It's pretty simple. Especially if the wall is an interior wall, and the wall runs
perpendicular to the floor joists.
Typical platform framing used in the US has the wall framing built laying down on the floor deck, and then stood up into position.
Wall sections assembled while laying down:
A 2x4 Sole Plate for what will become the lower wall end.
A multitude of 2x4 Studs spaced 16" on center (cheap places may allow 24" on center, yuck!).
A 2x4 Top Plate for what will become the top end of the wall.
Nails are hammered through the Sole Plate and Top Plate into the ends of the studs.
When the wall is stood up, the Sole Plate is nailed through the sub-floor (usually 3/4" plywood, but will be 1-by lumber in old houses) and into the top of the floor joists.
The Top Plates are then laminated on top with
another Top Plate that will tie wall sections together at the top. I'm leaving out all details of connecting the edges of wall sections together, as it is understanding the top and bottom that is important for fishing wires down from attic or up from basement or crawlspace.
So if it is an interior wall that runs
perpendicular to the floor joists, then it is a simple matter to drill UP in the proper place through the subfloor and Sole Plate, into the wall cavity.
What is the
proper place?
- Well, it is in the
middle of the Sole Plate (forwards/backwards through wall thickness), so there is low possibility of it being hit later by a nail driven into the wall for some other purpose.
- It is in the wall cavity away from a wall stud, you don't want to drill up into the end of a stud, as that won't help
So you will need to locate a few studs in the wall up in the room with a electronic stud finder (density) or a magnetic arrow type (finds drywall nails or screws that attach the drywall to studs, thereby locating studs).
- And due to the size of a drill's body, you can drill only so close to a joist.
-And avoid drilling right by an outlet.
You need to accurately project this info, or needed offsets from a known point, down into the crawlspace. Either by careful measurement or by drilling a small (1/8" is more than enough) hole somewhere where it won't show or can be caulked/filled later. Poke a thin wire through the hole for the reference. You could do this on the
other side of the wall since the room is tiled.
Just think it through before doing, assume you are fallible, and check measurements and concept twice.
Since the room is tiled, could you drill a reference hole in the room on the
other side of the wall.
IF the interior wall instead runs
parallel to the floor joists, then you will have to think and measure it carefully because the floor joist may block access to the Sole Plate above it.
Exterior walls are a bit more difficult due to insulation, access from below due to the run of the Joists or Band (Rim) Joist, etc. And if running down from the attic to near the floor,
may run into 2x4 blocking between studs in the wall. Blocking unlikely to be a concern when going UP a wall from down below, if you are only going a couple feet up from the floor. Blocking, if used (very rarely) is mid-wall.
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