Precision in Home Improvement, or: How to Trust a Laser Level?

Question

Even though I enjoy DIY, I also suffer from a certain amount of anxiety about it, especially when tolerances have to be tight. If the Internet's advice is that a certain thing needs to be square, or level, or plumb, I am never confident that I have achieved enough squareness, levelosity, or plumbitude. Should I fret about a 32nd of an inch? How about a 64th? Will the contractor I hire for phase 2 judge my handiwork in phase 1? These are the questions that keep me awake at night.

You'd think that laser levels would help my predicament. But they often don't, for multiple reasons:

1. How wide is the beam, Part 1: if I'm trying to cast a level laser beam across the entire span of a 40 foot wall -- with the laser pointed perpendicular to the wall at the 20 foot mark -- the beam is going to be fatter on the ends than it is in the middle. If I'm trying to draw marks along that wall that just kiss the bottom of the beam, marks at the ends of the wall will be ever so lower than the marks closer to the center.
2. How wide is the beam, Part 2: a beam in a bright room will seem narrower to my human eyes than the same beam in a dark room. Depending on the shape of the room, and the nature of the available lighting, some sections of walls can be less illuminated than others, making the beam on those less-lit walls seem fatter.

(To both of the above, you might say: "mark to the center of the beam, not the edge!" But what if I don't trust my hand-eye coordination to consistently mark the exact center of the beam at each point of reference that I draw? And what about when the pencil mark ends up just a skosh off of where your hand actually intended it to show up?)

3. How perpendicular is the laser to the wall you are projecting to?: Suppose you are projecting a plumb beam onto a wall , to mark the center of some feature that you are going to hang. (Perhaps a vanity mirror, that you have also marked the horizontal center of.) If your laser device is not perfectly perpendicular to the wall you are trying to mount the thick framed mirror on, there will be a parallax error between where the beam hits the wall, and hits the mirror frame.

4. How flat is the wall you are projecting to?: even if your laser level device is perpendicular to the wall -- when projecting a level beam, if your wall is out of plumb more on the left than it is in the right, the projected beam will appear to curve slightly upward (or downward) in the region that is out of plumb.

So I guess my overall question is: in residential finish remodelling, how do professionals decide what is an acceptable level of precision, and how do they achieve it in an imperfect house, with an imperfect environment, even with a laser level?

Answer 1

A good technical response by @martinbonnersupportsmonica but I’m going to broach the psychological side of the question.

In short, good enough is usually good enough. Do your best. Check your cabinet hanging at the last minute with a good quality spirit level (cough, cough stabila). Check that level against itself regularly. Things that want to be dead level/plumb: cabinets, doors, windows.

That said, making something look natural in its setting is important — more important than making it micrometer plumb. This means cheating with trim where you can, or splitting the difference when faced with existing conditions that aren’t perfect.

For example, a plumb upper cabinet butting into an out of plumb wall will have a terrible reveal. The solution might be a wide filler strip that tapers and cheats the eye.

Don’t fret about accuracy so much as visually pleasing results. You might know that something is off by a tiny bit, but almost nobody else will. And eventually, you’ll forget as well.

Answer 2

A quick search for "visible out of plumb" found this page, which is a detailed list of acceptable tolerances https://www.premierguarantee.com/insite/tolerances-in-internal-and-external-walls:

• Flatness of internal wall: maximum 5mm offset from a 2m straightedge;
• Plumb of internal wall: max 10mm out of plumb for a ceiling height of 2.5m or less;
• Level of ceiling: max 10mm over 2m;
• Squareness of corners: max 15mm deviation from a 500mm square.

If you are not comfortable with metric, 5mm is about 3/16th inch and 10mm is about 3/8ths. So not only should you not fret about a 1/32th, you shouldn't even fret about 1/8th.

I think the context of the page is "when will a building guarantee pay out", so these are quite generous tolerances — but it gives some sense of what is acceptable.

Answer 3

It's only a problem when it's a problem, and every structure has flaws.

• A relatively flat floor that's out of level 5/8" over 40' isn't a problem. No human can detect that. A wall that's out of plumb 5/8" in 8' is a problem. It can bee seen, and it probably affects many other things it relates to.
• A cabinet that's hung 1/8" out of level isn't a problem... unless it connects to a 20' series of cabinets which will magnify that to an inch.
• A roof deck with a 5/8" dip in one truss is a problem. It'll look terrible. A roof deck with a 5/8" swale over 6 trusses probably isn't noticeable and doesn't affect anything else.
• A drywall corner with a rounded-out tape joint isn't a problem... unless you're fitting a granite countertop that you'd rather not round out to match.

Every situation is different. There's no simple answer, and carpentry is as much art as science. It's mostly wood, after all, so no matter how precise you strive to be it'll confound you. That isn't something you can control. You'll have to be ok with that or go build computer processors instead.

Answer 4

The Six Month Rule

I always go for 120% accuracy, then settle for 80% when all is said and done.

The remaining gap will close back to 100% satisfaction all on its own in 6 months, when I am busy using the space for what it was intended and forget about all the flaws that nobody else sees either.

The Beholder Rule

For DIY work there is no hard standard. Level and aligned to the eye, smooth to the touch, and no glaring mistakes in sun light. If these criteria are not met it doesn't matter how accurate the work is. I use spirit levels, a laser level and numerous attempts at dry-fitting to ultimately arrive at a final install.

For Pro work there are precise standards because unhappy customers will find any flaw to complain about or to use as settlement leverage. No matter how great and smooth it all looks, if there is a technical flaw there is technically leverage.

Answer 5

Regarding the worry that the laser line thickness will be different at one place than the other: It won't be, or not enough that you can tell. Laser light is emitted as a collimated (i.e. parallel) beam rather than the diverging or focused-at-specific-distance beam you would get from most light sources. There will be some loss of that characteristic as it goes through the cheap level's optics, but generally not enough to matter.