February, 2010

Inspection News and Views from the American Society of Home Inspectors

Flat-Roof Inspection with a Focus on Modified Bitumen


Because of the costly damage they can cause, roof leaks surely top the list of home inspectors’ liability concerns. This is especially true when inspecting flat or low-sloped roofs.

My roofing experience began more than 30 years ago; my father and grandfather were both roofing contractors, so I practically grew up installing and repairing roofs. Now, 30 years later, with thousands of roofs under my belt, the task of accurately reporting or determining the cause of a leak in a low-sloped roof can still be challenging. After being called on numerous times as a consultant in litigation between homeowners and roofing contractors or regarding home inspection complaints, I wrote this article with the hope it will help ASHI members with these challenges, thereby better serving their clients.

The types of low-slope roofing seem endless: asphalt built-up, EPDM, PVC, Modified Bitumen and TPO, to name a few. Much of the information in this article could be applied to all low-slope roofing; however, the focus is on a worthy material called Modified Bitumen (MB) and on its residential use. Please note: Specifications and details vary from manufacturer to manufacturer, as well as for specific material types and for commercial use. This article is intended as a general guide for home inspectors — not an exhaustive reference. And, additional information covering flashing and general concerns will be published next month in the March Reporter.

What is Modified Bitumen?

Unlike and not to be confused with asphalt roll roofing, Modified Bitumen (MB) membranes are tough and resist tearing. When correctly installed, they make a reliable roof. To the contrary, asphalt roll roofing is thinner than MB, easily torn and the application usually involves nails and roof cement. An MB membrane is reinforced with fiberglass, polyester or a combination of the two, which gives it strength; puncture resistance and overall system integrity. The asphalt (bitumen) is the waterproofing element, which has been modified (or improved) with Polymers. The membrane can be surfaced with mineral granules or metal foil in a variety of colors or left unsurfaced (smooth). MB is often referred to as rubber or a rubberized roof, but it is important for a home inspector to know it is not the same as an EPDM (rubber) roof.

A variety of Polymer blends is available. For residential use, the two most common modifiers are: Styrene-butadiene-styrene (SBS) and Atactic polypropylene (APP).

• SBS polymer
gives the asphalt a rubber-like characteristic. Known to have good flexibility in cold temperatures, it’s more sensitive to UV radiation than APP. Usually, it’s applied with hot asphalt: however, it can be applied using a cold adhesive. To prevent premature deterioration from UV radiation, granules similar to those found on shingles or metal foils are used to surface this type of membrane.

•APP polymer
is known to perform better in hot weather applications, and it is more resistant to UV radiation than SBS. It’s usually applied with a torch (heat-welded); however, some manufactures have cold adhesives available. APP is usually an unsurfaced (smooth) material, but there are granular-surfaced versions available. It is generally recommended that unsurfaced membranes be painted with a reflective paint to increase its resistance to UV radiation. It is my experience that this greatly increases the life of the roof.

On homes, the membrane is usually single-ply, although multi-plies can be installed.

An inspector could make some general assumptions about the type of MB; however, I recommend simply describing it as MB. It’s difficult to differentiate types once they’re installed.

Several manufacturers have introduced self-adhering (peel and stick) versions of MB over the past 10 years. This article does not address this type, but it is important to know they are out there. They have the same basic installation details and defects as other types. I have seen peel-and-stick versions used to repair built-in gutters, as well as entire roofs. From experience, I believe their ability to adhere depends on the condition of the surface, including how well it was cleaned. Like tape, self-adhering MB will not stick to dirt, rust, etc.


Where to start

The goal of a roof inspection is to identify the following:\

All successful inspectors have their own roof inspection protocols. I recommend starting from below, as follows.

While on the ground, carefully view any perimeter edge flashing, noting any irregular appearance that deserves a closer evaluation once on the roof.

On a masonry home, note any efflorescence along the top of exterior walls, which could be related to a roof leak, or any masonry deterioration that could contribute to a roof leak.

