March, 2010
Inspection News and Views from the American Society of Home Inspectors

Flat-Roof Inspection, with a Focus on Modified Bitumen


Although I practically grew up installing and repairing roofs, I have to admit I still find it challenging to accurately determine and report the cause of a leak in a low-slopped roof. In the February ASHI Reporter, I focused on Modified Bitumen (MB), which, when installed correctly, is a tough, tear-resistant roofing material for a flat roof.

In discussing what I’ve learned about inspecting MB membrane roofs, I covered two frequently used types of polymer blends, the components of a residential roof system, and I described some common membrane roof issues such as transition areas, laps, blisters, wrinkles, movement and damage, promising to focus on flashing and several additional issues this month.


Most leaks occur at flashing. In my opinion, detailing flashing is more craft than science, requiring careful thought, design and installation. To function as intended, 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.

There is an endless number of flashing situations and/or potential errors that you may come across as an inspector, more than can be covered here. In fact, I have never found a book that fully covers the subject of flashing. This article will attempt to cover only the basics or the more commonly observed issues on residential flat roofs with MB membranes.

In low-sloped roofing, flashing can be metal or membrane. There should be flashing anywhere the field of the roof membrane is interrupted or terminated. Remember, the field membrane should turn up on vertical interruptions such as walls, curbs, etc.; then have a base flashing over it. The base flashing should always be of the same type material as the field membrane.

Metal should not be part of the base flashing. Metal should be used only on flanges of roof penetrations, around the perimeter, water shedding counter flashing or sometimes to protect the roof from mechanical damage and for decorative reasons.

The base flashing should turn up on the vertical interruption a minimum of 8". Eight inches is an industry standard and conforms to most MB membrane manufacturer’s specifications. Most manufacturers also have maximum heights for vertical flashing: 24" is common. The base flashing should be lapped a minimum of 3" at roll widths.

Often, you see base flashing installed in long lengths, which eliminates laps, but there is a greater risk of poor adhesion, so watch for that. Both inside and outside corners on walls should have a reinforcing ply of membrane; often, the reinforced plies in corners have unusual shapes, which are sometimes referred to as bow ties and footballs.

The top edge of the base flashing must be sealed water-tight and mechanically fastened, with the fasteners spaced a minimum of 8" apart on flashing up to 12" above roofline and 4" apart on flashing 12" up to 24" above the roofline. The base flashing and fasteners should have a counter flashing or coping that covers the top edge and fasteners. The counter flashing should extend below the fasteners and top edge of the base flashing a minimum of 4" and have a drip lip.

In some situations, the skylight or mechanical equipment itself may be used to mechanically fasten the top edge of the base flashing.

Even though frequently used to repair a roof or to add life to an aging one, roof cement and fabric is not considered an adequate substitute for base or counter flashing.

On homes, the counter flashing is often the siding or the steep roofing on an adjacent sloped roof. Base flashing membrane should never be installed directly over the steep roofing or the siding.

Most siding is water-resistant not waterproof and cannot be integrated into the roof system. Base flashing should never bridge over the corners; it should always be adhered tight to the substrate and cant strip. Bridged-over flashing is prone to damage and considered a defect.

Vertical angles

One important specification that often is not followed on an MB roof is that 90-degree vertical angles in the membrane should be avoided. Ninety-degree bends are prone to cracking and leaking. Any vertical angle more than 2" high must have a “cant strip,” which is a triangular strip usually made of Perlite, fiberboard or wood. The cant strip allows the base flashing membrane to make a 45-degree instead of a sharp 90-degree turn. The lack of a cant strip or the presence of a 90-degree vertical turn on an MB roof is a reportable defect.

Metal flashings are used around the perimeter as counter flashings, as copings, etc. Perimeter-edge flashing is often referred to as gravel stop. Metal flashings should be a heavy gauge (minimum 16 oz. copper, 24-gauge galvanize or .032 aluminum). The flange on perimeter-edge flashing (gravel stop) must be a minimum of 3.5" wide. The flange on other metal penetrations, such as a plumbing collar, must be a minimum of 4" wide. All metal flashing flanges must be stripped off with membrane, and at minimum the strip should be 8" inches wide.

