Installing baseboard trim

We’ve recently had the carpet replaced on the entire upper floor of our typical light-frame construction home. Before install I tore out all of the outdated baseboard trim and am going to install all new ~3-1/2" stuff.
My question is:
In typical 2x4 construction on the second level, is there a horizontal 2x4 running along the floor behind the base of these walls similar to being built on a slab or foundation wall? Reason being I’d like to use my nail gun to install the trim and having that horizontal member there would make it very easy. If not I’d have to go around the entire upper floor marking where all the studs are to nail into.

There is always (for 95% values of always) a base stringer in frame construction. You’re safe if you run the nail in that lower range and angle it a bit down for bite. Also, once you find one stud you can use a construction yardstick to locate the rest - most 4-footers have the 16-inch interval marked with a diamond or similar dot. You can run an upper nail where indicated, and if you miss, it’s no big deal.

I will say that I wish I could have back those painful hours of my life, nailing in baseboard and trim without nail guns. So, so, SO much easier that I’d recommend purchase of one even for one moderately-sized room. You’ll never regret the expense…

I did buy a Paslode cordless for just this purpose when I did the trimwork on the lower level. Worth… every… penny.

Just because there’s a sole plate doesn’t mean you shouldn’t nail into the studs as well. The baseboard will sit more flush on the wall that way. If you only nail into the bottom inch of the baseboard, the baseboard will come away from the wall at the top, leaving a gap. And, as mentioned, the studs should be at regular intervals, meaning you should only have to find one.

The studs are spaced 16"o.c. for the most part but there are a lot of short spans (24"-36") where i’ll have to find the lone stud.

some stud finders need to be calibrated (held at a known empty wall spot) to establish what is the sensing for that situation. some detectors need to be moved slowly in trouble spots.

you can verify or test trouble spots (for the 16" o.c.) by drilling (with a 1/8" bit) or nailing (with a finishing nail) at about 2 1/2 to 3 inches above the floor.

you should nail the baseboard at the top at the studs to keep the top tight, not so close to the edge or in the thin area which might split the wood. also nail the lower baseboard into the stud and not the bottom plate so the baseboard stays more even.