Awning Keder, Welt and Valance Rails: How Awning Fabric Attaches and How to Size It
On most retractable awnings, the fabric is attached by a keder (a bead sewn or welded into the fabric hem) that slides into a groove running the length of the roller tube, and a second keder at the outer edge slides into the front bar. The valance hangs from its own keder in a separate front bar groove. To replace fabric or a valance, you match the bead diameter to the groove and the flap thickness to the slot opening. This page explains the parts, how to measure them and what goes wrong.
Key takeaways
- Keder is the bead; welt is the US term for a corded hem that does the same job. Both lock fabric into a C-shaped groove.
- Measure two things: bead diameter (must fit the groove's inside) and flap or hem thickness (must pass through the slot opening).
- Common keder bead sizes on residential awnings and RVs run from about 5 to 9 mm (roughly 3/16-3/8 in); there is no single standard.
- Most retractable awnings use a front bar with two keder slots: one for the main fabric, one for the valance.
- The welt seam is the highest-stress line in the cover. Torn fabric along the roller tube is usually a pitch, tension or wet-rolling problem, not a bad keder.
What keder and welt are
A keder (the word comes from German) is a flexible bead with a flat flap attached. The bead may be solid or hollow PVC, a PVC-coated rope, or a polyester cord. The flap is welded or sewn to the fabric edge. In a groove with a narrow slot, the bead cannot pull out and the flap passes through the slot to the fabric.
A welt or awning rope hem is the traditional US version: the fabric edge is folded around a cord (cotton, polyester or a PVC rod) and sewn closed, so the fabric itself forms the flap. RV awning fabric is often attached this way into an awning rail on the coach side.
Both do the same job as screen spline but slide in from the end rather than pressing in from the face. That is why you feed fabric into a keder groove like threading a needle.
Where keder grooves appear on an awning
- Roller tube: the fabric's back edge sits in a groove along the tube. Some tubes use tape or a clamping strip instead. The tube itself is covered in torsion bar and roller tube.
- Front bar: the fabric's outer edge slides into a groove in the front bar (also called the lead rail or front profile). Details on that extrusion are in front bar and shoulder brackets.
- Valance slot: a second groove on the front bar, usually lower or on the face, holds the valance.
- Wall rail (RV and some fixed awnings): a keder rail screwed to the wall holds the fabric's back edge, with the roller at the front.
- Drop valance housing: some front bars contain a small roller with its own fabric attached by keder, which drops a vertical shade below the bar.
How to measure keder and the groove
- Bead diameter. Measure the bead with calipers across its round section, without squeezing. If it is sewn into fabric, measure the finished welt including the fabric wrapped around it, because that is what must fit the groove.
- Flap thickness. Measure the flap or the doubled fabric just beyond the bead. This is what passes through the slot.
- Groove inside diameter. Measure inside the round part of the groove at the end of the extrusion, using the inside jaws of calipers or a set of drill bits.
- Slot opening. Measure the slot width (the gap between the lips). It must be larger than the flap thickness but clearly smaller than the bead.
- Length. Measure the groove's full length, end cap to end cap, and note whether there are end stops that keep the keder from sliding.
| Bead diameter | Approx. inches | Common uses |
|---|---|---|
| 5 mm | 0.20 in | Light valances, small window awnings, some screens |
| 6 mm | 0.24 in | Valance slots on many front bars, RV awning rails with sewn welt |
| 7 mm | 0.28 in | Main fabric on roller tubes and front bars, RV rails |
| 8 mm | 0.31 in | Main fabric on larger patio awnings |
| 9 mm and up | 0.35 in and up | Commercial awnings, tensioned structures, heavy membranes |
Single-flap keder has one flap; double-flap keder has two, which lets two fabric panels join in one groove (common in commercial and tent structures, less so on residential retractables).
How to feed fabric into a keder groove
- Clean and deburr. Run a rag through the groove and check both ends for burrs. File any sharp edges at the entry; one burr can shred a hem over its full length.
