As explore the further mysteries of my subfloor I had some questions with respect to the 3/16" of flatness tolerance.


1. If lay have a gentle slope into a depression that goes to 3/16" of deep between two high points that are 10' apart, would you fill that space with patch, or only if it is greater than 3/16" deep?

2. If you have drop off in the sublfoor in a space confined by cabinets (kitchen) that is only 1/8" in say 5', but you know will be way over 3/16" extended out into your cabinets, should you deal with that?

3. If I place my 10' straight edge perfectly balanced halfway on a peak/hump such that it is 1/8" above the floor on each extremity of the straight edge, is that flat enough or should the hump be ground? Obviously if I place my straight edge flat on one side of the hump, the far end would be 1/4" above the slab. Is this a problem that needs to be addressed?

4. It seems to me that a floor would tend to be more tolerant of sub-floor flatness problems that appear perpendicular to the lay of the floor. In other words, long ways, the floor boards have fewer seams and they are staggered where the short side has more seems and they are aligned and would allow the run of boards to conform more readily to imperfections in the floor. Is this true and is it significant?


Thanks,

--David

I would venture to say by your questions that you know the answers, just looking for confirmation.

1) You are right on the max , 3/16" in 10'. It's a judgement call.

2) & 3) Basically both these descriptions are the same in 10'. IMO these are close, cutting hairs. Again a judgement call.

4) It IS more desirable to run the planks perpendicular to joists or minor subfloor irregularities.

3: It would be a simple matter to sand or grind out that 1/8th inch hump, so why not do it while you are doing the rest of the prep.

Floorologist,

I am not sure I fully know the answers. I designed questions that where meant to define what is meant by the 3/16" of an inch tolerance. I have the common sense, just not the experience to make the judgment calls. I guess I am not particularly worried about screwing it up as I am pretty sure I am putting more effort in prepping the sub floor than any sane professional would.

Dennis,

I am grinding down humps like in my example, but let's apply the same situation to an already ground hump. Let's say I have flattened out the hump and smoothed a swath about 20" wide over the center of the hump to a perfect flat. The ends of my straight edge eventually come off the floor 1/8" on each side of the straight edge just before hitting the wall.

Does the hump 'reset' the 3/16" tolerance to zero or is it cumulative across the 10' span? I hope this is clear.

Also, I tend to grind the humps out to just over twice the width of my 7" grinding wheel--about 20". This seems practical to me as the wider your flat space becomes, the more you have --usually a lot more-- to bring the level of the floor down. It is also the widest swath I can easily ground with repositioning myself laterally and you don't seem to save much floor patch after a certain point for the grueling work.

I am curious of how the pros do it. In general practice is it usual to grind a wider flat than my 20"? I know--judgment call-- but there has to be some general guidelines. In most of my floor it seems to make more sense to chop of the peak of the hump and bring the offending extremities up with patch as needed then grind a 16 sq ft area down 1/16 of an inch

Thanks,

--David

When I stated that "By your questions I would venture to guess that you already know the answers", that was a complement. You sound very thorough, and knowledgable.

When I talk about making a judgement call, yes, that comes with experience. But IMO certain situations are just "judgement calls". I should probably recommend sticking to the manufacturers tolerances strictly, regardless of cutting hairs. Depending on the manuf. and the product, they will waiver slightly on tolerance of flatness. The definition of the subfloor being flat within 3/16" in 10 ' is just that. Your out of tolerance on all your questioned areas, throwing out the use of a judgement call, they should be addressed. Usually the order of achieving flatness within tolerance is grinding high areas 1st to achieve the goal of using minimal amounts of filling compound.