I've been monitoring this site for quite a while and had a few questions about a new install in my 100 yr plus house. The house has two existing layers of hardwood ( top one is 1"1/4 and the one underneath is 2"1/2) over 1x8 shiplap in the opposite direction. I'm installing 4"1/4 pre finished ash overtop, and would like to run it in the same direction. Pulling up everything is not and option because the 1"1/4 is flush with an addition that has 5/8 plywood that was put on at a later date and the floor level in the kitchen that only has one course of hardwood floor. The 1"1/4 hardwood is in great condition with no gaps or bulges and was ran in the same direction as the old, so I was thinking it might not be a problem. The whole floor is very level and super strong and I'm not worried about weight. Any thoughts on this particular situation would be greatly appreciated. This floor is a big investment and I really want to get it right. Thanks!

I'd say if the floor is flat with that many layers you should be able to install about any thing in any direction you want. My only concern would be the breathability with so many layers of wood, and I presume more than 1 layer of felt underlayment. A certain amount of air must be allowed to pass between each layer in order to remain stable, and if you cover hardwood with another layer i'd say as an installer that could potentially create unstable conditions for the underlying wood floors. I'd definitely do moisture tests and make certain that there aren't alot of humidity/moisture variations.Any trapped moisture can be a nightmare. You certainly don't want the layers of flooring underneath moving/cupping under your new floor.

if being flush is such an issue, then why not just rip out one layer of oak, or both, and rebuild back up with plywood?

Thanks for the responses. I've thought about taking up the wood lots of times but it just doesn't seem to make much sense to me. I'm at the level I need to be at right now without having to do any of the labour or spending the money on multiple layers of plywood. There would be a lot of logistical issue building back up and if the floor is good and level with no squeaks right now I feel like I have a good surface to work from. I've looked to see if there was any felt paper under any of the original layers but can't find any, might have to pull a few boards. If what your saying Floorpro is that a layer of hardwood trapped between two layers of felt paper is a bad idea, what would be the alternative? Rosin paper or nothing? I'm going to borrow a moisture meter from a friend and start monitoring both floors. Thanks again for the reply's guy's!