We bought the house 6 yrs ago and have been using shims to level furniture. Now its time to replace carpet. When pulled old carpet up, found that was just laid over old porch. Old boards are cupped so luan doesn't help. What is the best way to level the floor. It's a 40x16' room with 19 floor to ceiling windows making a sunroom under a flat roof. There is 2" below the windows on the front. Have considered laying boards perpendicular to porch boards and putting plywood over then when gets to high end using leveling mud. Will that work?

The two ways to level a floor like this are:

1. Cut sleeper joists (joists that are tapered to lay on top of the existing floor and yet give you a level surface on top). Then install a solid subfloor - like 3/4" Advantec. Then install your new, finished floor.

2. Pour a mortar based leveling compound and once it cures, install your new floor.

You can combine both methods if it makes sense.

However - If you only have 2" below your windows and assuming that the window end is the side that needs to be raised then you may be mathematically eliminated from raising the floor at all. If it needs to come up by more than 2" to get to level then you can't do it without changing the windows - probably a deal breaker for a project like this. For all the work involved to raise the floor, it doesn't make sense to only make it closer to level than it was without being completely level.

So, it all depends on how far the floor needs to be raised.

I would say don't do anything without getting a view under that porch. if it's looking creepy, you can get a fiber optic inspection camera from Harbor Freight for $80 using a 20% off coupon, instead of crawling underneath.

I'm betting there is no foundation under that old porch. which on its face means you can't do anything with the space until this is fixed. I will also wager that the slap-on porch is rotting big-time, and probably needs to be torn off and redone.

if the thing is just leaning into the mud, but is solid, you can punch a hole on each side underneath, put a big ol' beam through the holes for the porch to rest on, pour a couple concrete pads at the ends of the beam, and use a couple 20-ton bottle jacks to slowly (as in over a day for a 2 inch drop) raise the porch level. then put blocking underneath of crosswise chunks of landscape timber on the pad to hold it level, put a pencil or a round stick on the end of each section of blocking, and remove the jacks (which sag, hopefully over time, although a failure can drop one NOW, like when you're underneath.)

if the pencils have fallen off, get the (heck) away from there and find out what's shifting.

otherwise, if all is solid otherwise, get a permit, dig a foundation, fill that dig with concrete, punch a little rebar into it, and stick some J-bolts in there after drilling the deck plate to receive them. put whatever the code authority requires between the concrete and the porch plate (metal plate, ground-rated treated lumber, rubber sheet), lower onto the new foundation, crawl under the porch and put washers and nuts on the J-bolts after a week or so.

now you can figure out what you want to do in the room above.

otherwise, you're throwing the remodelling money away. the porch, if I'm right, will eventually pull off and leave the room open.

way too many of those 20s-30s-40s slap-up porches have had the weather wall opened into them. uniformly bad move.

Demo the existing cupped boards and sister the joist and leveling them at the same time. Then either install new boards or plywood for floor.