- Q. On more than one occasion, I have installed 5/4x6 pressure-treated wood decking with the growth rings facing down. We screwed the boards down with stainless steel screws, but the edges still curl and warp very badly. How can this be prevented?
A.Chris Donnelly responds: Wood tends to shrink and swell about twice as much along its growth rings as it does across those rings. To see how this effects deck boards, let me illustrate two scenarios:
In the first, think of a regular flat-sawn board with the rings forming long arcs along the end of each board. As the board dries, the growth rings contract along their length. The longer rings will shrink from the shorter rings. Effectively, the rings are trying to straighten out, which cups the board towards the bark side of the tree. If the board is picking up moisture, the reverse happens. The individual growth rings expand along their length, with the longer rings expanding more than the shorter rings. This causes the board to cup towards the inside, or pith-side, of the tree.
In the second scenario, consider a board with one face toward open air and sunlight, while the other face is toward moisture coming up from the ground with poor air circulation under the deck. When one side of the wood is dry, and one side is wet, the dry side will tend to shrink and the wet side will expand. As a result, the board cups, regardless of the orientation of the growth rings.
If the problem is related to the first scenario, try using drier material, such as dried-after-treatment pressure-treated lumber (DAT) or kiln-dried material (KDAT). Or, choose vertical-grain material. I would not recommend laying out the boards to dry before installation, unless those boards are well constrained. The boards will still cup, leaving you with less usable material.
If the problem is related more to the second scenario, take steps to equalize the moisture differences on each face of the deck. Shading the deck, designing a deck with lots of underside ventilation, or even using a moisture barrier and gravel beneath the deck will all help.
Regardless of which scenario causes the problem, it’s critical to finish and maintain the wood. Proper finishing helps slow the gain and loss of moisture, thereby minimizing cupping. Apply a water-repellent or penetrating stain while the wood surface is dry, or choose pressure-treated wood with a preapplied water-repellent. In either case, apply a new coat regularly each year.
Chris Donnelly is a wood technologist and consultant in Northford, Conn.