The Need for Membrane Flashing Under a Threshold
Whether a manufactured stone with multiple joints or a solid board is used under the threshold of a door (featured photo), a flexible membrane flashing is necessary to stop the infiltration of runoff.
On the inside, runoff infiltration through a threshold usually takes on an appearance of moisture staining that emanates from the top of the wall (Figure 1). The heavy staining in Figure 1 indicates that the infiltration has been present and progressing for years. Additional evidence of a present and progressing condition is the severe decay of the band joist (Figure 2).
The solution: remove the door, install flexible membrane flashing and reset the door.
Easier said than done now.