What Fabrics To use?  
Waterproofing Fabrics
The inside of the pad can have a leakproof liner that works like a disposable pad does - in helping prevent the blood from soaking through the layers of the pad and onto your underpants. Some women do not need this, and some do. It depends on how heavily you bleed and how your flow is naturally distributed over the surface of the pad. If you tend to bleed in one small area, or "gush" then you may find that you soak through a pad too quickly if it's not waterproofed. A Water resistant fabric isn't completely waterproof, but will form a layer that is more resistant to leaking than the other fabrics in the pad.  
Most women are concerned about the "breathability" issue of waterproofing. Comparing a waterproofed cloth pad to a disposable isn't the same. A waterproofed cloth pad isn't going to be as sweaty as a disposable. The issue of letting the skin breathe is a debated one. If you suffer from thrush or excessive sweating then you might want to avoid synthetic waterproofing, and opt for something more "breathable".... but even with "PUL" its not like having a piece plastic underpants on.  
The other natural fibre layers in the pad will allow for some airflow through the pad. In a "turned and topstitched" pad, the waterproofing only comes partly up the side giving plenty of surface for airflow, and an overlocked/serged type pad has the sides (cross section) open too. Airflow doesn't have to be just from between the legs up. Some women feel a difference in a waterproofed pad over a completely natural one - others don't. The more open weave a fabric is, the more "breathable" it will be... but that also means less waterproof. So it is a trade off between airflow/natural fibres and leak protection.  
Which points do you consider most important  
- Thickness (do you want the pad to be as thin as possible)
- Waterproof or water resistant (if you have a lighter flow you may like to use something water resistant)
- "Breathability" (do you have problems with thrush or want a lot of airflow)
- Natural Vs synthetic
- Comfort/worry (Do you want to stress about leaking through a pad)
Some common waterproofing fabrics are:  
- PUL - This is a fabric (polyester or cotton) that has been coated on one side with a thin film of waterproof plastic (polyurethene).. so PUL stands for PolyUretheneLaminate. Its considered to be "breathable" - as the waterproof membrane lets no water through but will allow a little air through. This doesn't mean you can breathe through it - it means that according to the industry standard, it allows a certain amount of airflow. Just enough to lessen the sweatiness compared to a PVC or other non-breathable fabric (like what is used in disposables). It was designed for uses such as raincoats, where the fabric is designed to allow airflow to prevent the wearer getting too sweaty inside the coat. Pads with this should be basically waterproof. Some people don't like this as they feel the pad becomes as sweaty/non-breathable as a disposable pad and others find no sweatiness and can't tell the difference with a non-waterproofed pad. A lot of women need it to prevent leaks and make them feel secure - even if only on heavy days. As it is thin itself, and using it requires less absorbent layers, PUL waterproofed pads can generally be a lot thinner than other pads. The polyester version is most commonly used as it is thinner/softer than the cotton version. It can break down over time, and should not be washed with vinegar or chemicals that can break down the waterproofing. They should also not be put in the dryer or ironed. Examples of brands - 'procare', 'fabrite','gore-tex'.
- Fleece - Polyester fleeces (Microfleece or polarfleece) don't like to hold moisture, so can make a reasonably effective water-resistant layer on the bottom of the pad, as the blood tends to stay in the core rather than seeping through it. As the fabric is a more open weave there is more "breathability" than PUL, but its waterproofing is not as effective, so may not be suitable for a heavy flow. Some pad makers use fleece as a bottom layer for its non-slip effect, and some use fleece over the PUL for this reason. The thicker the fleece, the more water resistant it will be.
- Nylon - "Ripstop" nylon or other forms of nylon fabric can be used, though these are less water-resistant than fleece is. They are thin fabrics and rely on the fact that the fabric itself doesn't hold moisture and is a tight weave, so the blood is less likely to travel through. I would not recommend this for a heavy flow though, as it is only somewhat water-resistant.
Other - Other options I have seen are:  
- Waterproof table cloth type fabrics - I've found from past experience that waterproofed tablecloths do work well, but can have a more crunchy/crackly type of feel, and in a longer pad you might be able to actually hear that as you move. Most likely to be PVC, a non-breathable fabric.
- Waterproofed mattress protectors - Not really sure how these go, might be thicker than PUL - most likely a PVC or other non-breathable fabric.
- Umbrella fabric - I'm not sure of the composition, but I believe it would be a form of nylon - which would be less water resistant than a PVC tablecloth. Since as an umbrella works by the raindrops sliding over the fabric, it's not designed to prevent water from seeping through such as you would use it for in a pad. However, it would presumably work much like a store bought nylon, perhaps a little better. Some people use this.
- Wool - The natural option here is wool..... I don't know much about using this in waterproofing a pad however. Wool will actually absorb a little, and when the wool has lanolin in it, it will repel liquid. The wool used in pads seems to be a knit (perhaps slightly felted) and can usually be machine washed but may need special care instructions (I've read it can be quickly damaged by enzimes in some washing detergents)
In my humble opinion if you want waterproofing, its best to stick to some type of PUL (there are several different brands of this), as these are generally thin and quiet in a pad and work well. If you are after water-resistance (or refuse to use PUL), then Fleece is probably the best bet.
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