Checking for square: Framing tolerance?

[intrepid_dw]

All:

In laying out the wall locations for a storage shed, I'm checking my layout for squareness and need some real-world insight on the tolerances, if any, I should take into account when peforming my measurement check. I'm trying to avoid my inherent tendency to be mathematically anal, as I'm used to working on smaller furniture or shelving pieces where tolerances are more critical.

For a building that is just under 10' square, I have my diagonals matching down to about 3/16" difference. I've squared the corners using a conventional 3-4-5 technique, and each corner shows square.

As I measured my diagonals for the umpteenth time, I started realizing that over a 14-foot distance, it may be a foolish proposition to try to reconcile out those last three sixteenths (heck, I was able to show

1/16" inch difference at one point just in the deflection of one piece of wood from not being absolutely flat, to say nothing of any subtle sags in the measuring tape.)

In terms of specifics, at 117" (9'9") square, my diagonals should be

165-7/16". I have one diagonal to 165-1/2", and the other to 165-1/4". That's one sixteenth one way, three the other. Do I pursue an absolutely perfect diagonal measurement and chase those 16ths, or have I reached the point where it is close enough to press on?

Sorry if this is a naive/stupid question, but I figure I won't know unless I ask...

Thanks, intrepid

[Lew Hodgett]

Get the diagonals with in +/- 1/16" and get a beer.

It is the easiest way to get 90 degree corners.

The earlier you get a good set of measurements, the easier life will be at the end of the project.

Have fun.

Lew

[Doug Miller]

To put it another way: that's 3/16 out of 2647/16, or an accuracy of a little bit worse than one part in nine hundred.

Nobody will notice unless you tell them.

[NoOne N Particular]

If your "square" measurements are exactly 9'9" along the top, bottom, and each side of the same wall, then you should be able to get your diagonals exact.

But having said that, I am getting ready to build a 10x12 shed myself. I'm thinking that if all of my other measurements are exact (for example, exactly

144" width measured on the top of the wall, bottom of the wall, and exactly 96" high on both ends) I am anal enough that I would try for a little while to see if I could nail the diagonal too. I wouldn't spend much time on it though and if I couldn't get it in say 15 minutes, 1/8" would be plenty close enough and I would nail that sumbich down. Probably even 3/16 would be good. Like someone else said, no one would ever know and even I wouldn't remember for long.

Wayne

Wayne

[Tom Veatch]

The percentage difference in your diagonals is only 0.151% - that's less than two tenths of one percent, about 2 parts in a thousand. For my money, you've chased the 16ths enough. Press on! Tom Veatch Wichita, KS USA

[Robert Bonomi]

The closer you are to 'exact' measurments, the easier things are later on.

I grew up in a house built by an ex-Navy Seabee, with help from a bunch of his friends (who were paid mostly in beer). Everything was done 'by eye', if it _looked_ square,that was good enough. This resulted in walls that were as much as 3/4" different in length, if mesured at the floor or ceiling. and a wall that was well over 1" shorter than the one on the other side of the room. Made life _really_ interesting when taking up and relaying perimeter 'shoe', to accomodate carpet. Doing wallpaper with a strong vertical pattern was a *disaster*.

FWIW, your measureents indicate you need to move one of the 'long' corners by about 3/32nds. fix one side firmly in position, measure the diagonals, note which end of the parallel side has the 'long' diagonal, and which has the short one. move it that 3/32nds towards the 'short' end.

*OR*, given that the 'error' is less than 1/8", in "rough carpentry" construction, just "don't worry about it".

Putting things in perspective, your current 'accuracy' is equivalent to slightly better than 1/64" on a 10" measurement.

[Stephen M]

As my Brother In Law (who is usually the anal one) says: "thats why the title of the chapter is called 'rough framing'".

+/- 3/16" and it's going to wiggle more than that when you lift it into place.

Put on your first piece of sheathing before you raise the wall and use the factory edges and nudge to fit.

[HerHusband]

Intrepid,

The rule-of-thumb I always heard was to aim for 1/4" tolerances when doing rough framing, and that's what I used when we framed our house a few years ago.

Keep in mind, a lot of the lumber you use will be warped or twisted 1/4" or more, so trying to obtain more exact tolerances will be an effort in futility. Even if start with PERFECT lumber and make EXACT measurements, the wood will likely shrink or twist at least 1/4" after you're done building! :)

Of course, when laying out some of the basic lines, try to be as accurate as you can. Our house is 40'x40' and I found I couldn't get better than about 1/2" accuracy measuring the diagonal with a normal tape measure. You might be able to get more accurate with a transit or something, but over those distances the differences are almost insignificant.

Starting with a level foundation is also critical. Any errors here, and you'll be fighting it all the way to the roof.

Most things in carpentry/house-building are about dealing with imperfections. Check any room in your house and I can almost guarantee the walls will be out-of-square and/or out-of-plumb. Whether you're hanging cabinets, laying tile, or installing trim, it's all about dealing with the inherent variations in the building.

Believe me, I was a bit "anal" with my measurements when we built our house, and we still have walls that are out-of-plumb and out-of-square.

I think you're reached that point and beyond... :)

Anthony

[Larry W]

IME real-world framing carpenters don't read to 16ths, many don't read to 1/4, and some don't read to 1/2" If you're within a 1/4" I say go for it. Before the sheathing/siding goes on, you can shift a conventionally framed 10X10' structure that much by leaning on it.

[DanG]

Make sure, first, that the dimensions of your sides are accurate.

I expect large commercial buildings to diagonal less than a 1/2", dead on preferred.

Different issue: drop ceiling grid better diagonal at much less than a 1/16.

[jcatora]

Intrepid,

I'm a Registered Architect and I'll tell you how I look at it...

'Anal' probably doesn't go far enough in describing me (I've had tradesmen I haven't seen in 20 years *vividly* remember crossing my path). In this day and age of computers and CAD, you can imagine how I look at dimensional tolerance, etc.

However, one *does* have to be realistic and also consider not only construction time (or, time lost trying for that n'th degree accuracy), but also the final use / function of the space or building item in question.

That said, it's reasonable and realistic to look at it this way:

Try for 1/8" - Settle for 1/4".

You're right in the middle of that ---- as someone else said "Carry on".

-- john.

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