snake size

I have a house built in 1938. The upstairs bathtub has a drum trap located between the floor joists, about 10 inches from the tub drain. It is draining very slow. What size snake should I use to open the line? It runs across the bathroom floor to the vertical pipe. Is there some other method to open up the pipe and keep it from closing up in a few months? Thanks.

Snake size doesn't matter!

ummm... you should get rid of that drum trap! They are illegal in most places, and were used in combination waste and vent systems (for fixtures that are not individually vented). As far as I know, a snake will not go through these things... we have a classic situation of snake size not mattering (lol)! We'll wait to see what Terry or HJ or one of these guys have to say about it, I bet they'll have some better insight.

I can remove the cover of the trap and start from there. Would be nice to remove it, but would involve ripping out the ceiling in the 1st floor powder room to gain working room.

Then perhaps you'll just have to work with it, if you can get the cover off (which can be pretty tough to do sometimes, if it's corroded on there) - start by getting some oil on the cover, let it sit and get that puppy off. You'll then be faced with exactly what you would find in the ordinary drain: nasty, oily, greasy, bubbly, soapy, hair clog! Get the rubber gloves on, scoop it all out, and you should be good to go... good luck to you, and god-speed my friend!

Thanks for the response. I've cleaned the trap (about a year ago), which helped, however, if the horizontal pipe going to the vertical drain pipe is reduced in size-(after the trap) due to accumulated gunk and not much pitch, is there a particular size snake I should use that will scrape it away from the pipe wall? I've seen the size 3/8". Does this refer to the cable diameter or the expanded spring at the end? Thanks.

3/8 is the cable size. A 3/8 cable with appropriate cutter will do a 2" pipe easily, and you may have a cutter available for 3". Generally it is preferred to use the largest cable which will negotiate the fittings on the line.

snake

Don't put any end on the cable just put a little kink on the lead end and it will flop and cover the whole inside of the drain line. and it can manage the ells. easier too.
I'm assuming your going to use a power driven snake.