Trained Eye

 
 
 

Raising Garage

Question:

Good morning.

I read you column almost every Sunday. I have a question for you about a sinking garage. Our house, and probably the garage, was built in 1941. For some reason, they decided to build the garage floor out of wood. As you can imagine, it has rotted over the years and now is almost impossible to park on. The walls and roof are in great shape but because of the floor, the garage's bottom plates have probably sunk about 4-6 inches.

Now that it's one of the low parts of the yard, rain tends to flood it if we get a lot of precipitation. I'd like to grade the approach so that the water is directed away from the garage but have been told by concrete and asphalt guys that I should raise the garage before I do so.

Where on earth do you find someone who raises garages?

Thanks very much.

Answer:

The problem of moisture intrusion, moisture damage and rot is a very commonly seen concern during inspections on older garages. This is not exclusive to wooden or gravel floored structures, but regularly occurs on garages built on concrete slabs, as well. The suggestion you have been given may be the only way to salvage your old building, but proper repairs after lifting are very important.

Lifting older structures that are not secured to a proper foundation can be quite simple, but must be done with extreme caution. Garages are typically the simplest type of construction. A small rectangular building, with walls constructed of 2 X 4s and sheathed with wood or plywood. The roof structure is usually open rafters or trusses covered by roof sheathing and shingles. Because of the fact that the interior walls are open, in most older garages, access to the wall studs is readily available. Lifting the structure may require the use of several beams bolted to the studs, but that can be quickly accomplished if there is no interior sheathing. Several heavy-duty jacks and blocking will also be required for lifting the garage, but many foundation, concrete or general contractors will have access to such equipment.

Once the garage is lifted out of the dirt, it must be temporarily supported to allow for pouring of the new pad, grade beam and repairs before replacing it. This may be the tricky part, as good support will prevent a potentially serious accident if the garage falls of the jacks or supports. While the building is temporarily suspended off the ground, the bottom of the walls can be inspected to see the extent of the rot. This may vary from only a damaged bottom plate to major rot, well up into the studs and wall sheathing, from sitting below grade for many years. Whatever the situation, the damaged wood will have to be cut away before further repairs.

At this point there are two typical ways to proceed with the new garage foundation. The simplest way is to form and pour a solid concrete slab, with a thickened perimeter area and reinforced with steel re-bar. The second method is to excavate a trench around the perimeter and form and pour a steel reinforced grade beam. This mini foundation wall may be built on top of a concrete footing or on concrete piers. Once the grade beam is complete and the forms stripped, the concrete garage slab may be poured inside, again reinforced with steel.

The second method is superior, especially with poured concrete piers, but may be much more costly. The advantage of this method is that the grade beam will extend well above grade and prevent future flooding of the garage, even if the grading around is not substantially improved. The other benefit is that less repairs or modifications to the damaged garage walls may be required. Because of the height of the new grade beam, the bottom of the garage walls may be trimmed higher up, easily removing any damaged wood. A simple, single new pressure treated bottom plate may be the only modifications needed to the walls before lowering back on to the new grade beam.

If the old walls are to be placed on top of a new flat slab, only a few inches above grade, there may be additional studs and sheathing required as well as the new bottom wall plate. The rot may have penetrated the bottom of the studs and outside wall enough that a significant amount of wood will have to be removed and replaced. This additional material and workmanship cost may offset a fair amount of the cost saving of this slab-only method.

Whichever method is chosen, ensure that the contractors involved are well qualified and experienced in this type of repair. It should not be that difficult to find a good general contractor that works in conjunction with concrete contractors that will be willing to tackle this job. Always ask for several references, and ensure that the proper building permits are taken out so that the work is properly inspected. Also, modifications to outbuildings, such as garages, have to conform to current zoning requirements, which may be different than at the time or original construction. Check with the local municipal building office before commencing any work.

 

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