Stretcher bars are the wooden frame a canvas is pulled over, and the two things that matter about them are depth and timber. VanArtHub uses kiln-dried bars cut to each piece at its Davie Street workshop in Vancouver. Depth decides whether the bar stays straight under tension and how the edge reads on the wall. Kiln-dried timber decides whether the frame itself stays straight through a wet winter.
Depth, and why it is not just a look
A stretcher bar is under constant inward pull from the fabric. A shallow bar spanning a long distance bows toward the centre, which slackens the canvas at the middle of that side even though nothing has happened to the fabric. The longer the span, the deeper the bar has to be, or the more it needs bracing.
| Longest side |
Typical depth |
Bracing |
| Up to about 60 cm |
Shallow, around 2 cm |
None needed |
| 60 to 100 cm |
Medium, around 4 cm |
None usually needed |
| 100 to 150 cm |
Deep, 4 to 5 cm |
One cross-brace |
| Over 150 cm |
Deep, 5 cm or more |
Cross-bracing both ways |
Depth is also an aesthetic decision when the piece hangs without a frame. A gallery wrap on a shallow bar reads as thin and slightly unfinished against a wall. On a deep bar it reads as an object. If a piece is going to be framed anyway, depth is purely structural and the shallowest bar that stays straight is the right one.
Why kiln-dried matters here
Timber that has not been kiln-dried still holds moisture, and it releases that moisture unevenly over the following months. The bar twists. A twisted bar lifts one corner of the painting off the wall and puts a permanent ripple across the fabric that no amount of keying will remove, because the problem is the wood and not the tension.
In a climate with a long wet season this is not a small risk. Cheap ready-made bars from a craft supplier are frequently not properly dried, which is why a canvas bought stretched on them often looks fine for a year and wrong afterwards.
Keys
Keys are the small wooden wedges tapped into the inside corners. They are the reason a stretcher exists rather than a strainer. Tapping them in pushes the mitred corners very slightly apart, lengthening the whole rectangle by a millimetre or two and taking up slack in the fabric.
Two things about them. They should be tapped in stages, a little at each of the eight positions in turn, never one corner all at once. And they should be tied or taped to the bar, because a loose key that falls behind a hanging painting rattles and, on a large piece, can press against the back of the fabric from the inside.
Cross-braces
A cross-brace is a bar running across the middle of the span, and its only job is to stop the outer bars bowing inward. It sits slightly below the plane of the stretcher so it never touches the back of the canvas, because a brace in contact with the fabric prints a visible line through to the front over time.
Related
See how a canvas is stretched for the process, framing large art for what happens once a big piece is finished, and floater frames for canvas for the usual way a stretched piece gets framed.
Common questions
What depth of stretcher bar do I need?
It follows the longest side. Up to about 60 cm a shallow bar is fine. From 60 to 100 cm use a medium depth. Past 100 cm you want a deep bar and a cross-brace, and past 150 cm bracing in both directions. The reason is that a long bar bows inward under the pull of the fabric.
Why does kiln-dried timber matter?
Timber that has not been dried properly still holds moisture and releases it unevenly over the following months, which twists the bar. A twisted bar lifts a corner off the wall and puts a permanent ripple in the fabric that keying cannot remove, because the fault is in the wood rather than the tension.
What are the keys in the corners for?
They are wedges that push the mitred corners very slightly apart, lengthening the whole rectangle by a millimetre or two and taking up slack in the fabric. There are eight, two per corner, and they should be tapped a little at a time in rotation rather than one corner all at once.
Should keys be fixed in place?
Yes, tied or taped to the bar. A loose key that works its way out rattles behind the painting and, on a large piece, can press against the back of the fabric from the inside and print a mark through to the front.
What does a cross-brace do?
It stops the outer bars bowing inward across a long span. It sits slightly below the plane of the stretcher so it never touches the back of the canvas, because a brace in contact with the fabric prints a visible line through to the front over time.
Are ready-made stretcher bars from a craft shop any good?
They vary, and the usual weakness is that the timber has not been properly dried. That is why a canvas bought stretched on cheap bars often looks fine for a year and wrong afterwards. In a climate with a long wet season it matters more than it would elsewhere.
Does bar depth change how the painting looks?
Only when it hangs without a frame. A gallery wrap on a shallow bar reads thin against a wall, while a deeper bar makes the piece read as an object. If it is going to be framed, depth is purely structural and the shallowest bar that stays straight is the right choice.
Can old bars be reused when re-stretching?
Sometimes, if they are straight and the corners still move. Lay the frame on a flat surface and see whether all four corners touch. If one lifts, the timber has twisted and reusing it puts the same ripple straight back into the painting.
