Objection 1. It would seem that venial sins are unsuitably designated as "wood," "hay," and "stubble." Because wood hay and stubble are said (1 Corinthians 3:12) to be built on a spiritual foundation. Now venial sins are something outside a spiritual foundation, even as false opinions are outside the pale of science. Therefore, venial sins are not suitably designated as wood, hay and stubble.
Objection 2. Further, he who builds wood, hay and stubble, "shall be saved yet so as by fire" (1 Corinthians 3:15). But sometimes the man who commits a venial sin, will not be saved, even by fire, e.g. when a man dies in mortal sin to which venial sins are attached. Therefore, venial sins are unsuitably designated by wood, hay, and stubble.
Objection 3. Further, according to the Apostle (1 Corinthians 3:12) those who build "gold, silver, precious stones," i.e. love of God and our neighbor, and good works, are others from those who build wood, hay, and stubble. But those even who love God and their neighbor, and do good works, commit venial sins: for it is written (1 John 1:8): "If we say that we have no sin, we deceive ourselves." Therefore venial sins are not suitably designated by these three.
Objection 4. Further, there are many more than three differences and degrees of venial sins. Therefore they are unsuitably comprised under these three.
On the contrary, The Apostle says (1 Corinthians 3:15) that the man who builds up wood, hay and stubble, "shall be saved yet so as by fire," so that he will suffer punishment, but not everlasting. Now the debt of temporal punishment belongs properly to venial sin, as stated above (Question 87, Article 5). Therefore these three signify venial sins.
I answer that, Some have understood the "foundation" to be dead faith, upon which some build good works, signified by gold, silver, and precious stones, while others build mortal sins, which according to them are designated by wood, hay and stubble. But Augustine disapproves of this explanation (De Fide et Oper. xv), because, as the Apostle says (Galatians 5:21), he who does the works of the flesh, "shall not obtain the kingdom of God," which signifies to be saved; whereas the Apostle says that he who builds wood, hay, and stubble "shall be saved yet so as by fire." Consequently wood, hay, stubble cannot be understood to denote mortal sins.
Others say that wood, hay, stubble designate good works, which are indeed built upon the spiritual edifice, but are mixed with venial sins: as, when a man is charged with the care of a family, which is a good thing, excessive love of his wife or of his children or of his possessions insinuates itself into his life, under God however, so that, to wit, for the sake of these things he would be unwilling to do anything in opposition to God. But neither does this seem to be reasonable. For it is evident that all good works are referred to the love of God, and one's neighbor, wherefore they are designated by "gold," "silver," and "precious stones," and consequently not by "wood," "hay," and "stubble."
We must therefore say that the very venial sins that insinuate themselves into those who have a care for earthly things, are designated by wood, hay, and stubble. For just as these are stored in a house, without belonging to the substance of the house, and can be burnt, while the house is saved, so also venial sins are multiplied in a man, while the spiritual edifice remains, and for them, man suffers fire, either of temporal trials in this life, or of purgatory after this life, and yet he is saved for ever.
Reply to Objection 1. Venial sins are not said to be built upon the spiritual foundation, as though they were laid directly upon it, but because they are laid beside it; in the same sense as it is written (Psalm 136:1): "Upon the waters of Babylon," i.e. "beside the waters": because venial sins do not destroy the edifice.
Reply to Objection 2. It is not said that everyone who builds wood, hay and stubble, shall be saved as by fire, but only those who build "upon" the "foundation." And this foundation is not dead faith, as some have esteemed, but faith quickened by charity, according to Ephesians 3:17: "Rooted and founded in charity." Accordingly, he that dies in mortal sin with venial sins, has indeed wood, hay, and stubble, but not built upon the spiritual edifice; and consequently he will not be saved so as by fire.
Reply to Objection 3. Although those who are withdrawn from the care of temporal things, sin venially sometimes, yet they commit but slight venial sins, and in most cases they are cleansed by the fervor of charity: wherefore they do not build up venial sins, because these do not remain long in them. But the venial sins of those who are busy about earthly remain longer, because they are unable to have such frequent recourse to the fervor of charity in order to remove them.
Reply to Objection 4. As the Philosopher says (De Coelo i, text. 2), "all things are comprised under three, the beginning, the middle, the end." Accordingly all degrees of venial sins are reduced to three, viz. to "wood," which remains longer in the fire; "stubble," which is burnt up at once; and "hay," which is between these two: because venial sins are removed by fire, quickly or slowly, according as man is more or less attached to them.
Source: Summa Theologica (New Advent)