Assembling stretcher bars seems a little mysterious the first time you do it. The bars don’t seem to be able to meet at the corners, because at first they don’t look cut at an angle. But as soon as you know the process, it is really very simple to put them together.
Stretching your own canvas allows you to control the quality of materials and customise the size of your stretched canvas. You begin the process by assembling your stretcher bars into a frame on which to stretch your canvas. Jackson’s stock three sizes of wooden professional stretcher bars (the profile is measured by depth from the wall): 43mm, 21mm, 18mm. The profile you choose will depend on what you want it to look like, how you want to frame it, and also the strength you need because larger canvases may warp from the pulling of the canvas if the bars are not heavy enough.
What they look like
When your stretcher bars arrive in the post they look like this. Left to right: 18mm, 21mm and 43mm.
The 18mm and 21mm bars may seem to be flat on one end so it looks like they won’t fit together, but don’t worry they will.
You can assemble them without tools, but a mallet (or hammer and scrap of wood) and a measuring tape or stick help.
Also – They come with little wooden wedges. The wedges (or keys) are used later on, after you have stretched the canvas over the bars, to add a bit more tension at the corners, before you start painting. Read more about canvas wedges.
The width of the bar is cantilevered (slanted downward towards the centre of the canvas) so that the canvas doesn’t rest across the whole of the width of the wood but only touches the outside rim and floats above the rest, so when you paint the bar shape will not imprint on the canvas.
You can see that bevel in the profile of the bars here.
The corners have slots cut into them so that they fit together at a right angle and make strong corners.
The 18mm and 21mm bars have one end of the bar (A) with a slot in it that looks square at the end and the other end (B) has an angle cut and a thin piece of wood meant to insert in slot A of the other bar. At first they may seem not to be able to fit together.
(The 43mm shape is the same on both ends of the bar and it less confusing.)
Fitting them together
Fit them together by making sure a (B) fits into an (A)
Keep the bevelled slope of the bars to the inside of the square of your canvas frame.
Fit them together by inserting (B) into (A) while insuring that all bars slope to the inside of the square.
After you have slotted all your corners together, and inserted any centre bar you might be using, then use a mallet to tap them fully tight. They slot together tightly so although you might be able to fit them together by wiggling them with your hands, you will probably need a mallet for the last bit of insertion. If you use a regular hammer instead it will dent the wood, so you will want a small scrap of wood between your hammer and the stretcher bar to protect the bar. Dents in the bar might show as dents when the canvas is tightly stretched ever them.
(Centre bars provide extra stability for larger sizes. Both CSP18 and CSP21 have a slot for a centre bar starting at size 30 inches, CSP43 has double slots starting at size 80cm. The double slots allow a centre bar in each direction, one crossing over the other.)
The final step before stretching canvas on the bars is to insure your stretcher frame is square. One way to do this is to measure the diagonal measurement in two directions (like an X), them wiggle and adjust your square until the two measurements are the same.
Here is a video that Lisa did earlier that may help and in which she shows a method of using a CD case to check the square of the assembled bars instead of a measuring tape.
After you have assembled your bars the next step would be to stretch your canvas over the stretcher bar frame you have just assembled.
Click on the underlined link to go to the current offer on Stretcher Bars on the Jackson’s Art Supplies website.
Postage on orders shipped standard to mainland UK addresses is free for orders of £39.
Hi, I’m trying to assemble the stretcher bars
I bought on order 3729520 and the centre
bars seem much too short. Have I ordered
the wrong thing or have you sent the wrong
thing? Thank you very much.
Hi Jackie
The first time you assemble them can be confusing.
Centre bars are smaller than the stretcher bars because they fit inside the rectangle, whereas the stretcher bars are sized based on the outside of the rectangle they produce. They should be the same length at the bar they are parallel to minus the width of the two bars they are attaching to. If you go ahead and assemble your stretcher then what I am saying will be clearer.
You could also take a look at this comprehensive article Everything You Need to Know About Stretcher Bars