Technical Details: Installing T-Joints on Modified Bitumen Roof Systems
As the use of modified bitumen roof systems has
increased over the past 20 years there have been significant changes to
material technology and material application procedures.
As the use of modified bitumen roof systems has increased over the past 20 years there have been significant changes to material technology and material 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 for it is at this point that three layers of material are adjoined as one. In a typical modified bitumen system application this condition presents itself at several locations. The most prominent concern at these points is if they are not properly adhered moisture can enter the system.
Most modified bitumen roof systems are constructed of two layers of felt: a smooth surfaced base sheet and a granulated cap sheet. Proper seam and T-joint application is required at both (or all) layers of felt. In modified bitumen systems all seams are overlapped and staggered so that no seams align. This can be accomplished by offsetting the end laps a minimum of three feet. Offset the top layer of felt by staggering the laps between the laps of the bottom layer. The sheets are overlapped at the side laps and end laps a minimum of three inches. Proper application procedures vary with the type of attachment methods: (torch applied, hot bitumen, or cold adhesive) and by manufacturers requirements. However, there are two application techniques that could add to the successful completion of the T-joints and the roof system.
Industry studies have indicated that cutting the end laps at the overlapping side (outer) edge on a radius of “dog ear angle” improves the adhesion capabilities at the T-joints. This is primarily due to the fact that these cuts minimize the material thickness at these junctures. Without these cuts the adjoinment 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 and open laps at unadhered locations allowing for the free flow of moisture into the system. The cuts should be completed on a radius of three inches at the side laps and three inches at the end laps. This provides a uniform thickness throughout the entire side lap.
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 and cold adhesive application. In addition, apply pressure directly to the end laps and 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.
As the use of modified bitumen roof systems has increased over the past 20 years there have been significant changes to material technology and material 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 for it is at this point that three layers of material are adjoined as one. In a typical modified bitumen system application this condition presents itself at several locations. The most prominent concern at these points is if they are not properly adhered moisture can enter the system.
Most modified bitumen roof systems are constructed of two layers of felt: a smooth surfaced base sheet and a granulated cap sheet. Proper seam and T-joint application is required at both (or all) layers of felt. In modified bitumen systems all seams are overlapped and staggered so that no seams align. This can be accomplished by offsetting the end laps a minimum of three feet. Offset the top layer of felt by staggering the laps between the laps of the bottom layer. The sheets are overlapped at the side laps and end laps a minimum of three inches. Proper application procedures vary with the type of attachment methods: (torch applied, hot bitumen, or cold adhesive) and by manufacturers requirements. However, there are two application techniques that could add to the successful completion of the T-joints and the roof system.
Industry studies have indicated that cutting the end laps at the overlapping side (outer) edge on a radius of “dog ear angle” improves the adhesion capabilities at the T-joints. This is primarily due to the fact that these cuts minimize the material thickness at these junctures. Without these cuts the adjoinment 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 and open laps at unadhered locations allowing for the free flow of moisture into the system. The cuts should be completed on a radius of three inches at the side laps and three inches at the end laps. This provides a uniform thickness throughout the entire side lap.
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 and cold adhesive application. In addition, apply pressure directly to the end laps and 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.
