The existing detached garage that I used to build my accessory dwelling unit (ADU, or commonly known as in-laws unit) was constructed in the 1940s. Since the garage is advanced in its age, it has its share of structural issues that need to be resolved in order to turn it into a code complying livable space. In this post and future posts, I will break down some of the structural issues I encountered in this ADU project and how I fixed them. This post (Part 1) will focus on the roof of the structure. Future posts will focus on other structural elements such as the walls and foundation of the ADU.
The roof of the existing garage was in an overall good shape at the start of the construction. The existing roof consists of composition shingle on top of 1/2″ CDX plywood and supported by wood joists spaced at 2 feet on center. At the perimeter, the roof has a gutter and downspout system that directs rain water from the downspout into a system of buried drain pipes that empty into the public drainage system at the front of the house.
If the existing structure that you are considering for your ADU does not have a gutter/downsprout system, I highly recommend you put one in for your project. You do not want rain water to collect around the edges of your foundation because this might lead to differential settlement and could potentially damage your structure’s foundation.
As mentioned above, the existing roof is held up by wood joists. The advantage of the wood joist system is that it provides an attic space that you can use to install and hide equipment from plain view. If you are building an ADU from scratch, I also recommend you consider wood joists instead of beams as roof members because the wood joists allow you to have an attic space to add equipment (i.e. furnace) and run electrical wires and pipes as needed through this hidden space.
For my ADU project, there was only one minor structural problem with the existing wood joists. Since the joists spanned from exterior wall to exterior wall without an internal support, a few joists had too much deflection (i.e. sag) that prevented proper installation of the ceiling below the joists. There were two approaches I used to fix the sagging joists. At locations where joists crossed over an interior wall, I used that interior wall as a support point to help prop up the roof joists to the correct height. At particular location where a joist did not span over an interior wall, I connected a sister beam to the bottom cord of that joist to increase its capacity and reduce sagging (see picture below). Both approaches worked well and eliminated the sagging of the joists and allowed the ceiling to be installed.
In Part 2 and Part 3 I will discuss more major issues that I had with the existing walls and existing foundation of the ADU.
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.