T-Joints occur on modified bitumen roof systems at the interface of the end laps to the side laps. This is a critical juncture in the roof system.

As the use of modified bitumen roof systems has increased over the past 20 years, there have been significant changes to material technology and application procedures. Most of the changes have occurred from experiences gained in the field over time. Modifications have been applied to correct concerns and problem areas. In some instances, minor application changes were all that were required to correct troublesome points.

One application modification that has proved to be successful has been in the installation of membrane T-joints. T-Joints occur on modified bitumen roof systems at the interface of the end laps to the side laps. This is a critical juncture in the roof system because it is where three layers of material are adjoined as one. In a typical modified bitumen system application, this condition presents itself at all end laps. The most prominent concern at these points is that if they are not properly adhered, moisture can enter the system.

Most modified bitumen systems are constructed in two layers: a smooth-surfaced base sheet and a granulated cap sheet. Proper seam and T-joint application is required at both (or all) layers of the sheets. In modified bitumen systems, all seams are overlapped and staggered so that no adjoining sheet seams align. This can be accomplished by offsetting the end laps of the adjoining sheets a minimum of 3 feet. Offset the top sheet by staggering the laps between the laps of the bottom sheet. The sheets are overlapped at the side laps a minimum of 3 inches.

Application procedures are similar with each type of attachment method: torch applied, hot bitumen or cold adhesive. However, there may be variations depending on the manufacturer’s requirements.

The following are some application techniques that could add to the successful completion of the T-joints and the roof system. The first recommendation is to limit the number of T-joints in the system. Vulnerable areas can be controlled if they are minimized. This may require some pre-application planning. The easiest way to limit the number of T-joints is through the application of full rolls of modified bitumen. Depending on the length of the roof area, this may reduce the need for a number of T-joints. This may not be possible, however, when cutting the rolls is required to relax the sheets prior to application.

Industry studies have indicated that cutting the end laps at the overlapping side (outer) edge on a radius — or a “dog-ear angle” — improves the adhesion capabilities of the sheets at the T-joints. This is primarily due to the fact that the cuts minimize the material thickness at these junctures. Without cuts, the adjoining of the three sheets at these points produces a thick juncture, which could be in excess of 480 mils. Thicker junctures at isolated points in the seam could contribute to wrinkles or open laps at unadhered locations, allowing for the free flow of moisture into the system. The cuts should be completed on a radius of 3 inches at the side laps and 3 inches at the end laps. This provides uniform thickness throughout the entire side lap and reduces the possibility of openings at these points.

Adhesion at these junctures is critical. To ensure proper adhesion, apply the membrane sheets in accordance with the manufacturer’s requirements for torch application, hot bitumen application or cold-adhesive application. In addition, apply pressure directly to the end laps at all edges of the T-joints. Pressure should be asserted at these points directly after installation and can be applied with a trowel or broom. On hot bitumen applications, a heated trowel may be required to increase adhesion capacity.

These are minor modifications that could add to the success of the modified bitumen roof system and thereby decrease the number of callbacks. And best of all, they are not time consuming practices, so they will have little effect on the project’s bottom line.