In Part 1 of this series about the conversion of my detached garage into an accessory dwelling unit (ADU), I discussed the structural issues relating to the roof of the garage. In Part 2, I discussed the structural issues relating to the walls of the garage and how I dealt with them. In this Part 3, I will discuss structural issues I had with the foundation of the structure and how I fixed them.
There was no existing structural drawings for the garage foundation so I had to figure out the existing design by field verifying it. From my visual inspection, I determined that the foundation for the existing garage is slab on grade. However, I still needed to figure out the thickness of the slab and whether it can support the exterior walls of the ADU. The first thing I did was I dug out a small area of dirt next to the exterior face of the slab to see if I can get to the bottom of the slab. I was able to dig down to around 12″ before I found the slab’s bottom. This was good news because it meant that there was enough concrete foundation support for the exterior walls and I can at least think about keeping a portion of the existing slab. My big worry was that since this is just a garage, it might have a very thin (4″ or so) slab for its entirety. If this were the case, I would not be able to reuse this slab to support the walls of the ADU. What I would need to do then is go through a very expensive process of shoring up the existing garage to pour a new footing all the way around. At that point I would probably have torn down the whole existing structure to build a brand-new ADU.
There were a couple of complications with reusing the entire existing slab for the ADU. First, there were many large cracks propagating throughout the slab (see picture below). The reason for this was that the existing slab did not have regular steel reinforcement in it. Some of the smaller cracks were due to concrete shrinkage but the larger cracks were most likely due to stresses created by soil movement underneath the slab. These large cracks are not only bad structurally, but they also allow space for water or animals to go through them. The second issue with reusing the slab is that by code there needs to be a vapor barrier between the slab and the soil beneath for occupied space. Garages typically don’t have vapor barriers below the slab because by definition it is not an occupied space. To complete my investigation, I broke off a part of the slab to see if there is a vapor barrier underneath it. I found that there was no vapor barrier beneath the slab so the interior part of the slab cannot be saved even if it was in perfect condition.
My structural investigation determined that the existing slab cannot be reused since it had many problematic cracks and it did not have a vapor barrier beneath it. However, the exterior edge of the existing slab can be kept since it was generally in good condition and was thick enough to support the exterior walls of the ADU. I created a structural detail for the Contractor to show that the interior part of the slab can be saw-cut out and replaced with a new reinforced interior slab. The detail also showed that the existing exterior slab perimeter can be connected together with the new interior slab by epoxy dowels to create a continuous slab on grade foundation for the ADU.
As you can see, there were quite a few structural issues even for a small ADU since this was an existing structure. However, if you have some knowledge of structural engineering, you can easily work through them and create additional value for your home.
If you are looking to get in touch with a Contractor to help you with your ADU project, reach out to me at [email protected] and I can give you recommendations for some excellent Contractors.Disclaimer: The structural issues identified in this post are specific to my ADU and may or may not apply to your specific situation. Please use your own engineering judgment or consult with a civil/structural engineer for your specific situation.