Rugs, Carpets and Carpeting - Carpet install over plank flooring

I'm considering having carpeting installed in my children's second floor bedroom which currently has loft/plank flooring. The flooring is 1 1/2" thick T&G pine which also serves as the ceiling for the first floor (it's a log home with exposed beams(joists) on the first floor ceiling.

Anyway, my question is.....should I just do the padding and carpet install right ontop of the plank flooring or should I attach a new plywood subloor to the plank flooring first and then place the carpet on that.

I'd like to avoid adding to much extra height to the floor because I'll then have to adjust the room entry door and twin closet doors so if I should be adding a plywood subfloor will 1'4" be sufficient?

Thanks!

Tom

I've been out of town and just noticed this was not answered. I hope it isn't too late, but, here goes, just in case. My experience over the years has not been good with carpet over the sort of floor you describe. The planks tend to cup over time and there will be a low spot running the length of each plank in the center. This causes a trampoline effect with each plank. When the carpet is stretched, it'll be resting on the high spots at the edges of the planks and spanning the low spots in the center of the planks. The areas it spans will have a void there and the carpet is acting sort of like a trampoline in those areas. Each time you step on the trampolined areas, it will stretch the carpet down into the low spots and begin to stretch out the backing and will, in some instances, cause it to begin to delaminate. Within a relatively short time, humps will develop in the carpet as it looses its stretch. Plywood is the best solution. If it were my floor, I'd use half inch, but I suppose you may get away with quarter inch. But, do you really want to take a chance on "get away with" and run the risk of having to do it all over again?

If there is movement in the planks then no. Install pplywood subfloor panels, then the carpet.

If the planking is 100% solid and firm with no movement or gaps, then its ok to isntall carpet and cushion over top.

Thanks guys!

Tom