panels, concrete, and in more rare instances Gypsum Buildings( from a certain era).

Wood decks can be found both in residential and lower rise commercial flat roofs. To note is that on wood substrates; leaks generally appear on the interior of the building, in fairly exact, or close proximity to where the flat roof is actually leaking.

Steelpan decks typically have a boxed rib design with flutes (channels) between the raised boxed portions of the panel itself. On steel decks; the leak’s origin point may be far from where the water actually is seen to enter the building’s interior, simply because water can travel many feet along with the flutes of the steel decking, beneath the actual water-proofing membrane. The leak will usually appear internally, where a metal deck panel will reach a penetration point (such as a pipe opening), or where meeting and overlapping panel. Such leaks will follow a line usually in the same direction as the directions of fluting runs.

Concrete decks are typically used in high rise construction but can be also be found on certain low rise commercial, and custom build homes in the GTA. Like wood; concrete is fairly similar in nature in that where a leak originates externally; it will generally appear on the interior of the building.

Related Content: Important Cost Considerations When Installing New Commercial Flat Roofing.

Third Concept: It is important to learn or identify what type of water-proofing system exists on one’s flat roof building. Also, it is important to learn if there are multiple layers of water-proofing systems which have been installed.

Identifying the type of water-proofing system is important knowledge because there are differing repair actions which can be applicable to a specific type of system. Being able to communicate such information to the contractor can help speed up the repair process.

Identifying the number of layers of waterproofing materials is important for two reasons.

When a flat roof leaks, water can travel between the layers and can provide a false impression of where the origin of the leak is. Also, trapped water between roofing layers can freeze in winter months and be a potential source of winter-related flat roof leaking. Warm interior air can collide with colder exterior temperatures, resulting in a circumstantial leak situation. The best solution under such circumstances will involve cutting out the wet areas of the roof if it can be repaired, or tearing off all of the roofing layers if the roof requires replacement.

Fourth Concept: Why does my flat roof leak all of a sudden, or under specific weather conditions?

Often a flat roof will appear to be in good order, or it may have been replaced less than ten years previously. The question a building owner can ask is why all of a sudden am I having a leak when no observation of such previously.

The answers can be many, but the starting point of investigation are some basic inspection points.

The roof drains are a common source of circumstantial leaks. Check that they are free from blockages such as leaves or debris. Next; check drain connections immediately below the roof grade. In many instances, a rusted pipe coupling, or blockage at an elbow beneath the roof can cause a roof drain to back up under a load of heavy volumes of water. Blocked internal roof drains can dam water up on the roof, and such ponded water can enter at perimeter roof areas under metal flashing details.

Similarly, exterior gutters associated with flat roofs, can back up and cause leaks usually appearing to run down walls at related interior locations.

If the field area of a roof appears to be in good order, check all protrusions through the roof such as HVAC equipment, pipes, stacks, chimneys and skylight curbs as examples. Careful examination may reveal breaches, splits, or deficient flashing details which can allow water entry.

Related Content: Section-by-Section Flat Roof Repairs for Commercial Properties.

Fifth Concept: The field of the roof (the mass area of the flat roof itself); can be easily inspected for a condition with some basic knowledge points.

On tar and gravel roofs (also known as built-up roofing or BUR); walk the roof in a grid pattern to identify areas which feel spongy underfoot. This can signal trapped moisture beneath the water-proofing membrane, even if there is no evidence of water appearing on the ceiling beneath.

Common signs of deterioration include air or water blisters protruding from the roof, or splits or fissures in the membrane where bare felts may become exposed.

It is important to consider the age of the roof. Older BUR systems have lasted as long as thirty years, but more typically survive fifteen to twenty years in modern times.

Observe earthen and loose gravel which speaks to the age of the assembly, as newer BUR flat roofs have gravel firmly embedded in the asphalt pour, with loose but clean gravel at the surface.

With respect to two-ply modified bitumen water-proofing system (which appears as a rolled sheet good, having a granular finished surface much like shingles); inspect for de-bonded seams in the material, blisters or wrinkles in the membrane, spongy areas, and cracking in the finished surface of the materials.

Single-ply assemblies such as TPO, EPDM and PVC are termed single-ply systems, which essentially can be thought of as large sheets of material (much like laying a carpet). Leak failures typically result at adjoining seams which are typically hot air welded, and or glued with specific products designed for the membrane itself.

Punctures are a common source of failure but often can be easily repaired by a pro.

Sixth Concept:  I can’t see the roof because it is covered over.

Sourcing leaks on flat roofs which have wooden roof decks, or are covered by external thermal insulation (termed a PMR-Protected Membrane Roof or inverted roof assembly), can be tougher because the materials covering the water-proofing membrane must be removed first.

Other covered roof systems include Green Roofs, which can be covered in vegetation, having a root barrier sheeting and fleece layer beneath, to absorb rainwater.

While protected membrane roofs can last longer than exposed flat roof assemblies; they are considerably more expensive to install and repair.

The best advice involves selecting a reputable flat roofing contractor that can safely access the roof and conduct inspection methodology, reporting , repairs or flat roof replacement.

Always select a roofing contractor that carries the specific insurance required to complete such work and who is financially stable as a business. Experience has taught that many people do not do their due diligence where this element is concerned.

To learn more about commercial, industrial or residential flat roofing systems, care, and replacement; visit:

Heritage Contractors
Cadar Shake & Shingle Bureau
Select Shingle Roofer
Canadian Condominium Institute