ADU on existing garage slab
Hello (and thank you for this opportunity!). Reading an older post (2011) about building in Massachusetts on a slab.
Currently speaking with a builder about converting an existing detached 2 car garage into an ADU.
My county and town finally are now allowing these to be built, but
I’m so concerned about a couple things. One is water supply from current and main home’s well into this build.
Water lines freezing is my big concern. And just a build in general on a slab is, too. Am I overthinking this?
Electric water heater will be placed in the unheated garage, and I still need to have a water filtration system installed.
Any concerns I should be asking, or be very aware of?? Other than frozen water lines??
Thanks SO much!!
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Replies
Millions of houses are built on slabs. It's not my favorite way but it's workable.
The plumbing is going to have to be trenched, and a hole made in the slab to bring it into the house. This is not a big deal at all for anyone who calls himself a plumber.
A bigger deal than the water supply is the sewer line. It flows by gravity only and has to slope downward through its entire run. If the ADU isn't higher than the low point of the sewer in the house it gets complicated in a hurry, you have to pump the waste. This isn't insurmountable, it just creates a maintenance item and the maintenance is unpleasant.
Unless your zoning creates incentives to reuse the existing building, I wouldn't assume that is the best path forward. Framing is the cheapest part of a building, garages are framed differently than houses, it may end up being more cost-effective to tear the garage down and reframe a residence than to try and rework it.
Thank you for the feedback/response. I had a 2 story on a slab in FL. Millions maybe, but not all in MA, which was my initial concern.
The septic has been surveyed/inspected for this. It is lower than the garage, and has (thankfully) perfect pitch.
My concern isn't the water heater itself being cold in the garage, but the line coming up to it, more-so, especially if there is a deep freeze.
Most all of that structure will come down.
Thank you again!
The line has to be buried below the frost line. I used to live in MA, but it's been a while, my recollection is 39" is the depth you need but I'd check that. It's around that though. A plumber will know, and he'll also know how far from the edge of the slab it has to be.
In houses on piers or above an open crawl space it's common to have an insulated chase for the pipes. This can be as simple as a 8" pipe surrounding the plumbing and filled with insulation.
My understanding is MA has gotten pretty strict about enforcing the energy code. I would find out what they want before going very far.
DC, thank you kindly for your replies! So helpful, and much appreciated!
Home Owner,
The problem is no doubt mine, but I'm confused as to what you a proposing to do, and what's currently there.
Leave the garage and build an ADU above it? Convert the garage into living space? Can you also flesh out what the existing foundation consists of? A slab with stem-walls around it?
Hi..no, not above, but convert current garage to living space. Currently has a concrete slab floor with what I think are stem walls? There is a concrete edging all around the perimeter, is that what is considered the stem wall? Sub-floor will be built upon that existing slab.
Believe the plan is to tear all down but the basic frame and rebuild (ie: use existing footprint)
to home structure vs garage structure code.
Did I answer sufficiently?
Homeowner,
Yes, thank you. That's much simpler than what I was imagining. I don't see any real impediments. Insulating the waterlines where they ascend into the new living space from below the frost line can be done a few ways. Perhaps the easiest and simplest is to run them in a sleeve surrounded by foam inside another larger sleeve. If you leave the top open they will take some heat from the space above. You can see an example on the drawing in this link: https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
Malcolm, cannot thank you enough (and glad I gave you something to work with lol). That set-up you mentioned is what I was thinking, that they could insulate that portion from frostline to unit.
Appreciate your time, sir! Thank you again!
Cheers. Good luck with your build.
having done something similar on one addition (framed a primary bedroom/bathroom suite over an existing slab) your process will likely be to trench for all the waste lines in the existing slab (cutting the slab running waste and patching concrete). The main water line will also come from below but then all other plumbing should be fed from above or through walls to avoid cutting the slab more. I would suggest putting a 8-10 mil vapor barrier down over the slab and wrapping/taping all the penetrations with compatible tape. Put a minimum of 2" EPS foam over top that and then two layers of 3/4" plywood overtop of that (the second layer run perpendicular to the first). Let the first layer "float" above the foam with a expansion space around the perimeter and glue and screw the second layer to the first. Google "concrete-free slab" for some other examples. The rest is straightforward. Note this assumes that all your load bearing is on exterior walls. Gets a bit more complicated if not but still doable.
I think from saving the old slab are not worth the costs.
The old slab is almost certainly sloped toward the door so that when snow on the car melts the water will flow out the door. When they go to build on the slope it complicates every cut because nothing is square to the floor.
The edge of a garage slab is almost always raised under the wall that will make attaching the baseboard a challenge.
You are going need drains pipes under the floor. That mean you are going to cut patch at least some of the concrete.
A garage slab is set at grade and ideally the house would be several inches above grade. One never knows what day the 500-year rain event will happen and when it does you want to be one of the highest points around.
Why limit the location and the dimensions to save chump change.
Walta