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Old 11-28-2017, 05:30 PM   #1
uncreative_username
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Leveling products which won’t shatter when drilled into for sill plate attachment

Greetings,

First post here, thank you in advance for any time, thoughts and suggestions you may have for this project. Here goes:

Below are some pics of the room I am planning to build inside my garage. The garage has a sloped floor that drops a total of 1-3/16” over 15’ (grade is not constant) to the overhead door. The blueprint page shows the garage and foundation as it currently exists (i.e., without the new room), and relevant details such as depth of slab, where footings exist, etc.

For a few reasons, I need a straight and level floor on which to attach the sill plates, as opposed to shimming or otherwise modifying the framing to fit the space. As such, I need to find some way to level the floor (15’ x 17’ - 255 ft^2) that will allow me to attach sill plates to the new concrete / substrate without cracking / shattering it. Given this goal, I assumed the way to least disturb the concrete is to attach the sill plates with 1/2” retrofit / straight bolts and epoxy (and avoid using expanding anchor bolts, Tapcons, etc.). My understanding is most SLC’s will shatter when they are drilled into, especially in thin areas.

Given this room will be adjacent to structural posts, I assume planing the slab is out of the question. I can’t imagine the home was built with a slab thickness any greater than the minimum to meet code. (Of course, I plan to involve a structural engineer in this process down the line as well.)

What about drilling 1/4” into the existing slab at each and every place where attach a bolt to sill plate? Obviously this would require careful planning to ensure no conflict with vertical framing members etc., I’m up for it. Then glue down a 5/8” O.D. PVC round rod, then pour the new slab, then drill out the plastic for a nice neat hole for the bolt to be fitted.

Other points of concern:

The north garage bay and the northern wall of the new room - I need to keep this north bay UN-leveled, and maintain the existing slope, so a car can park here and be compliant with code. So I need a plan that does not extend any concrete more than a few (3-6?) inches further north of the room’s northern-most sill plate.

Note how the two eastern walls of this new room are very close to the red structural posts. I assume it’s against code to cover these base plates and bolts with new concrete.

The HVAC boxes on top of the new room are resting on the room’s ceiling, NOT attached to the ceiling joists in the existing garage. These boxes have a combined weight of 1700lbs, hence the inner walls being load-bearing.

Finally, to keep costs as controlled as possible - if a speciality / expensive solution needs to be used for the sill plate area, could I do that for only the area surrounding the sill plates, shown in light blue in the pics, and then use something cheaper like deck mud for the inner area of the room? Proper bonding?

Assuming you made it this far, thanks for reading this not-so-brief post, as well as your thoughts and suggestions!
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Old 11-28-2017, 07:04 PM   #2
wwhitney
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I didn't fully digest all the details you provided, but you mentioned new load bearing interior walls on an existing slab. Is there an existing footing under that slab in the desired location? If not, you'll need to cut out the slab to install a new footing for your load bearing wall. Which would moot your original question, as you could pour your new footing with its top at the desired elevation, and with preinstalled anchor bolts.

Cheers, Wayne
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Old 11-28-2017, 07:35 PM   #3
JerseyDIYguy
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I think I understand what you're trying to do, but I might be off a bit...

Could you simply attach 2x4 sill plates directly to your existing slab, frame your walls, and then pour SLC inside your new sill plates using the sills as a dam? Your sills would be on a slope, but that's relatively easy to deal with. After you pour the SLC your sill plates would be partially below the top of the finished SLC but that shouldn't matter.

Another idea would be to get some galvanized threaded rod, drill holes into your existing slab, epoxy the threaded rod into the holes, and then pour the SLC so that the threaded rod projects a few inches above the finished SLC. Then you'd just drill and install your sill plates on top of the SLC surface using the threaded rods.
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Old 11-28-2017, 08:15 PM   #4
jadnashua
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A taper jig might let you make a long, tapered section to put underneath the new plate so it was level...then, you wouldn't have to make each stud a different length with one end at an angle, then pour your slc. Rather than SLC, you could do a bonded mudbed and likely save a bunch of money, but not necessarily time.
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Old 11-29-2017, 09:22 AM   #5
uncreative_username
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Thanks for the replies! I know it is a lot to take in, and being a neophyte, I don't really know how to succinctly communicate the problem.

There are footings present at some parts of the east walls at the red support posts in animated drawings, and light green on the blueprint page. I will double-check, but I believe the structural eng. I initially spoke with said that the load was not enough to warrant footings, the existing 3.5" slab should support it (and would provide the exact calculations to show it.)