Photo: At only slightly more than 2” high, this base flashing was improperly installed.

Photo: This is a defect because the flashing is not adhered to the brick.

Photo: The membrane has separated from the perimeter flashing. The flashing was not primed, possibly a contributing factor to the failure.

Failure points

In my experience, one of the most common defects and failure points on an MB roof occurs at the perimeter-edge flashing. Usually, the membrane loses its adhesion to the metal, often because the metal-edge flashing was not cleaned and primed with an asphalt primer as required. All metal flashings must be secured (mechanically fastened down), cleaned and asphalt-primed so the membrane flashing will stick to it. When not primed, it holds for a while, but rarely functions long term.

In my experience, the second most common defect on MB roofs, especially on homes, is the failure to strip metal flashings. The perimeter flashing, as well as any flange on installed metal flashing, should be sandwiched between the roof field membrane and a top cap sheet or a strip of membrane (often referred to as being stripped off by roofers). Commonly, the field membrane is improperly sealed over the perimeter-edge flashing, as well as other flanges on installed metal flashings. Sometimes, if the flashing was primed, this works and sometimes not, but technically it’s incorrect if not sandwiched and stripped off.

Always carefully check the seal at the perimeter flashing. Be careful with how much you pull up on the membrane because if it’s not primed, it will separate easily from the flashing. You should be able to visually determine whether or not it’s sealed. Although difficult to determine, it is important to know that the perimeter flashing must be adequately secured to resist wind forces.

From my experience, these are the basic or the more frequently observed flashing issues you’ll encounter when inspecting residential flat roofs with MB membranes.

More issues

Last month, I listed laps, blisters, wrinkles, movement and damage as membrane defects to watch for when inspecting an MB roof. In addition to problems with the membrane, I’ve found the following to be frequently encountered issues with a residential-membrane roof:

Mechanical equipment
should never sit directly on the roof membrane. The vibration of the heavy equipment slowly cuts into the membrane, leading to a leak. Although the equipment often is found sitting on two 4"x4"s or similar material, it should sit on a flashed curb or on a steel frame with angle iron legs penetrating the roof.

Guard railing
is common on homes where the flat roof also is used as a sun deck.

Leaks often occur around the guardrail posts. Rarely are posts properly flashed, and the stress applied to the railing makes the post connections prone to leaking. Sometimes the post are split, damaged and not painted, all compromising the waterproof joint. The membrane used around the base of the post often makes that prohibited 90-degree turn on MB roofing.

Recently, I inspected a newly installed MB roof that had 4"x 4" guardrail posts. There were leaks from eight of the 10 posts. Railing posts or vertical supports should be set in a metal-sleeve flashing detail with the flange stripped off. Always closely inspect guardrail posts and the ceiling below them.

Pitch pans (or pitch pockets) are metal fabricated flashing components that are used around irregularly shaped roof penetrations such as angle iron or sometimes around guard railing posts. Historically, pitch pans were filled with hot pitch or hot asphalt, creating a watertight seal. More common today, the pans are filled with pitch pan fillers (a cold formula designed for that use) or with roof cement. Carefully inspect pitch pans for cracking, separation, voids, etc. They should be sealed and should not hold water.

such as plumbing or mechanical system vents should be at least 18" from walls, curbs, edges or other penetrations so that there is enough space for proper flashing.

often open out to a flat roof on a home. The door threshold transition is an area prone to leaking. Usually, the door threshold is not wrapped as well as it could be. Often, the roofing membrane is just trimmed off around the threshold and caulked, and the caulking is not watertight. Always closely inspect doorways, as well as the ceilings below.

A General Guide

This two-part article is intended as a general guide for home inspectors. Please note: Specifications and details vary from manufacturer to manufacturer and for commercial use.

Although it is not an exhaustive reference, hopefully it will help home inspectors meet the challenge of inspecting low-slope and flat roofs, specifically those with a Modified Bitumen membrane.

All photos in this article are by John Cranor.