Do You Need to Stain the Undersides of a Deck?
Bringing up some debate is whether or not you should stain the undersides of a deck. It is common and advisable to stain the weather-exposed surfaces of a deck to protect them from water and sun damage. But do the undersides of a deck also need protected?
Protecting the underside of a deck from the sun is not an issue since the underside of a deck is always shaded, but what about protection from moisture? Moisture is absorbed into unprotected wood causing it to swell. Once the wood dries out, it shrinks back to its normal size. Over time this constant swelling and shrinking causes deck boards to warp, crack, and split.
Obviously, the exposed upper side of a deck is much more susceptible to moisture and water damage caused by rain and even morning dew. Although the underside of a deck may get a little wet during a good downpour, it certainly is not as likely to get wet.
The dilemma with applying stain to the underside of a deck is time and the cost of doing it. In some instances it is almost more work than it is worth. Staining the undersides of boards can be very labor intensive and will double the stain cost of the deck-staining project.
There is also some debate about moisture being trapped if all sides of deck boards are stained. This varies by opinion but all boards should be able to breathe. Whatever the case, it seems that just staining only the top exposed surfaces of a deck make the most sense.
One acceptable reason to stain the undersides of a deck would be for appearance especially on an upper level deck where the underside is more noticeable.
In the experience of most deck staining professionals, staining only the exposed surface of a deck is adequate in protecting the wood. If you want some of the beams, posts or more exposed underneath portions of the deck stained strictly for appearance then that is up to you. But as far as protecting the wood, it is not necessary to stain the underside areas of a wood deck.
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