Prior to getting on the roof, thoroughly inspect interior ceilings, walls and attics for evidence of leakage and/or damage. Signs of a leak on a ceiling don’t mean the roof is leaking directly overhead. Often, flat roofs have layers such as base sheets, more than one ply, old roof, rigid insulation or metal decking. The leak may originate in one spot and zigzag down until it finds the path of least resistance before it shows up, often many feet away. On a metal deck, for example, it is possible that water entered on the front high side of the home and trailed down the metal to the rear low side before it found its way down to stain a ceiling. Experienced roofers hired to locate and repair leaks always determine the type of roof decking, asking whether the leak showed during the rain or sometime after. This information helps them find the leak.

What about Structure?

Structural defects can affect roof performance. Often, I find residential additions with inadequate flat roofs. The once-flat roof has deflected downward several inches into the shape of a bowl, causing cracks and noticeable bowing in the interior ceiling. If it’s a drop ceiling, you can see the bottom side of the structure by removing the ceiling tiles. The decking should not be rusted (if metal); even slight rust should be reported and further evaluated. Rusty roof decking can be an indication of high humidity in the building; but more often indicates faulting roofing. What appears to be surface rust on the underside of metal decking can actually be significant damage, so be careful walking on a rusty metal deck.

As we focus on MB, please be aware there are many structural issues and roof deck materials not covered in this article.

Why walk the roof?

Opinions differ on whether to walk on roofs, but I don’t know how anyone can adequately inspect a flat roof without walking on it. When inspecting any roof, always err on the side of safety, always know where you are on the roof and never walk backwards. As you walk around inspecting the roof, pay close attention to how it feels underfoot. Often, rigid insulation is found with MB roofs, although it is less common on homes than in commercial buildings. It is impossible to visually detect wet insulation, and it’s difficult to detect it from feel. A sunken or spongy-feeling area may indicate the insulation is wet and damaged. Sometimes, you can hear the sound of water squishing, or you may see water squirt out from the membrane when pressure is applied. Be careful not to step on blisters or anything else that could damage the roof.

What are the components of the roof system?

Home inspectors, probably more than anyone including roofers, understand that the home is a system of components where one component can directly affect another. The roof system is no different. Roof system components include the following:
If one component fails, it adversely affects the entire roof system.

The waterproof system of any flat roof has three main parts:
  1. the membrane (or the cap sheet),
  2. the seals (or the sealants that form a waterproof joint between two waterproof materials), and
  3. the flashing.
The majority of roof failures (leaks) occur at laps, flashings and seals, a small part of the total roof. So, careful attention should be given to where the membranes lap, or meet a vertical angle, such as around a skylight, plumbing vent stack, mechanical equipment curb, parapet wall, house exterior wall, or terminate at the perimeter edge or at an installed guard rail.

What are common membrane issues?

1) Laps are critical on flat roofs and should be examined for full and uniform adhesion. On an MB roof, you should see the following:

Examine the laps for indications of short laps, overheating, voids and/or seal deficiencies.

2) Blisters
are commonly caused by moisture entrapment within the roof system, but may also be due to poor adhesion, air entrapment and/or vapor. Blisters should always be reported as a deficiency, but the cause and/or the decision whether it should be repaired is best left to a roofing contractor. If not moisture-related, not in a lap and not within a walking path, small blisters are probably insignificant and commonly ignored by roofing contractors.

3) Splits can be an indication of stress or movement within the structure, and, if so, flashing will usually have also failed. In large buildings, stress is controlled with the installation of expansion joints, which are seldom used in homes. Splits are caused by a number of reasons in addition to stress, including faulty installation.

4) Wrinkles (or ridges) in the membrane can be due to movement in the membrane, but also can be an installation flaw. Wrinkles usually end up splitting.

5) Movement, slippage or migration of the membrane is not commonly seen on homes unless the slope is steeper than normal. Usually, it’s related to thermal forces, the lack of adhesion and/or the use of a membrane not rated for a steep slope. Movement of the membrane can contribute to splits, flashing failures and wrinkles.