- Lubricate lightly. A dry silicone spray in the groove makes feeding much easier. Avoid oil, which stains acrylic fabric.
- Use two people. One feeds the keder into the groove, keeping it straight and the fabric flat, while the other pulls gently from the far end. Long awnings may need three.
- Keep the fabric square. Mark the center of the fabric and the center of the tube, and check that they align when fully fed. An off-center cover rolls unevenly; see awning fabric rolls unevenly.
- Fit end stops. Refit end caps or set screws so the keder cannot creep out of the groove over time.
The full fabric replacement process, including tension, pitch and arm reset, is in replacing awning fabric.
Valance rails and valance options
A valance is a short strip of fabric, typically 6-10 in (150-250 mm) deep, that hangs from the front bar. It hides the front bar, trims glare when the sun is low and gives the awning its finished look. Most residential front bars have a dedicated valance groove, commonly smaller than the main fabric groove.
| Style | Look | Practical notes |
|---|---|---|
| Straight | Clean, modern | Least flutter, easiest to make and replace |
| Scalloped or wave | Traditional | Edges fray faster if not heat-sealed or bound |
| Bound edge | Contrast trim | Binding tape can fade at a different rate from the fabric |
| Drop valance (roller) | Vertical shade below the bar | Adds low-sun shade, adds weight to the front bar and needs its own crank or motor |
| No valance | Minimal | Cleaner on cassette awnings, less low-angle glare control |
A drop valance is one of the most useful awning accessories on west-facing patios, because late-afternoon sun comes in under the main cover. The shade calculator shows how much a given drop adds to the shaded area.
Common keder and welt failures
| Failure | Likely cause | Fix |
|---|---|---|
| Fabric torn along the roller tube | Rolled up wet repeatedly, over-tensioned arms, UV-weakened thread | Restitch or replace cover; dry before retracting |
| Keder creeping out of groove end | Missing end caps or set screws | Refit end stops |
| Hem pulled through slot | Undersized welt cord, slot spread by impact | Remake hem with larger cord; check slot with calipers |
| Valance flapping loose | Valance keder too small for its slot | Remake valance with correct bead |
| Fabric stitching rotted near the welt | Cotton or non-UV-stable thread | Restitch with PTFE or UV-stable polyester thread |
RV awning rails
RV awnings attach the fabric's back edge to an awning rail on the coach wall, with the roller at the front. Rail grooves and welt sizes vary by manufacturer and era, so measure the rail and the old fabric rather than relying on a nominal size. Feeding is the same process: clean, lubricate lightly, keep the fabric square. More on RV systems in our RV awnings guide.
Frequently asked questions
Can I replace just the valance without replacing the awning fabric?
Yes, on most awnings. The valance slides out of its own groove on the front bar. Measure the bead and the slot, and have a new valance made in matching fabric.
What is the difference between keder and piping?
Piping is decorative cord sewn into a seam. Keder is a structural bead designed to lock into a groove and take load. They can look similar but serve different purposes.
Why is the new awning fabric so hard to slide in?
New fabric is thicker than worn fabric, and the welt cord may be slightly larger. Clean and lightly lubricate the groove, deburr the entry and feed with two people. If it still binds, the welt may be oversized for the groove.
Do cassette awnings use keder?
Most do, on both the roller tube and the front bar. The cassette just adds a housing around the rolled fabric.
Can a canvas shop sew a welt onto my new fabric?
Yes, most awning and canvas shops can. Bring the old fabric or a measurement of the groove and slot, and specify the cord size and thread type.
How deep should an awning valance be?
Many residential valances are 6-10 in (150-250 mm) deep. Deeper valances block more low sun but catch more wind and can flutter noisily.
Sources and standards consulted
- Screen mesh specification sheets (fiberglass, aluminum, polyester and PVC-coated meshes) (manufacturer documentation)
- Spline and screen frame channel specifications (manufacturer documentation)
- Retractable screen and zip screen component catalogs (manufacturer documentation)
See how we research for our sourcing and verification process.