Reparations
There are two type of parquet floor: raw wood that needs to be sanded and varnished right after the installation and the pre-varnished. Of them both the easier (and cheaper to repair) is the raw one cause it is manufactured acording to a specific standard and it is very easy to find replacing materials for any given floor since all tiles are of the exact same size regardless of the manufacturer that produced it. Of course, once the repairs are done, the floor still needs to be sanded and varnished (at least the whole room) because the new wood (that was never sanded) is thicker than the old one (that has been sanded perhaps many times) and needs to be leveled down to the rest of the floor. If only the repaired spot is sanded instead of the whole floor, a huge difference of color between the repaired spot and the rest of the floor would spoil the whole thing.
It goes otherwise with pre-finished parquet floor because there are tongues and grooves that seem to change from one manufacturer to another and the size of the tiles are different also, which makes any replacement material very difficult to get (unless you happen to know the maker and the brand of what you need and assuming that it is still available!). On the birghter side, once the repairs are done the work is finished; no need to eans and varnish right now. Though it is possible ("usual" would be a more acurate word here) to see a color missmatch between two batches of parquet produced at different times.
Unlike parquet, there is not much of a difference between pre-finished and non-finished wood when talking about strip wood floring. What you need to check is the width of the boards (the most common are 2¼" wide). It is fairly easy to find any wider also. Things are becoming a mess when you need thinner (though 2" wide are still easy to find). Some floor have boards that are 1¾" or 1½" wide and these are very difficult to find (sometimes downright impossible!). In the later case it becomes necessary to have the wood produce and that is, you guessed it, very expensive!
Repairs on a stripwood floor may vary greatly from one damaged area to another seemingly identical in size, wood type and all. What is important to note here is the orientation of the strips of wood compared to the hole to cover. As an example, lets imagine that someone removes two different walls in two different rooms. They both measure 10ft. long and under the walls there was no floor so we need to fill the gap! Now room one's wall was oriented in the same way the floor is so no problem there: the repairs will be calculated on 10sf. (I realize here that 10ft. long times 4" to 6" does not gives 10sf. but it'll still be calculated this way). Now room two's wall was oriented the other way (across) so now we are looking at something more like 60sf.! How's that possible? Quite easily actually, here's how: unless we "fill" the whole and that is all (and very ugly also) we will need to insert the new boards of wood through the old ones to make the patch seamless wich means that we will have to remove up to 3 feet each side of the hole, therefor 10ft. long times 6ft. wide (3 on the left plus 3 on the right) totals up to 60sf.
Just like the parquet, engineered floor can be difficult to find. Make bay man'a hand it is not always easy to find the maker, it still available? And if yes, it is also very likely that there will be some color missmatch with the old floor.
Engineered floors are usually sound proof, which means that the membrane will be damaged when we remove the damaged pieces of wood and therefor will need to be changed as well. Keep in mind that a considerable sum is to be expected when dealing with engineered wood repairs.
Sub-floor repairs are rarely a problem (except in the case of major repairs that touches to the very foundation of the house, in which case you need to deal with a carpenter). It is always expensive to fix the sub-floor (or part of it) because it never comes alone: the resurfacing needs to be done also.
One kind of repair that are often asked for is to get rid of the "cracking" noises emmitted by some old floors. One has to know that is is not the floor resurfacing that cracks but the sub-floor itself. When the house was build, the sub-floor was nailed down into the supporting beams. Time went by and the nails got loosened a bit (realy a tiny little bit) but enough to allow the tinyest movement of the sub-floor againste the supporting beams and this is what causes that ennoying noise. The only way to fix this situation is to "screw" the sub-floor back into the supporting beams (a screw will not loosened with time), which will stop the movement of the sub-floor against the supporting beams and in turn will stop the darn noise.
The best way to go with that is very expensive because it consists of the removal the whole floor resurfacing, rescrewing the sub-floor back into the beams and reinstalling a new floor resurfacing...
Another way requires an access to the sub-floor from below (through the basement ceiling) and the screw the "supporting beams" back into the "sub-floor". Not quite as expensive as the first choice but it is a very physicaly hard work to do therefor quite expensive.
The cheapest way to do it would be like this: we will screw the sub-floor into the beams "through" the hardwood resurfacing! A whole lot of pretty big holes will be drilled through the resurfacing (big anough to clear the head of the screws) all over the floor. One screw will be put in every hole and screwed into the sub-floor and beams. The holes are then plugged with a piece of wood (the same kind as the floor) which is held tight there with glue. After all that, the floor needs to be sanded and varnished. This kind of work is not really desirable because it'll leave the floor full of little circle, unevenly spreaded (more where there were lots of crackings and less where there was less) and they can be quite noticable.
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