Composite Doorway Sills 

Below most exterior doorways there is a kick panel (door sill) installed to support the bottom of the door frame and provide a finish to the bottom of the doorway. The dooe sill material is ¾ inch in thickness and extends the length of the doorway. The width is usually 6 inches that extends to the door stoop or deck. Flashing is installed behind the kick panel to protect the material underneath from water damage. Usually the door sill or kick panel is cut from  pine or pressure treated wood. Pressure treated wood is better than plain pine wood because it is resistant to moisture. Regular pine wood will deteriorate in about ten years where pressure treated pine will withstand the moisture for at least twenty years .

Pressure treated pine is being replaced by composite lumber which is much better than pine or pressure treated wood. Composite lumber is totally resistant to moisture and will last for decades. Composite lumber is primarily made from recycled wood products and plastics. The manufacturing of composite lumber is one of the prime uses for recycled plastic containers and waste wood products. A resin is used to bind the material together. It is used to produce many different building products such as decking, door and window frames, exterior siding and, moldings. Composite lumber does not include toxins that are used in pressure treated wood. The composition is usually a 50/50 mixture of waste wood products and plastics. The density of composite lumber is higher than regular lumber but is also more flexible.

Eventually the kick panels beneath your doorways will require replacing. I recommend that you use composite doorway sills rather than pine or pressure treated wood. To replace your sills simply remove the old material and install the composite sill material which can be obtained from most home supply centers. It comes in various thicknesses and lengths. It can be cut using a standard skill saw with a fine tooth blade. It is easy to work with and unlike regular wood it will last for decades. 

See:

Composite Siding

Composite Wood Decking