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Situation: We have a seriously sloped floor, lowest points are in the far corners of an external wall of what appears to be a newer (~30 years) addition on a much older home (1945). The inside dimensions are 13.5'x 9.5'.The house is new to us and we don't know if the addition was ever leveled or perhaps was a quickly walled in former lean-to porch with an intentional slope (ceiling has a similarly angled slope). There isn't sufficient crawl space for much inspection, but enough to see joists running perpendicular from the concrete foundation of the main house outward to (4) 6" piers at the far and lowest end (the 13.5'span) with a far beam (5-1/8x2-1/2") resting on its 5-1/8" side atop the piers. Two layers of 3/4" plywood subfloor atop the joists. We thought best to level from the inside with angled joists atop the existing subfloor layers (just deep enough to bring level), then new 3/4" OSB before installing underlayment and snap-together laminate.

Question: How can we ensure that the floor is at its lowest point vs wondering if it started level and has been sinking over time and/or will continue to sink more in the future, possibly damaging our interior efforts? Is it worth spending 3K for a professional to install new larger piers, gain some amount toward leveling and stabilizing before completing the interior work or will we never know if that money was worth it? We don't have a lot of cash, so trying to make the best choice for the house, peace of mind and the budget.


To answer all of the questions: the slope is just under 3" from where the floor attaches to the house to the farthest point away from the house. Correction from my first post, the ceiling does have a paralleling slope, but at a sharper angle than the floor(ceiling to floor is 95-1/4" where it attaches to house and 88-1/4" at the far end where the floor slopes). The walls show no signs of cracking, two sideways sliding windows, one on side wall close to foundation wall, one on wall opposite foundation wall. Neither window have issues opening or closing. We've only been in the house < 1 year, so don't have comparison data. The attic above this room appears to have been added later along with a pitched shingled roof.

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There is really no way to reliably guarantee what is happening with your situation there. And no way we can ascertain that without being there and inspecting it.

Probably your best bet would be to contact several experienced trades-persons and have them do an inspection and recommendation discussion. Get some informed opinions from experienced people. It will still be up to you to decide how you want to proceed.

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If you don't have a lot of cash then I'd go with sleepers on 12" centers with 5/8" ply to negate the sloped portion. Use some primer and SLC to take care of any smaller areas / dips.

If it took 77 years to drop 3" you should be good for long enough that your laminate floor is worn if/when you need to adjust it again.

Even if you think the foundation for the closed in porch or addition isn't perfect, fixing that will require exponentially more work and money such that re-doing your laminate install will be a minor cost.

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    1/2" ply seems a bit thin to install laminate onto. Even with only 12" spacing it would flex a lot.
    – brhans
    Oct 24, 2022 at 4:03
  • I see this has been edited to 5/8" rather than 1/2" - but I don't think that's enough. I would use 1" OSB. Nov 23, 2022 at 14:38
  • 1" that seems excessive. floor loads are designed and normally handled with 5/8" ply on 16" oc joists. some people go the extra and put 3/4" but 1" when you are just adding sleepers and the end product is LVP which is more tolerant of flex than hardwood seems way over kill. If you were doing tile I could see 1" to prevent grout cracking. Nov 27, 2022 at 21:44
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How can we ensure that the floor is at its lowest point vs wondering if it started level and has been sinking over time and/or will continue to sink more in the future

Look at the walls. Usually, walls are perpendicular to the floor.

If the building was originally level, then the walls should have been built vertical. So grab a bubble level and check the floor and all the walls.

If it was built straight & level then one side sunk into the ground, there are two possitilities:

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Left- The building kept its integrity, and the floor remained perpendicular to the walls. Therefore, the walls will no longer be vertical.

Right- The building did not keep its integrity and turned into a parallelogram. In this case the walls will still be vertical, but they won't be perpendicular to the floor anymore. This can happen if the building is held by one side to a much stronger building with better foundations, like your house. In this case, if the foundations that aren't on the side of your house sink, but your house does not sink,

This latter case is a bit more complex, because maybe it was built this way. So in this case you'd look at stuff that usually has nice perpendicular angles and likes to remain that way, like windows, sheets of drywall or plywood in the walls, etc. If the rectangular walls of the building are turning into parallelograms because one side is sinking, then you can expect the openings for windows to follow, but the glass will stay rectangular, so the windows will be stuck. Likewise the screws holding drywall or plywood sheets to the walls may pop. Outside, if it is clad with wood, you may find the edges which were originally aligned no longer are. You can also look at stuff that needs to have a certain slope for proper water flow, like the roof gutter. If it slopes the wrong way, that's a clue.

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