In preparation for floor refinishing I removed the shoe molding in my 100 yr old home. Getting ready to reinstall it now (mostly came out intact). But, it was originally installed with 8d finishing nails, like 2.5 inch long nails that went into the studs I guess. I don't think I have the carpentry skills to repeat that! But, I can't just use small nails to nail the shoe molding to the baseboard because the bottom of the baseboards is about a half inch above the floor, leaving a gap. In other words behind the shoe moldings was mostly air. So what to do now? I have thought about filling in the gap with little strips of wood, or caulk, or drywall mud, or something, but it would have to be something that can hold a nail. Any suggestions welcome. Also, I am wondering why the baseboards were not just installed flush with the floor. Thanks!

 

Is there some reason you cant nail it to the floor?

 

I just nail it to whatever is there. Whatever you can find to sink a nail into will work. It doesn't take a lot to hold it in place so hit what you can. You can nail at a shallow angle into the baseboard or at a very steep angle sown into the floor. Don't bother trying any goop in the gap below the baseboard. It won't hold a nail.

The core purpose of the baseboard and shoe molding is to finish off the bottom edge. Baseboard can't be bent to follow undulations in the floor so you'll often find gaps underneath. Shoe molding and 1/4 round is much more flexible and is used to follow the fine imperfections and cover the gap.

 

Perfect time to pick up a brad nailer, 1 1/4 brads would work perfect!

 

I'd use a nail gun to attach the shoe to the floor although you could hand nail it. Use painter's putty to fill the nail holes and caulk the top of the shoe mold to the baseboard [assuming it's painted]
btw - welcome to the forums!