Should the underside of a deck be stained?

Question

When one stains a wooden deck, should they stain the underside of the deck too?

Answer 1

This question is somewhat opinion based, but I'm going to answer anyway. If it's just a stain or oil, rather than a more solid surface/coating finish like Varathane, you're probably OK to just stain one side.

With a solid coating, the problem is the wood will absorb moisture at different rates on the top of the deck from the bottom. This can lead to cupping, warping because wood "moves" with moisture. If you are in an area that's dry most of the time, that would be less of an issue. Here in Western Washington, where we get a lot of rain and it can be quite humid at time, it's a consideration when finishing wood that will be outdoors. Personal note: I would never again build a wood deck here, too much maintenance and short life span.

EDIT: Giving credit to JimmyFix-it's comment, I'm adding to my answer.

Using a solid surface coating like Varathane or Urethane, etc. usually ends up being a disaster in just a few years if the wood is directly exposed to weather (rain, snow, sun), even without seasonal wood movement. It will crack, peal, split and to re-finish you have to sand most of it off! Penetrating oils also require maintenance every few years, but often just a good cleaning followed by application of the oil. If you want that "new wood" look, it might require a light sanding first, but much easier to deal with than a solid surface coating. Products generically referred to as "wood brightener's" can also be used to remove stains, mildew, etc. but I find that a light sanding works pretty well.