Since most fabric sites categorize and describe fabrics by structure first and then fiber content, we'll go ahead and make the bullet points fabric structures and then list the commonly available fiber contents. Laminates (mainly PUL) and durable water repellent (DWR) treated materials are the only waterproof materials suitable for diapermaking. However, the DWR materials will only remain waterproof for the number of washes specified by the manufacturer, so they are best for diapers that will not receive regular laundering, like swim diapers. There are a number of waterproof materials that aren't suitable for diaper making, so if you don't see it listed here, it is either very new or probably not suitable for diapers for a variety of reasons, usually durability. There are many water resistant materials that work for diaper making, but some of them require special treatment such as lanolizing or hand washing. These include animal fibers like wool and cashmere, as well as some polyester fabrics. Some water resistant fabrics can be obtained by upcycling garments and other items, and these should always be washed and tested before cutting.
A special note about vinyl: most vinyl is not durable enough for diaper use. It does not hold up to laundering well, it tends to rip at the stitch lines under stress, the seams have to be sealed with either waterproof seam tape or a silicone based sealer, and most of it is stiff and uncomfortable or thin and fragile. Much of the material labeled "vinyl" is PVC, which releases chlorine and other compounds at body temperature or sometimes even lower, like room temperature. I personally do not feel that is safe for use next to the skin or for any baby products. Here is the Material Safety Data Sheet for PVC. There is a newer "vinyl" that is PEVA/EVA, which does not have or release chlorine, but it is even less durable than the PVC in most cases and may still release unwanted chemicals. Here is the MSDS for EVA.
- Laminates - Generally, a knit or woven fabric joined to a plastic film by heat and/or adhesive processes under high pressure. Polyurethane is one of the most commonly used plastics for this, because it is flexible, relatively sturdy under stress, self-healing with sufficient damp heat, and not known to release any harmful compounds under normal use. There are some PUL (polyurethane laminate) and TPU (thermoplastic polyurethane) materials that are FDA approved for food storage use. A wide range of fabrics can be laminated with polyurethane, but the best for diaper making are polyester based. This includes the commonly available polyester knit PUL as well as other polyester fabrics like minky, the second most common. Cotton based PUL has an increased risk of wicking and delamination. 1 mil polyester PUL is the most commonly available and most commonly used in diaper making.
- Interlock and sweater knits - Medium to heavy weight knits are the most likely to be water resistant. Look for very tightly knit or fulled/felted fabrics, or full/felt them yourself. Some polyester and acrylic knits are moderately to very water resistant even without water repellent treatments, so wash and test these before cutting. Wool and other animal fibers such as cashmere and alpaca are moderately to very water resistant when lanolized, and wool interlock yardage is reasonably commonly available online. Animal fibers may also be upcycled. More info on animal fibers below.
- Fleece - Medium to heavy weight fleece is most likely to be water resistant, and it must be tightly knit. If you can see through it, even just pinholes, it is not likely to be water resistant enough for a water barrier. Water resistant fleece is available in polyester, DWR polyester, and DWR nylon. DWR treated fleece is usually waterproof for the number of washes specified by the manufacturer, and moderately to very water resistant after that. However, very thin DWR treated fleece (100 wt or microfleece, for example) may eventually become wicking rather than water resistant! At this time, it is best to wash and test a sample of any polyester fleece before buying a lot or cutting into the yardage, because many previously water resistant brands are being produced with a much looser knit that may not be water resistant. Wool and other fiber fleece is uncommon.
- Wovens - any weight woven can be water resistant if tightly woven enough or treated with DWR materials. Smooth wovens are more likely to be water resistant. DWR treated wovens of any fiber, including normally absorbent materials like cotton, will be very water resistant to waterproof for the number of washes specified by the manufacturer. However, they may eventually become wicking or absorbent as the DWR treatment washes out. Some polyester and acrylic fabrics are water resistant even without DWR treatments, so wash and test before cutting.
- Other knit fabrics - any weight knit can be water resistant if tightly knit enough or treated with DWR materials. DWR treated knits of any fiber, including normally absorbent materials like cotton, will be very water resistant to waterproof for the number of washes specified by the manufacturer. However, they may eventually become wicking or absorbent as the DWR treatment washes out. Some polyester and acrylic fabrics are water resistant even without DWR treatments, so wash and test before cutting.
Here is more information about wool and other animal fibers. Also see the upcycling for woolies post. Wool is absorbent, wicking, and allows for rapid evaporation, as well as being moderately to very water resistant when lanolized. It is less absorbent when lanolized, but still allows for rapid evaporation and wicking, plus the lanolin reacts with urine to make a soaplike compound that makes lanolized woolies self cleaning to a degree. To get the "magic" properties of wool, the fabric should have at least 50% of the animal fiber as wool. Other animal fibers, such as cashmere and alpaca, are more hairlike, less absorbent, less wicking, hold more heat, and do not hold lanolin as well as sheep's wool. However, they still work for diaper covers, just be aware that other animal fibers may not full/felt and will usually need to be washed and lanolized more often than wool.
Look for items with at least 80% animal fibers (wool/merino/lambswool, cashmere, alpaca, angora from goats or rabbits, mohair, exotic varieties like opossum), no plant fibers (cotton, bamboo and other rayons, ramie, linen), and less than 10% nylon. Up to 20% polyester, acrylic, or silk generally still lanolizes well, although the higher the silk content, the more likely it is for the cover to feel damp on the outside. Anything that feels soft enough to you will be fine, and lanolizing will make it even softer.
It isn't necessary to felt thick or firmly knit/woven materials at all, but you may want to gently full/felt in warm or hot water on the permanent press cycle with a small amount of detergent for ease of care. If you are upcycling or the fabric you are prepping is fuzzy, put each item or about a yard or two of fabric in a large pillowcase or sweater bag. If you have a top loading washer with an agitator, you can and should check every 5-10 minutes during the wash cycle to see how much the items have felted. You may need to add tennis balls or a pair of sneakers to increase the agitiation if little or no fulling occurs in one cycle. It's very possible to felt something, especially a loosely knit or woven item, so much it loses all its stretch! I usually use my front loading washer, the permanent press cycle, and hot water.
Sew with 100% polyester or wool thread, and use a stretch stitch or hand finishing stitch like the mattress or baseball stitch used to finish handknit items. Lanolize before use; I like to run a sinkful of very hot water, dissolve a large dab of nipple lanolin in it, and put the inside-out woolie in that until the water cools. Roll up in a towel to gently squeeze out the excess water, and lay flat to dry.