6) Damage. As with any roof, MB is susceptible to damage. The causes of damage are numerous such as punctures from fallen tree limbs, foot traffic, dropped tools, sharp objects, vibrating roofing installation mechanical equipment, fasteners backing out of substrate puncturing the membrane, etc. Also, rooftop mechanical equipment such as kitchen exhaust fans can discharge cooking oils, etc., which can quickly degrade a roof membrane. Any potential discharge must be controlled to prevent roof damage.

Photo above: This new roof is an example of poor workmanship. The base wall flashing was not adhered to the roof membrane.


Photo above: Another example of poor workmanship on a new roof. A poorly adhered lap was improperly patched with roof cement and the scupper was improperly installed.


Photo above: This roof is no longer watertight because a lap has failed.

Determining age

Determining the exact age or estimating the remaining service life of any roof is tough, and Modified Bitumen is no exception. Although the ASHI Standards and Code of Ethics requires us to report on a system that is near the end of its expected service life, it does not require providing life expectancies. Modified Bitumen manufacturers offer warranties that range from five to 20 years. In my opinion, manufacturers use long-term warranties as a marketing tool because they know most premature failures are not due to the material.

The actual service life of any roof depends on the quality of the material, environment, foot traffic and especially workmanship and maintenance. If conditions and workmanship are good and if maintained, MB might be serviceable for 12 years. In my opinion, it is risky to anticipate more or to state remaining life, never forgetting that MB failures (leaks) almost always occur at terminations, flashings or penetrations.

Like other asphalt products, MB dries out and cracks. Surface cracking (or the alligator appearance) is common and will appear on smooth-surfaced MB after a few years, especially if not coated with a UV protective coating. It is difficult to describe a worn out MB roof, but indications of advanced age include severe, deep surface cracking; extensive granule loss; and rusted flashing.

Flashing: a topic unto itself

Most leaks occur at flashing. In my opinion, detailing flashing is more craft than science, requiring careful thought, design and installation. Good flashing details must be able to move with the stresses, hold up to the forces of nature and remain watertight — a difficult task even under perfect design and conditions. As an inspector, you will come across what seems like an endless number of flashing situations and potential errors — more than I can begin to include here. Next month, I’ll cover some of the basic flashing issues related to MB roofs.

What to look for at transition areas

On any roof, transition areas deserve careful inspection. These areas, where different materials come in contact with each other — often where low slopes change to steep slopes — are stress points in the roof system.

The seals, both roofing materials and/or any flashing, should be thoroughly inspected for cracking, splitting, open gaps or deterioration that could have an effect on the waterproof roof system. Installation errors are common in transition tie-in areas.

For example, there should not be metal flashing in the upward transition to a steep roof. The field membrane should be turned up a minimum of 18" on the steep slope and, at minimum, a 30" strip of base flashing should extend up and under the steep roof covering for a minimal of 24" and out over the low-sloped field membrane for 6".

Using metal flashing at upward shingle transitions is technically incorrect because the sheet metal will expand in temperature swings, tearing or splitting the membrane.

On the contrary, when a flat roof is at the higher elevation and the steep roof comes up to it, there must be a perimeter-edge flashing detail that extends down a minimum of 4" over the adjoining steep roofing, i.e., shingles.

However, in this case, the face of the flashing should not be nailed. The shingles should extend under the face of the flashing and any nails should be covered with the perimeter-edge flashing. As with all perimeter-edge flashing, the face should be anchored down with cleats to resist the wind forces. However, I never see that detail on homes.

Photo above and illustration below: A deteriorated roof. The roof pictured above is patched where it leaked. It leaked because the transition between the MB roof and the shingle roof was done incorrectly. The illustration below depicts the
correct way to make this type of transition.


Illustration courtesy of Certainteed.

