Flats, prefolds and fitted diapers require a cover to be waterproof. Covers are generally made from either PUL, wool, or fleece.
PUL or TPU covers
Many diaper covers are made from a thin, thin, water-resistant fabric such as PUL or TPU. (PUL is short for polyurethane laminate, and TPU stands for thermoplastic polyurethane). These fabrics don’t absorb liquid, but they do provide a water-resistant backing over flats, prefolds, or fitteds. They also provide an extra layer of protection against the dreaded poop-splosion. To use a PUL or TPU cover, simply put on the diaper underneath, and then snap or Velcro the cover on over the diaper. While some covers come in multiple sizes, most covers nowadays come in just two sized — newborn, which fits babies from about 5 – 16 pounds, and one sized, which fits babies from around 10 pounds till potty training. Many covers that are coming out nowadays are “tuckable”, meaning they have two flaps on the inside to hold an insert. You can fold a flat or prefold into a rectangular shape and simply lay it inside the cover. This is a fantastically quick, easy, and absorbent system.
Wool covers may seem intimidating at first, but they’re a great choice for day or night use. Wool is antimicrobial, somewhat self-cleaning, water-resistant, and can absorb 30 percent of its weight in liquid. Wool covers can be machine or hand-knitted wool, made from interlock (felted wool) or upcycled from wool sweaters. For many people, a wool cover over flat or fitted diaper is the most bulletproof overnight option. Wool can come in a soaker (cover), shorties (short pants), or longies (long pants). Some people use wool shorties or longies as clothing or pajamas. Wool needs to be hand washed and lanolized, and can take a day or two to dry. If it’s only wet, you can go 2 – 4 weeks between laundering. However, if needs to be washed if it gets soiled. Because of this, many people choose to start wool after their baby is out of the newborn phase, since newborn poop is runny and more likely to get on covers. See our page on wool care for an overview of washing and lanolizing wool.
Fleece is water resistant and easy to use. Unlike wool, you can throw fleece into the washer and dryer, making it a nice option for newborns. However, unlike wool, you do need to wash it with every use. Like wool, fleece comes in soakers, shorties, or longies and can also be used as clothing or pajamas. Fleece covers are usually made from anti-pill or windpro fleece.