Fixing moisture issues for detached garage/studio
I live in a century home on Long Island, NY that had a detached garage in the backyard. At some point past owners finished it up and converted it to a studio with carpeted floors. The studio is built on a concrete slab (not sure if theres any footings). The studio is right at the property line and so theres concrete on front, back, and right side while there's soil on the left. There's 4x4x4 PT lumber as the sill/bottom plate for the walls to stand on and I believe it is sitting square at the edge of the slab. The wall is then constructed such that sheathing is nailed to the studs down to the bottom plate, then foam board, and finally vinyl siding. All of this directly at grade so no concrete wall for the bottom plate to sit on. The stud bays have fiberglass insulation between them. Outside, there's a few conduits to run electricity to the shed.
The first problem is that one of the past owners put up a garden bed on the soil side. This meant that the soil side siding was actually below dirt. Over time from moisture it seems the bottom parts of the sheathing have rotted and have given way to termites and carpenter ants.
The second problem is that the monolithic concrete slab in the front of the studio cracked thereby creating a slight negative slope towards the structure. During heavy rainfalls the water pools and thus causing the same set of problems.
I was hoping to just use this space as a home office so trying to limit drastic changes to the structure while still being able to repair and prevent further damage.
I've called an exterminator who will drill holes in the concrete + trench the soil and do termite treatment. After waiting 48 hours we'll resume repair work with the following plan:
The Plan (Soil Side):
- Snap a level line 9.25” above the final dirt grade, removing/cutting away the rotted lower siding, foam, and sheathing.
- Install a 3/4" PVC trim sheet in place of the cut out sheathing. Top half screwed to the studs and bottom to the PT lumber. The sheet will go down to grade.
- Use liquid flashing like Flashmate to flash the seam between the concrete slab and PVC trim
- Slip aluminum z-flashing strip over the trim and under/behind the older sheathing, overhanging the PVC trim to shed water
- On the "older" wall right above the Z flashing strip, install new vinyl starter strip and reuse vinyl siding.
- On the inside if any insulation was cut, place foam board in the stud bay and fill with expanding foam. Will look for insect resistant products here.
- On the outside perimeter, dig a 1' wide and 4" deep trench that slopes away. Plat heavy duty landscape fabric and fill with river rocks to prevent muddy splash back and help with drainage during rains.
The Plan (Concrete Sides):
Follow the same plan as above however we'll cut the PVC trim just enough to leave about a 1/4" gap between it and the concrete slab. For that gap we'll use Sikaflex construction sealant.
For the corners we'll just overlap the trims together instead of doing a miter cut
Any glaring red flags with this plan?
Thanks in advance!
