Thread: Novice needs help building a simple arch from 3 pieces of wood

Hi Brian. Best method is to use a 3:4:5 triangle (AKA Pythagoras theory). Measure from the inside corner of where the horizontal and vertical timbers meet out along the horizontal piece and make a mark at 30cm. Measure from the same spot down the vertical 40cm and make another mark. Now measure the distance from mark to mark. When this is 50cm the corner is square. You will need to attach the intersecting timbers to eachother to ensure things don't move and affect your measurements, however you will have some difficulty maintaining their "squareness" within design because the joins will be able to pivot. And using 1.9cm timbers does not allow for multiple fixings. A half lap joint would help.

Welcome to the forum. You don’t mention what the arch is for and why it has to be absolutely square but I suspect it will be easier to make it square than keep it that way while you instal it. There will be a lot of flex in the stakes and the point are is tiny. There is a huge amount of leverage.
But leaving that aside, one option for perfect layout with minimal equipment is to use basic Pythagoras mathematics. Get some strong twine and use it to mark out 3 - 4 - 5 triangle. If you don’t know about Pythagoras google it. The longer the sides of the triangle the more accurate you will get.

Good luck

Bruce

You could measure the diagonals, both need to be the same. Keeping it square when you move it is more of an issue I would think..
Cheers,
Mark

Another variation of Pythagorus is 5, 12, 13. or multiples thereof.

5 on one side of the right angle, 12 on the other and diagonal will be 13 if the angle is correct at 90 degrees.

Originally Posted by Brian33

Hi all,

I am trying to build an arch like in the picture below, and my question is, how can I construct it so that it is perfectly square and parallel?

The arch is made up of 3 long pine stakes, 1.9cm (0.7") thick, and 170 to 200cm (66.9 to 78.7") long, with the shortest stake placed on top of the other two stakes which will be screwed in at both intersections. The horizontal stake will be 1.9cm away from the end of both vertical stakes (you can see a bit of the blue stakes peeping out).

what you actually have sketched is more correctly known as a portal frame, not an arch.

Others have described how to use a 3-4-5 triangle to ensure the corners are square.

To keep the corners square while the frame is erected, you could reinforce the corners with MDF "plates" that can be removed after the frame is in place.


The size of the sticks you plan on using suggest that buckling of the vertical (blue) sticks will possibly be an issue for you.
Especially if the sticks are carrying any vertical load.


Also, unless attached to a wall, the frame will not be very stable.

All the above advice is good but if we knew what you were aiming for we could perhaps give more detailed advice. In general a rectangle needs 2 sides to be the same length and 2 ends to be the same length then if the diagonal corner to corner measurements are the same then the corners will be square. If you have a reasonably sized square Then check the inner corners are 90*. Never mind the sticking out bits.
Regards
John

Mustn't have been too important as the OP hasn't been back since joining and posting on the 26th of Sept.

Brian as mentioned use Pythagoras
But I'd measure width and length and calculate the diagonal over the longest side. More accurate and then no need to be concerned for bowing of that long length.
If you haven’t a calculator use this.(not sure if links work on this site)
Diagonal of a Rectangle Calculator

I'd add (screw or brad) diagonals to your frame to keep square.
One each way,forming a cross

I find it easy to keep open end separated by cutting a piece of timber exact inside dimension at top and putting that in gap at floor.
Next. Is the base it sits on.
Is that dead level?
Have some thin spacers on hand to elevate the lower end.
Vinyl, icecream or other food container scraps

Hope that makes sense