Tapering the sills, as well as just framing in place seem to be more problematic - I didn't go into reasons why it wasn't ideal to frame with on the sloped floor in the first post, just that it was (I was trying to keeping the initial post from becoming a novella). It's no problem to explain why if you think it would be helpful to discuss.

Quote:
Another idea would be to get some galvanized threaded rod, drill holes into your existing slab, epoxy the threaded rod into the holes, and then pour the SLC so that the threaded rod projects a few inches above the finished SLC. Then you'd just drill and install your sill plates on top of the SLC surface using the threaded rods.
So this is exactly what I had thought of, which then changed into the proposal with the PVC rod placeholders. The only issue here is that there’s no way to epoxy around the newly formed concrete, so the only epoxy would be what was below the slab already. Is it OK to install the rods like this? I.e., wouldn't code require the entire submerged part of the rod to be epoxied? The PVC plan would have the added benefit of not going too far into the existing slab (e.g., only 1/4" or so), and not risk drilling entirely through the slab, or disturbing the structural posts as much.

Thanks again,
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Old 11-29-2017, 11:22 AM   #6
wwhitney
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If I'm deciphering your plans correctly, there are two new interior walls to be built, one running N-S and one running E-W. The downhill gradient on the existing slab is to the west. Based on the ceiling framing, it appears that only N-S wall will be load bearing.

I'm still concerned about the need for a new footing, but I'll leave that aside for now. My initial impression is that it would seem like a simpler option to install your bottom plates to the existing slab. Then if you need a level finish floor in your new room, just level the area inside the walls.

The load bearing N-S wall is running perpendicular to the gradient. Is the slope at the location of the bottom plate of this wall greater than average? Because the average is 1-3/16" over 180", or only 1.5/64" over the 3-1/2" width of a 2x4. So I would think you could either install the bottom plate ignoring that slope, or possible use some thin patching material (not SLC) to flatten out just a 4" to 6" wide strip where the bottom plate will go. Then use tapcons to attach your bottom plate to the slab.

As for the E-W wall, it appears to be non-load bearing. Why does the bottom plate need to be level? It would be simplest to just install the bottom plate to the existing slope and varying the length of the studs in the wall to get a level top plate.

Cheers, Wayne

PS An alternative would be to form up a new concrete curb under the footprint of your bottom plates, say 4" wide (maybe 6"?) and maybe 2" to 4" tall at the N-S wall. Then you could ensure that the top of the curb is flat and level. The curb should be mechanically bonded to the existing slab, e.g. you could install a series of partially driven tapcons or other anchors into the existing slab and pour the curb around them.

I'm not recommending a new curb, it seems like a lot of trouble, just throwing it out there as an option in case it appeals to you or solves some other issue that might arise in your project.
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Old 11-29-2017, 12:45 PM   #7
JerseyDIYguy
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Quote:
So this is exactly what I had thought of, which then changed into the proposal with the PVC rod placeholders. The only issue here is that there’s no way to epoxy around the newly formed concrete, so the only epoxy would be what was below the slab already. Is it OK to install the rods like this? I.e., wouldn't code require the entire submerged part of the rod to be epoxied? The PVC plan would have the added benefit of not going too far into the existing slab (e.g., only 1/4" or so), and not risk drilling entirely through the slab, or disturbing the structural posts as much.
My take is that you'd want to drill into the slab a good distance (2"-3") and rely on epoxying to the slab for strength rather than relying on epoxying to the SLC. My (admittedly limited) experience with SLC is that it's not as sturdy as the typical concrete used in slabs. I also don't see a big issue if you accidentally drill all the way through the slab since you'll be filling the hole with epoxy before setting your threaded rod.
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Old 11-29-2017, 06:17 PM   #8
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Wayne, The design is sort of a room within a room, and will be a (small) sound editing room. There are two outer walls, the furthest N and E walls, respectively, that are non load-bearing. These are just partitions - their sole purpose is to help isolate sound to/from the inner room. Then there are four walls comprising the actual room. The sound isolation requirements have made a lot of things more difficult than I had initially anticipated.

The bottom plates for the N-S running walls could probably be leveled out OK with some patching. The slope is by no means constant. Just for a general idea of it, I've attached a pic of measurements taken at 5 different points on the eastern-most wall, measuring the fall every foot until hitting the west wall or garage door. I need to think through the framing on the slope, maybe it can work. I thought about the curb as well, but thought it was needlessly complicated. I will revisit this.

Warren, I think you're right, or at least it makes sense to me, that new SLC would be much weaker than current slab, so better to epoxy bolts into as much of existing slab as possible.

Thanks for offering up these suggestions, it's exactly what I came here for. Both new ideas as well as helping me to think through these issues with a bit more clarity.
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