Troubleshooting - onsite

Masonry Construction, Dec, 2002 by Norbert V. Krogstad

Terminating through-wall flashing

Q How should you terminate masonry through-wall flashing at door jambs and at first-floor windows that extends to the base of the masonry wall and that's recessed from the face of the wall?

I know that end dams are recommended; however, how can end dams be installed in these cases when the brick masonry is typically returned beyond the face of the window at the jambs? Should the end dam be placed in the first mortar joint in front of the window or door? In this case, what will happen to any leakage that may occur in the space between the end dam and the face of the window or door?

A In locations where the doors or windows are recessed, I typically recommend using a three-sided end dam. The three-sided upturned leg of the end dam should return out beyond the exterior face of the window or door. Water that reaches the flashing in this location cannot leak into the interior; it will be diverted to the exterior in front of the perimeter sealant joint for the window or door.

The three-sided end dam should extend far enough beyond the face of the window or door frame so that the perimeter sealant joint can be bonded to the exterior face of the flashing end dam. It is important that this flashing end dam be installed tightly to the face of the masonry so that there is not a significant gap that would be unsightly and make the installation of the perimeter sealant joint difficult.

Causes of diagonal cracks

Q I am making repairs to a building that was constructed in the early 1950s. The masonry walls have diagonal cracks extending up from the ends of the steel lintels over the windows. These cracks occur on both sides of the lintels and are worse at the top floor of the building.

Do you have any ideas as to what is causing these cracks?

A Diagonal cracks that extend up from the ends of the lintel are often caused by corrosion of the steel lintel. The corrosion product of steel will occupy 10 to 20 times as much space as the steel itself. This expansion generates tremendous pressure when confined and is capable of bending the steel angles, breaking apart the brick masonry, or lifting the brick.

Corrosion that builds up on the top surface of the steel lintel lifts the masonry. Diagonal cracks form at the ends of the lintel because this area is the weakest plane. The cracks are more likely to occur when there are only a few feet of masonry above the head of the windows because there is less weight to resist the expansion pressure generated by the corroding steel.

Where there is greater confining pressure, such as at lower levels on a building that does not contain horizontal expansion joints, the pressure generated by the corroding steel causes the portion of the angle above the window to deflect downward and the brick at the jambs to crush.

To repair this problem, it is often necessary to replace the lintel with a new one. Typically, three to four courses of the exterior brick masonry wythe will need to be removed in order to perform this repair.

Temporary support must be provided for the masonry immediately above the angle. This support is often obtained by using steel rods at close spacing that are drilled into the backup to serve as shear pins or by using a shore to support the masonry from a lower floor or the ground. You should work with a structural engineer to develop an effective method of shoring for your particular set of conditions.

I recommend replacing the lintels with new, galvanized steel lintels; however, painted mild steel lintels may be used. If a proper corrosion-resistant paint coating is applied, the finished appearance of the lintel can be used to match the window system. A portion of the lintel is often exposed above the window and will match the surrounding materials better if painted. Galvanized angles can also be painted; however, special surface preparation is required to develop proper paint adhesion.

After the lintel is replaced, flashing must be installed above it to protect the angle from water and to prevent water leakage problems in the wall system below. The flashing should extend beyond the ends of the lintel and should have upturned ends--called end dams--to prevent water that reaches this flashing from flowing off the ends and remaining within the wall.

Mortar or not?

Q Some experts recommend that brick units be placed dry on flashing--such as above shelf angles--rather than setting them in mortar.

If the brick are not set in mortar, what holds the units in place? Won't they slide out away from the wall?

A I usually recommend that brick units be laid on flashing without any mortar. The mortar on the surface of the flashing may interrupt the flow of water out of weep holes. When units are installed on flashing without mortar beneath them, there is no mortar to interrupt the flow of water out of the weep holes, and water can flow underneath the brick.

Placing mortar underneath the brick unit does not prevent it from sliding on the flashing. Mortar will not bond well to the flashing, and with most flashings, there is no bond between the flashing and the shelf angle or foundation.