The Need for Membrane Flashing Under a Threshold

Posted by in Siding, Water Sources | August 22, 2016

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.

Figure 1 Moisture staining emanates from top of wall where rain runoff is infiltrating under threshold.

Figure 1 Moisture staining emanates from top of wall where rain runoff is infiltrating under threshold.

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).

Figure 2 Band joist exhibits severe decay and discoloration from repeated wetting.

Figure 2 Band joist exhibits severe decay and discoloration from repeated wetting.

The solution: remove the door, install flexible membrane flashing and reset the door.

Easier said than done now.

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