Interfacing weights explained: light, medium and heavy

Interfacing comes in light, medium and heavy weights, and the rule is that the interfacing should be the same weight as the fabric or lighter, never heavier. Lightweight interfacing suits lawn, voile, viscose and blouse fabrics; medium weight suits shirt collars and cuffs, waistbands and facings on poplin, linen and medium cotton; heavy and craft weights suit bag bases, hat brims, belts and structured pieces in denim or canvas. Fuse a 10 cm (4 in) test square to a scrap of your fabric before you commit the real pieces.

Quick reference: which weight for which fabric?

Interfacing weights are rarely printed on the bolt; the shop names are light, medium and heavy, and the numbers above are a guide to what those names usually mean. Hold the fabric over the interfacing on the shelf and let the two hang over your hand together: the fabric should still fold, with more body than before. If it holds a crease like card, the weight is too heavy.

What does interfacing do?

Interfacing is a layer added to the back of a pattern piece to give it body, keep it from stretching, support buttonholes and hold an edge straight. It is cut from the piece it supports (a facing, a collar, a waistband, a bag panel) and fused or sewn to the wrong side before the piece is assembled. It is never seen from the outside, which is why it is easy to overlook, and why the wrong weight is the most common reason a collar feels like cardboard or a neckline facing flops.

How do you match the interfacing weight to the fabric?

Start lighter than you think. The job of interfacing is to support the fabric's own hand, not to replace it: a lawn blouse with a medium interfacing in the button band has a stiff stripe down the front that never softens. A medium cotton with a light interfacing at the neckline is barely changed, which is right for a neckline, and firmer at the waistband, where two layers of medium are better than one layer of heavy.

Three questions settle it:

1. How light is the fabric? Fabric under 100 gsm takes light interfacing; 100 to 200 gsm takes medium; above 200 gsm takes medium or heavy. Reading fabric descriptions explains gsm if your fabric does not state it.
2. What should the piece do? A facing should stay flat and not stretch: light. A collar or cuff should hold its shape: medium. A waistband should not crumple: medium, or a firm woven waistband interfacing. A bag should stand up: heavy or foam.
3. Does the fabric stretch? Knits and stretch wovens take a knit (tricot) interfacing that stretches with them. A non-stretch interfacing on a knit neckline makes a rigid band that pulls the seam.

Woven, non-woven or knit interfacing?

Each weight exists in three constructions, and the construction matters as much as the weight.

• Woven interfacing has a grain and moves like fabric. Cut it on the same grain as the piece. It is the best choice for garment facings, collars and waistbands in woven fabrics.
• Non-woven interfacing is bonded fibers with no grain, cheaper and easy to cut in any direction. In light weights it can feel papery and crack along a fold; in heavy weights it is the standard for bags.
• Knit (tricot) interfacing stretches across the width and has the softest hand. Use it on knits, on drapey wovens such as viscose and Tencel, and anywhere you want support without stiffness.
• Weft-insertion interfacing is a knit with extra threads woven across it: stable in length, soft in hand, used in tailored jackets.

Fusible interfacing has glue dots on one side; sew-in interfacing is basted inside the seam allowances. Fusible vs sew-in interfacing covers the choice; in short, sew-in is for sheers, velvet, sequins, metallic and heat-sensitive fabrics, and fusible for the rest. The glossary entry on fusible explains the glue side.

Which weight for each part of a garment?

• Button bands and plackets on a shirt or a button-front blouse: light on lawn, medium on poplin. The band is usually interfaced on one half only, so that the fold is crisp and the inside layer soft.
• Collars and collar stands: medium woven on shirt fabric; a firmer collar interfacing for a stand that must hold up.
• Cuffs: medium, cut to the outer cuff only.
• Waistbands: medium, or a purpose-made firm waistband interfacing with fold lines marked. A pencil skirt or an A-line skirt with a faced or banded waist needs it on the outer half of the band.
• Neckline and armhole facings: light, even on medium fabrics, so the facing does not add bulk at the edge. Keeping a facing from rolling out depends on understitching more than on interfacing.
• Pocket openings, zip openings, buttonholes: a small strip of light or medium interfacing in the seam allowance stops the opening from stretching.
• Hems of jackets: a light strip the depth of the hem gives a clean, soft edge.

Which weight for bags and hats?

A tote bag in canvas needs a medium interfacing fused to the outer panels, plus a heavy or foam piece in the base if it should stand up by itself; the same bag in quilting cotton needs heavy interfacing on all outer panels and medium on the lining. A zip pouch is fine with medium on the outer. A bucket hat wants medium to heavy in the brim and medium or none in the crown, depending on how soft a hat you want. The notions list of a Sewly pattern includes the interfacing for pieces such as waistbands, facings and bag panels, so you can check the amount before shopping.

How do you fuse interfacing properly?

1. Pre-shrink woven and knit fusibles: soak in warm water for 10 minutes, roll in a towel, hang to dry without wringing. Non-woven interfacing does not need it.
2. Cut the interfacing from the pattern piece. For light and medium weights, cut it the full size and trim 3 mm (1/8 in) outside the seam line; for heavy weights, cut it without seam allowances to keep bulk out of the seams.
3. Place the glue side (the slightly rough, dotted side) against the wrong side of the fabric. Glue side up melts onto the iron.
4. Cover with a damp pressing cloth and set the iron to the fabric's setting, usually wool or cotton for a fusible.
5. Press, do not slide: lower the iron, hold for 10 to 15 seconds, lift, move to the next area with an overlap. Sliding shifts the layers and leaves unfused strips.
6. Let the piece cool flat before you move it. The glue sets as it cools.
7. Check the bond on your test square by trying to peel a corner. If it lifts, press longer or hotter; if the fabric shines, press cooler with a thicker cloth.

Warning: Bubbles that appear after the first wash come from skipping the pre-shrink step or from pressing too briefly. Pressing again from the right side through a cloth often flattens them, but it does not always.

What are the common interfacing mistakes?

Too heavy is the mistake that cannot be undone: a fused interfacing does not come off cleanly. Interfacing that stops inside the piece with a visible ridge on the right side comes from a hard edge on a light fabric; pink the edges or use a lighter weight. Light non-woven that cracks along a fold should be replaced by a woven or a knit. And an interfacing that was not tested on the fabric is the cause of most of the rest: fuse the square first, every time.