Square but not Equal…Help me Bro!
Alright…got started on a 22×22 carport today…measured the diagonal and adjusted actual jobsite dimensions for the posts until diagonals were the same. Set and plumbed all posts, cut off at correct height, and installed double 16″ LVL beams on top. Went to measure ceiling joists and one side was 2″ longer than other.
We ended up pushing both end posts on the long end about 3/4″ in (out of plumb) toward each other to “fix.” The joists are hanging over on the short end and work out toward the long end. Looks like crap!
I made my pattern rafter (the roof is a simple 4/12 gable) and just took the middle between the short and long sides…run is 10-11 1/2. I’ll see how they fit from one end to the other (and if the ridge runs level) tomorrow.
I just dont understand how if the diagonals were equal I was 2″ longer on one end than the other, unless I accidentally made an “exact” trapezoid. In any case I don’t think it will shoe in the finished product but the lack of perfection really upsets me on a project so straitforward.
Replies
Did you try squaring it up by creating a right triangle?
You could use a 15' 20' 25', the largest variation of a 3'4'5' that will work with your dimensions?
Congratulations, you made a perfect trapezoid. I've done it, too!
You should have measured with diagonals and 3-4-5 triangles.
Live and learn -
Did you square it up by using the 3,4,5 method (Pythagoran theorem), or just pull diagonals? Sounds like you did what you suspected, & created a trapezoid if you just pulled the diagonals.
Any chance of moving the posts in from the corners so that the beams cantilever past each corner post a bit? This might help mask the difference.
One more thought... Is this being built on a new slab? If so, is the slab close to square?
So, all your legs are the same length and the diagonals match. That's at ground level.
Did your plumb bob sway in the wind?
No bob? Misread the vial on your level?
LVL's as beams - are they setting plumb on top of the posts? A slightly unsquare cut on the top of a post will cause the LVL to cant. Doesn't take much to compound errors.
Yeah, the equal cross measurements technique only works if you have a parallelogram to begin with -- you gotta make sure that opposite sides are equal first. And it's easy to screw up if you don't get the measurements to the same relative reference points all-around.
It sounds like you set the posts just from the diagonal measurements, without checking the distance to adjacent posts. No telling what shape you got that way.
Hey can you measure the posts at bottom? It sould be 22' x 22' at the bottom and at the top. Plus the dianglonal would be 31' 1 3/8". If the bottom of posts is wrong maybe it is not to late to adjust. Posts out of plumb is hard to hide, but making ridge and overhang strait is very important. I am building a 22' x 22'8" carport, and i used 4" steel posts that I will wrap with some trim? Use a string on your overhangs and make it look good to your eye. A old saying in these woods is straiter then a wedding dick. LOL
Measuring the diagonals is the second step in squaring up. You have to measure the length of the walls first. Opposite walls have to be the same length. You won't be square if you measured the diagonals without checking the wall length.
The 3 4 5 method is extremely touchy. A 1/16" can make a big difference in accuracy. It's best to make the measurements as large as possible, such as 15' 20' 25'.
The best way is to calculate the hypotenouse for the building, (aa) + (bb) = (cc)
If you don't go back and fix the problem, it could effect the placing of plywood and roofing material. You don't want to have to cut the plywood on an angle or have the roofing lines going on a slant.
Beat it to fit / Paint it to match
Yes, I believe I made a perfect trapezoid. Joists were hanging over on one end, and flush on the other. Rafters started out great at the short end but ended up having to tack OSB "extensions" to the plumb cut on the rafters at the other end. (Please don't rag me about this--I know it's crappy!) We got the ridge level, and with all the trim inside the beams I don't think the plywood ceiling will look crooked. I know now that I should have went with the original dimensions I got off the slab and created a right triangle. I guess that's what I get for being in a rush.
Anyway, we got felt on the roof and the gable end sort of framed up ( a lot of high wind code stuff has to be done to gables here in AL) so we'll be installing the subfascia and Hardi trim tomorrow...roof will be done Sat.
Thanks for the assurance, and I'll remember this next time!
Jason, there's no such thing as a 'perfect trapezoid'. You made just any old trapezoid. Check it out: Take two sticks of equal length and cross them. These are your diagonals. The ends are the four corners of your building. Now slide the sticks back and forth and up and down. Notice that there are two degrees of movement and that the intersection has infinite points. So do the corners, and hence the shape of the building.All that is to say that there are infinite possible trapezoids for any set of equal diagonals. But only one is a rectangle. That's the one with opposite sides being equal and a right triangle at all four corners. So first you measure up your equal sides, then you square up the corners (3-4-5 method or just use your calculater and the pythagorean theorem). Voila, a 'perfect rectangle' every time.Next time Jas, do the figuring and the research BEFORE you build ;-).
Trying to fix the mess you made after the fact is usually a disaster, sometimes impossible, and always shoddy. Plus it makes you look totally incompetant.
Here is your perfect trapezoid. Three sides at 22-0, one at 22-2, top adjacent angles equal, but different from, bottom adjacent and equal angles.
With geometry like this, basically a whacked-off isoscoles triangle, you get equal diagonals as shown.
Calculation for the diagonal of a true 22 x 22 square, yields a diagonal of 31-1 11/32", rounded to the nearest 32nd.
My diagonals were 361", but I was measuring from inside to inside on my posts...I snapped lines 5 1/2" in from the edge of the slab to get all my post bases in line.
jason.... if the diagonals were equal.. and it is not square, then one or more of the side lengths is wrong..
can be a result of layout creep, or an error in measuring..
when i started out .. i could easily get a 40'-0" house to build out to 40'- 3".. it was like falling off a log..... oh yeah... the diagonals were right on
Ok so now that I know the cause of my problem, I gotta say that I did this job differently than when I normally set beams; I set the posts first and then set the beams because I didn't know if I could get any extra manpower to heft those double 16" LVLs. Turns out I borrowed three extra guys, so I could have set the beams first (like I normally do), squared and leveled them, and then cut the posts under them. That's what I get for trying to change my ways LOL.
I doubt anyone will ever realize that the thing is out of whack a little bit, but I did lose some sleep over it the other night. Of course, being so young in this game, I don't have the luxury of years of experience. Thank goodness for you guys here at BT.