Parapet walls

Technically, parapet walls are not part of the roof system; however, they can compromise its waterproofing function. They should be inspected for deteriorating or improperly installed copings (the sheet metal or terra cotta tile on top of the wall). The copings should be secure, overlapped approximately 3" and sealed watertight. Inspect exposed masonry for cracked or open mortar joints, and inspect walls covered in wood, vinyl, stucco or other materials for deterioration or anything that could allow water to penetrate them.

Evaluate drainage, check for debris

All flat-roof inspections should include an evaluation of the drainage. In most cases, poor roof drainage is due to poor design or roof deflection. The weight of ponding water can cause damage to the structure and the roofing system. Many older commercial roofs are notorious for standing water and some old-timers argue it is a good thing because it keeps the roof cooler during the summer. In truth, the potential harm far outweighs any conceived benefit. What’s more, building codes and all manufacturers of roofing require positive drainage. Modified Bitumen membranes are especially not suited for ponding conditions.

As you walk around inspecting the roof, look for signs of “ponding.” If it is dry, low areas usually are filled with dirt. The dirt can have a degrading effect on the membrane. You may even find vegetation growing in dirt-filled sunken areas, with its roots growing into laps of the membrane.

Inspect the drains and/or scuppers. Drains should be clean and have a minimum 3" opening with a grate to prevent clogging. Ponding is defined as water that remains on the roof for more than 48 hours after precipitation ends. Poor drainage or severe ponding should always be reported for correction, but incidental water on a flat roof is generally considered acceptable.

Poor drainage can be corrected by adding drains in low spots, pumps or tapered insulation under the membrane. If the roof is totally enclosed by walls, pay careful attention to overflow drains. If the primary drains were to clog, there must be a way for the water to drain off and prevent overloading of the roof structure.

Note excessive debris on the roof. It can impede drainage, clog drains and/or gutters and sharp debris can puncture the roof.

The scuppers (the square or rectangular holes cut through a wall to allow water to run off) receive a lot of water flow and are another potential place to have a leak. They and any associated flashing should be carefully evaluated; even ones made of copper develop pinholes.


Photo above: Modified Bitumen membranes are especially not suited for ponding conditions.

Checking seals

Joints between the membrane and flashing or some other waterproof material are sealed with flexible waterproof materials. Sealants also are used to cut off water along the top of base flashing or at the counter-flashing reglet. Accept-able uses of sealant include around sky-light caps and plumbing collars, and on coping joints, etc. However, sealants, caulks and/or mastics are not flashing substitutes. Overuse is an indication of poor workmanship. Sealants should be carefully evaluated for drying, cracking and/or deterioration, keeping in mind failure can be difficult to detect.

No Leaks: Then what?

More often than not, an MB roof installation will have a reportable issue. Some are significant; others are debatable, especially if it has withstood the test of time without leaking. It is difficult to know where to draw the line, but hopefully, this article will help. In closing, I suggest always advising clients that flat roofing requires annual inspection and maintenance.

Watch for part two of this article in the March Reporter. It will focus on flashings for MB roof installations and some general issues with this type of roof.

Inspecting Roofs: Steep vs. Low-Slope

When inspecting a low-slope roof, it’s important to remember that unlike steep roofing, which relies on water shedding to keep water out, low-slope roofing must be completely waterproof. We’ve all seen deteriorated shingle roofs that continue to shed water because the slope and overlapped shingles keep the water moving down and off the roof. In contrast, on a low-sloped roof, the smallest hole or slightest installation error can result in significant leaking. A small hole on a steep roof might mean a ceiling stain or a few drips; a small hole on a flat roof could cause a water fall, collapsed ceilings, ruined floors, etc. 


Beyond the Scope

There are a variety of devices (outside the scope of a home inspection) that can be employed to confirm or find leaks. Moisture meters work well and are used routinely by many inspectors. Infrared photography, gaining popularity among home inspectors, can detect the signatures of wet insulation. Both devices are only as good as the trained user.


All photos in this article are by John Cranor.