Private property is not something demanded by the natural law for we do not find in things the nature of being owned by particular individuals. Rather we find that things are good for men generally rather than a particular man. Therefore, it is in the nature of things that they are for the good of men that is the common good.
Now private property comes as an addition to the natural law as an invention of human reason and as competent to humans as rational. This in virtue of the necessity of private property for its better use and the peace of society. It is natural for men to possess things in virtue of their reason and it is necessary for the common good that they procure and dispense with them as their own possession.
Aquinas states: “The second thing that is competent to man with regard to external things is their use. On this respect man ought to possess external things, not as his own, but as common, so that, to wit, he is ready to communicate them to others in their need.” That is a man makes use of things that he possess for the sake of the common good, ready to provide to those in need. This needs some more thinking on my part.