I hope you can help me because no one else can. I have a double detached garage with siding. The problem is that the cement pad is all one piece extending to the sidewalk along side of the garage. The rain runs from the sidewalk right into the garage & is rotting the wood in the garage. The siding of the garage is about 1/2″ above the sidewalk. I tried using blow-in foam insulation but there is nothing for it to stick to.
Many garages are built with the walls directly on top of the poured concrete slab, but it is unusual for the slab to extend out to become a full sidewalk. This will inherently cause a problem with moisture being wicked up from the concrete by the wooden framing and siding on this side of the garage. This appears to be exactly what is happening in your situation. There may be several things that you can do to minimize this to try to prevent further rotting of your garage walls.
I am surprised that blow-in foam insulation has not provided some relief for the water intrusion. This is an excellent product that is waterproof and will often help to seal large gaps, such as in your case. If installed properly, it would not be a permanent solution, but should have helped, somewhat. Before going further, the remaining foam should be removed by cutting with a utility knife to clean out the area under the garage wall. The bottom plate, normally 2 X 4, may be quite rotten and should be probed with a screwdriver or awl to see the extent of the moisture damage. If it is extensive, the plate will have to be cut out before proceeding. In advanced cases, the moisture damage may have travelled up to rot the bottom of the wall studs. In this situation, damaged studs, or sections of studs may have to be cut out and replaced as well.
To accommodate further repairs, the bottom piece or two of wood siding on the exterior will have to be removed on the problematic side of the garage. Once this is complete, the bottom wall plate can be cut out with a reciprocating saw and a new pressure treated bottom plate installed by toenailing to the existing studs. The pressure treated wood will resist moisture damage and rot, even if the water leakage is not completely eliminated. It will also prevent “wicking” up of water from the concrete, which may damage the wall studs, as well. The new plate can be attached to the concrete slab with concrete screws or shot in with a power nailer.
Once the bottom wall plate is replaced, a metal flashing may be installed on the exterior of the wall to prevent further seepage of water. This should be a 90 degree right angle flashing, normally a 4 X 2, or larger. This flashing should have the narrow edge placed along the concrete and be caulked along the underside, to seal it to the slab. The wider edge should be nailed directly to the wall studs above the new bottom plate. Once in place, the flashing should be covered by reinstalling the previously removed siding. If this siding is beyond salvaging, new siding may be installed and painted to finish the job. Installing the flashing behind the siding will prevent moisture and melted snow from wicking up the siding and damaging the wall, as well as minimizing seepage underneath. This flashing may lift from the concrete over time and may require re-caulking, periodically.
One thing not yet discussed is the presence of excess water on the sidewalk, in the first place. The source of the water may be minimized by better grading and eavestroughing maintenance. If the soil around the sidewalk has settled or is sloping toward the garage, this may be levelled to help channel water away. If the eavestroughs are older, damaged or leaking they can be repaired to help eliminate standing water on the sidewalk. The downspouts may be relocated and extended to the driveway or back lane to reroute the water away from the garage. This is assuming that proper eavestroughs and downspouts are installed, which are often missing on older garages. Installation and regular maintenance will be the biggest factor in eliminating seepage. Regular removal of snow from the sidewalk in the winter will also help prevent damage.
Most moisture intrusion into buildings, including garages, can be prevented or minimized with attention to drainage of rainwater and melted snow. A common sense approach should be used to determine where the source of the water is and how it can be managed. Once damage is done to the wooden structure, as in your case, the repairs to the wall will have to completed, but better drainage water management can still be accomplished.