While the following advice will be most helpful to clients who are building or planning a new deck, it can also be helpful to understand why many common deck care problems occur on existing decks.
UNCOATED END GRAIN AND BACKSIDE MOISTURE INTRUSION
It is the nature of wood to absorb and store moisture through natural design. Capillaries and cells of wood are simply nature’s waterways, and when exposed to moisture, dry wood will seek and absorb water. Moisture intrusion into wood is predominately through saw cuts and exposed end grain. In a very simple test, you can take a piece of deck lumber and stand it on end in about an inch of water. Within 12-24 hrs, moisture elevation will be measurable up to 3 feet into the length of the board.
Unsealed end grain on saw cuts leads to a legacy of costly maintenance, and splitting and cupping of deck lumber. As wood dries, it shrinks. Any change in moisture content to various parts of the board will cause expansion and contraction cycles to the wood that lead to cracking and splitting.
Seal all end grain before and during installation.
Seal backside prior to construction, as many times, underside is inaccessible after construction.
If you make a saw cut, re seal the exposed end grain.
Backside Coated wood prior to instalation.
HIGH MOISTURE WOOD
Most cupping and splitting is the result of rapid drying of wood, as decks and structures are constructed of wet lumber. The more rapid the wet to dry cycle, the more pronounced splitting and cupping will be. Most of the wood that is used in new construction is high in moisture content, and improperly handling that issue can lead to a lifetime of various deck care issues and costly problems.
Warping and cupping usually are caused by uneven shrinkage between the top surface and the bottom underside surface of deck lumber. The cupping of individual boards is aggravated because the top surface is usually at a lower moisture content because of exposure to the elements compared with the protected bottom surface. This means that deck boards installed wet will likely warp the most, especially when installed during hot months.
Wet wood must be dried in a controlled fashion, by racking and stacking wet wood for a few weeks prior to construction. Stack your lumber with 1 or 2” spacers between layers, boards 1” away from each other. Even if indoor storage is not available, stack the wood on any flat surface (a couple of wood pallets would do), then spacer the wood for air movement, and cover with blue tarp or other covering. Be sure to stack some weight on top of the stack to keep the drying boards flat. Dry to under 20% moisture range, and you are then ready to pre-coat the back side, followed by end-grain coating as saw cuts are made during construction. More...
When wood is planed (smoothed) at the sawmill, resin, sap and sawdust is “polished” onto the surface of the wood. This mill glaze will repel most wood coatings, and if coated over, can lead to a legacy of maintenance issues. Mil glaze is apparent if you can look at an angle across the wood, and you see a shiny surface. You can also test by splashing water on the wood, and seeing if it soaks in or holds out.
Do not stain over mil-glazed wood. While mil-glaze can weather off over time, the best approach is to remove by washing, and in some cases, sanding.
Wash with TSP or a strong oxygen bleach cleaner and scrub then sand where appropriate. Sanding is not advised for some treated lumber.