People have been using pressure treated wood for about 70 years. However, we still know very little about such popular outdoor building material. It is a softwood lumber, which is chemically treated so that it can resist decay, rot and termites. Chemical preservatives are forced into the wood fibers. Such kind of woods are very good option for building fences, decks, sheds, swing sets, picnic tables and many other outdoor projects.
However, if you consider different varieties of pressure treated lumber then they are not created in a similar way. Their resistance power usually depends on what kinds of preservatives and chemicals are used. Some of the lumbers are not meant for ground contact and hence they must only be used above the ground level.
Therefore, while making an application of these pressure treated woods, you need to check its label or stamp for its strength and types of chemicals used. It is expressed in terms of numbers and higher this number, more resistant the wood is.
Literally it may sound simple, but there is fair amount of confusion while using such pressure treated woods. Most of the experienced users of such woods can often get confused and cannot say how this wood may change with time and environment.
This confusion is further enhanced when industry stopped using CCA, which is chromate copper arsenate for residential use. The CCA was introduced during 40s and used as an effective preservative. CCA was discontinued due to various health concerns because of chemicals and since 2003 it has been totally stopped. However, for docks, telephone poles and in large scale commercial projects, CCA is still used.
If you are using it for above ground use then CCA retention level should be 25 while for ground contact it should be 40. Retention level may also vary based upon the preservatives used.
Nowadays pressure treated woods are treated with many different inorganic chemicals rather than by using any arsenate. Few other chemicals used are
- Copper Azole
- Copper Quaternary
- Sodium Borate
- Micronized Copper Quaternary
Such newer treated woods are little less toxic but they contain higher levels of copper and hence it can be more corrosive as compared to CCA treated limber.
Some of the manufacturers of pressure treated woods recommend using only hot dipped galvanized screws, nails, anchors, bolts and connectors while using this material. Since these chemically treated woods can be corrosive to aluminum, it is better to go for vinyl or copper flashing in a protective rubber membrane.