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Fresh PT wood arrives at the lumberyard green (in color) right from the factory, but over time weathers to a gray color. Some stain and sealing products will not work on new PT wood because of the high moisture content caused by the manufacturing process. See our article on PT wood in the article index for more info... https://www.naturalhandyman.com/iip/infxtra/infpre.html
If your deck is over a year old, it has sufficiently dried out, so you can use any exterior stain or preservative with confidence, whether it is intended for PT wood or not!
Cedar is a fine natural decking product and many people prefer it for aesthetic reasons. Cedar, like the other naturals decking material redwood, does not shrink appreciably or warp. It has a long life and a natural resistance to insects and fungus. However, this resistance is not permanent and the actions of the sun and weather will eventually take a toll on the exposed surfaces. Therefore cedar decking should have an annual application of a clear sealer for maximum life. Of course, it can also be stained or painted.
If you would prefer a tougher decking with a longer life than cedar, two options are pressure treated yellow pine or one of the synthetic decking products such as TREX or plastic lumber. Pressure treated wood decking can be purchased in two thicknesses… nominal 2" thickness (actual 1 1/2") or 5/4" thickness. Pressure treated (PT) wood will not rot BUT should be sealed annually to protect it from the weather and the sun. Over time, unprotected PT wood tends to crack and splinter without this added protection. Since PT wood can shrink and twist as the preservative evaporates… fresh PT wood is installed while still somewhat "wet" with preservative… it is not uncommon that a few deck boards may need to be replaced within a few years of installation. This twisting can actually pull nails and split boards! To minimize these problems PT wood should be sealed immediately after installation with a special PT wood sealer. This slows down the evaporation and thus minimizes shrinkage. If you plan of using a stain or paint, it is wise to give the surface at least six months after the initial sealing for proper adhesion.
TREX boards, available in 5/4" and 2" thickness', are composite products made from 50% recycled plastics and 50% real wood fibers. This combination makes TREX somewhat more stable than plastic lumber (at a given thickness) and more durable than any 100% wood product since it is virtually impervious to moisture. TREX is not rigid enough to be used as a structural support… it is strictly for decking purposes. TREX does not require any preservative or treatment and weathers to a gray tone. Though not necessary, it can be painted or stained if desired… but this will add to your maintenance chores! TREX is available at most lumberyards and some home stores.
Plastic lumber is made from 100% recycled plastic and is available 1" and 2" thickness' for decking, and is also available in a 1" tongue-and-groove style. Plastic lumber is not only available as decking… an entire deck can be built from plastic lumber components! Plastic lumber is totally impervious to moisture so no sealing is necessary. It is available in a wide range of colors, which is good because it does not hold paint or stain well. Finally, since there is no toxic component as with PT wood, it can also be used indoors in showers, damp basements, etc. One downside is that this product is less widely available than TREX or the other wood products.
One critical issue in choosing a retrofitted decking material is the "center-to-center" (c-t-c) distance between the supporting floor members, or joists, of your existing deck. Every decking material has a maximum c-t-c distance for adequate strength without excessive flexing. Cedar, PT wood and TREX in 5/4" thickness have a c-t-c maximum of 16. 2" thick TREX and PT wood can be installed with a maximum c-t-c of 20". Plastic lumber of one inch thickness requires a maximum of 12" c-t-c because it is not as rigid as PT wood or TREX. However plastic decking is available in 2" thickness that can be used on 16" centers. Keep in mind that if your decking is diagonal, you may need more reinforcement under the decking since the actual center to center may be 20" or more.
Another critical difference between these products is in the installation. Though all products can be installed using screws or nails, there are differences in each product's reaction to temperature changes. Cedar does not shrink or expand significantly so board spacing during installation remains constant. Pressure treated wood tends to shrink as the moisture from its chemical treatment evaporates, so most installers leave no gaps between the boards… shrinkage will take care of that! Both TREX and plastic lumber will expand and contract with temperature changes, so allowances have to be made in positioning the boards to allow for these changes… or else the decking may buckle! The temperature during installation is also important… installation at low temperatures requires more initial spacing than installation at high temperatures.
All the decking products mentioned can be installed with decking nails or screws, though the high-tech pullout-resistant nails used for PT lumber are unnecessary for the other products
By blocking do you mean support under the deck or actual blocking between the joists to stabilize and straighten them? If you mean blocking between the joists, it won't do any good since the frame will tend to twist to conform to the ground without extensive support underneath. If you mean support under the deck every 16", that may be a little overkill.
You could use 2x4's for this deck, but I would advise against it. The deck will be much more rigid and stable with a 2x6 frame. And frankly, I just like the appearance of a 2x6 frame... plus it is a nice "step" height... especially if you slightly bury it to blend with the ground. With the 2x6 frame, you really only need to support underneath the center of each 10' joist or at about 6' and 12' for the 17' joists. Even that is a little overkill, since a 2x6 frame with 16" centers can span over 9' safely. However, since this type of ground-level platform has a distributed load rather than spans, the extra supports are good!
A trick I have used to make easy, solid supports under ground-level deck joists is to dig out about a square foot hole about 4 - 6 inches deep under each support point and fill the hole with concrete. Make a well around the joist with dirt so the concrete height is slightly above the bottom of the joist. The concrete will expand slightly when set making a strong, stable support pad. For the perimeter of the deck to keep the concrete concealed at least on the outside of the platform for aesthetic reasons.
The "natural" in "Natural" Handyman refers to talents, abilities and aptitudes, not philosophy (like a "natural athlete"). I am a repairer/writer, not a writer/writer. If I tried to run a home repair business that did not use toxic chemicals, I would not have a business… at least outside of the most affluent areas where folks can afford the luxury of "natural" products. Lubricants, solvents, adhesives and paints virtually all have a dangerous component. Without them, it would be impossible to do the repairs that people want done every day. