Dampness at bottom of exterior walls

[Kent Verge]

My current rehab has an addition with a concrete foundation at ground level. The exterior walls have moisture at the bottom that led to mildew on the bottom part of the sheet rock. I removed the bottom 2' of sheet rock throughout and removed the damp insulation. There were also some nasty cockroach nests inside along with some geckos (they eat the roaches).

I know that there is an issue with drainage/grading outside that I need to address, but since the addition is at ground level, should I also add Z-flashing under the siding? Is there anything else I need to do about the roaches (other than a chemical barrier), or will resolving the moisture issue keep them out?

The cottage in the back is also on a slab at ground level, so it probably has the same problems.

This room is also where the previous owners fed the feral cats -- we caught 29 of them. They had sprayed on the walls, so I hopefully removing the sheet rock will help with the urine smell too.

[Brian Rossiter]

Building codes require the top of the foundation to be at least 6 inches above grade for this very reason.  The best thing to do is to regrade the soils around the house so that the top of concrete is at least 6 inches above the new grade and slope it away from the house at about a 2 per cent grade (2 to 3 inches every 10 feet).  Make sure downspouts drain far away and don't put any plant drip lines near the foundation.  For the love of God, please don't build a mound around the house. 

If you can't achieve the slope away from the building because the site is dead level with the street drainage, you can build a retention pond or drainage swale at the edge of the property.  Consulting a site civil engineer would be invaluable in this case.

I'm not well-versed on pest removal, but once you have the wall opened up and have removed any damaged insulation and wood, an exterminator should be able to help get rid of the roaches and geckos if you're not sure about doing it yourself.  After all the problems are fixed, you shouldn't have any more pests in the walls (that's "shouldn't," not "won't").