Building Homes or Extensions/changes in floor level
Recently, about 4 months ago, I bought an 1870 (or earlier) balloon framed house with a sag in the second floor main beam that spans across 15' of the living (first floor) room and I have to assume comes across the wall to support the second story landing. The sag most likely existed for a long time, as I did not notice any cracking or obvious gaps in the plaster and first generation drywall (multiple layers of wallpaper on it) that made up the wall in the second story room that is most affected by this, even though this room has a clear sag in the floor toward the wall over the beam in the middle of the room.
The main (sagging) beam looks to have been reinforced with two engineered wood pieces (1 inch thickness, plywood look to them) on either side of the beam. On either side of the room there are two jack posts (~9.5' - ceiling height) holding up the main beam, with what appears to be a large post (I assume 8"x8"x8-9') under the reinforced pieces with a hole in the middle for a lighting fixture switch - this is only on the interior wall. Another, possibly relevant piece of information is that with cold dry months, the rel. humidity @ house is ~20-25%, summer likely to be ~65-100% - no central AC in this house - last owner mentioned they did not like AC, although he BS'd a lot.
I have been hearing occasional cracks and pops and became concerned about the beam that supports second story landing lengthwise, perpendicular to the sagging beam in the living room. The end of it near the stairs (as apparent from the fascia board) appears to have a gap between it and where it used to be (at some point) and the hardwood floor has some warping (I did not see this when I bought the house, but it does not mean that it wasn't there - I noted it about 2-3 months ago). It does not appear to have widened by much, if any, since I first saw it, but I did put some scotch tape with markings on it to track any further changes recently. Also, the second story landing has a kind of a bounce to it if walking briskly or doing some calisthenics on it (only to feel the bounce - I do not do this regularly).
Additionally, I just noticed about an eighth-quarter inch gap under the quarter round shoe along baseboards in the room over the sagging beam that I do not think was there earlier, particularly near the chimney that is on the room side of the beam/landing - I have done some renovations in the room, including peeling wallpaper, putting on veneer plaster and redoing the closet, also over the beam. The gap freaked me out, and I started investigating the issue by taking off some shelves in the living room I was eventually planning on taking out, and making a few holes in the drywall under the beam to check out the structural integrity. I have attached the photos of the jack posts and the internal structure of the beam/reinforcement assembly. I did not notice any obvious structural deficiencies across the length of the beam, with the exception of what I think are too few posts (although not fully sure about this).
My question is - is this a major structural concern, or can the low relative humidity have an impact on the floor in the 1/8-1/4" range with the concomitant pops and cracks that I hear? The dilemma that I have is - should I tear out all the drywall and plaster to get a good look at the structure ASAP or can this wait? Also, I am thinking about putting in another post next to the inside wall jack post to ensure that the interior post does not buckle, as I am still not fully convinced that there are other structural elements through here.
There are a lot of possibilities causing the problem. My concern is the house sitting on bad soil which contains peat moss or sand. Your said "I just noticed about an eighth-quarter inch gap under the quarter round shoe along baseboards" is another concern. I am sure you would have notice a 1/4" gap before you bought the house. If you are fairly certain the gap was not there before, then there is a good chance the problem is below that part of the house. There is now way a house sinks that quickly unless there is an ongoing problem like a slow water leak or bad soil underneath. Are the incoming water lines made of lead pipe or have they been replaced? What about the sewer lines. Have they ever been replaced or updated or are they the old clay tile sewer pipe. What size is the engineered laminate beam? 1x12, 1x8 for example. What is the size of the floor joists? For example... a 2x8 floor joist 14 to 16 on center is running more than 10 to 12 ft without a support beam holding it up, the floors will sag. Now under that support pole should be a 3x3ft concrete pad around 15 to 18 inches deep, depending on the floor load. You say there are support posts that were added? That does no good if they are put on a 4 inch inch thick concrete basement floor. When I was building in one area of a town, I ran into soil that acted like jello. There was soil clay in most of the hole I excavated for the foundation but in about three areas about 2x2 ft, you could actually bounce on it. Luckily these spots were no where near sections that supported the house. In the same town there was another section that an old river bed with silty sandy soil that had been covered truckloads of clay in the early 1900's and in 2006 new homes were built on there and they have had a problem with the homes ever since. Now since the house has been there since 1870, it sound like the problem started not to long ago. That is why I asked about the possibility that water may be gettting under the house some how. I did a two repairs last year caused by water. Both involved clay sewer pipe. Both places had their rain gutters draining into a pipe going into the ground. In both cases the drain pipe cracked under ground and suck the first house's concrete slab floor about three inches and the other house had a side driveway they suck 4 inches because of a slow water leak. You should not be hearing pops when you are in the house. as far as the guy who sold it to you, he could have temporarily patched cracks like the one you spotted along the baseboard every few months. The same with the supports. Are they the metal adjustable poles? I am sorry I can't give you a more definite answer, but there are to many variables that may be the cause.
Ok, can you measure the size of the floor joists, tell me how far apart they are run or the distance between the poles or foundation walls support them. Do you have the old style sewer lines. They should be a reddish color. Do you have the old soft lead water pipes supplying the water to the water meter? You did say you have a concrete floor in the basement, right? And the answer to your second floor bounce. Sometimes builders back then would use what was available. It is possible the second floor floor joists are to small. The best example is...most of the Cape Cod style A frame house built after World War Two used 2x4's instead of 2x6's to support the roof. 2x6's where 2x8's should have been used on the second floor. Once again, that may no be the cause, but it is very possible that is the problem upstairs. if you want, get me the measurements I need so I can give you a better answer to the problem