Wood vs. Engineered Wood?

No contest in this case. If the home was built in 1905, put real hardwood in. This preserves the integrity of the home's building materials. Plus, as you can see from the age of these floors, higher grades (i.e. thicker) hardwood can be sanded several times; you usually don't get even one chance to sand engineered wood. The hardwood layer at the top of this multilayered product is simply too thin.

Level Floors and Subfloors

There are ways to level a subfloor, ranging from replacing parts of the subfloor with thicker or thinner sheets of plywood (or planks, see below), to pour-on leveling compounds that harden to make the entire floor level. What you use to level uneven areas depends on what the subflooring is. If it is plywood or concrete, the compound works. But more likely with this home, on floors at or above grade, there will be wood planks laid at a diagonal across the floor joists. So in your home, the layers were probably joists first, then subfloor planks of wood, then hardwood with tongue & groove joints. You cannot effectively use a pour-on leveling compound on a plank subfloor, as there are cracks between the planks. However, you can lay a new hardwood floor directly on a wood plank subfloor as long as those planks are sound and relatively level. Replace individual planks that have been damaged by water or insects, are otherwise compromised, or even missing (it happens!), with either new planks of lumber, or with plywood of the same thickness as the planks. Any lumber you install, for either the subfloor or finished flooring, should be brought in to your home several days prior to installation, so the materials acclimatize to your typical interior temperature and humidity.

Unlike for ceramic or porcelain tile, the subfloor does not have to be perfectly level. And unlike vinyl sheeting, planks or tiles, the hardwood won't "telegraph" minor differences in height, and minor surface irregularities, so the subfloor doesn't have to be absolutely perfect; it should be sound, strong, uniform and essentially level. Your flooring contractor can give you specifics on what upgrades your subfloor may need once the original hardwood is removed.

Joists

If the floor is significantly not level in parts, or if certain areas feel spongy when you walk on them, I recommend you have your flooring contractor, a home inspector, or structural engineer inspect the subflooring and especially the floor joists. You don't want to invest in a new floor, only to have it become damaged because some joists weren't attended to.

Transitions

Transitions (a.k.a. thresholds or molding) between existing rooms and the new floor ought to be made of the same wood and same finish (stain, polyurethane, etc.) as the new flooring you are putting down. As wood ages, the finish becomes darker, so trying to make the transitions match the finishes of existing floors is very difficult and not recommended (what will match those floors right now won't match later after your transitions age up a bit). Eventually, the transitions wouldn't match either the new or the old floors!

Transitions come in four types: reducer (when the two floors being joined are of different heights), seam binder (when the floors are the same height, and the boards from each are very close to one another), T-mold (when the two floors are the same height, but there is a crack between the two rooms' flooring) and stair nose or stair edger (when the flooring needs to curve around the front of a stair's "tread"--the flat part of a stair upon which you tread!). The transitions for your particular project can be ordered when you order the hardwood, so make sure to include measurements for those doorways/entrances. The contractor can tell you if he/she anticipates there will be a height difference or gap between the floorings, which would require those particular transitions.

Dry Air Concerns

Consult a local lumber supplier or flooring contractor familiar with your local climate conditions to discuss finishes or other tips for keeping your wood floors healthy in the dry winters.

Sounds wonderful! Have fun!

Thanks for your thoughtful input, pcnb4! It pretty well lines up with what we've been thinking but I thought it was worth checking if there are any real advantages to engineered flooring as it's been awhile since we did any significant flooring after the addition we did to this house 10 years ago.

Yep, you are exactly right about the subfloor being planks on the diagonal. We actually feel pretty lucky, that although we know there are some leveling issues to address, we have a solid subfloor to work with. There are a number of similar vintage houses in our neighborhood that don't really have much in the way of subfloors (it's bizarre--we've had friends who pulled up their hardwood flooring and found they were directly laid across joists!). I also like your great overview of transitions--we have a threshold of the reducer type when somebody redid the bathroom and put in tile.

The house is pretty unique and we love it. We've restored almost all of the original windows (many of which were painted shut or had pulleys that had broken) in a big job a few years ago. The most unusual feature in the home is a bank of "pocket" windows in the kitchen--they actually disappear UP into the wall (vs the more typical side-hanging pocket door we are all familiar with). Although we are pretty dedicated to preserving the historical integrity of the house, we may be proposing to either replace these (it would be nice to have some insulation in that wall here in Colorado and there aren't any locks on them) or at the least move them up (and sacrifice some of the "pocket") to allow for a more functional kitchen layout when we remodel.

I'm in a similar situation as you in Denver and found this post helpful...thank you for putting this out there and for pcnb4's thorough response.

Your friends with no sub-floor likely pulled the sub-floor thinking its was just hardwood. Many of the late 1800's and very early 1900's homes around here have tongue and groove sub-floors that look just like hardwood flooring. In fact these sub-floors are frequently finished and treated just like hardwood flooring (my upstairs level included). I had to cringe and laugh a little when I read that.

@kad284, good luck with your floors in Denver. I just met with the historic preservation people up here and in discussing the 20% tax rebate for historic restoration here in Colorado (here's thelink in case it's useful), and if we want to have a chance at this program, we're likely going to need to replace the flooring with "like kind" meaning the SAME species of wood. In our case, it's likely heart pine, red pine variety. Hoo boy--this is going to be a little more interesting than we thought!

You're probably right about our friends' sub-floor being tongue and groove that was finished to look like hardwood flooring. I guess using the sub-floor as the main floor was the shortcut of the late 1800s / early 1900s! ;)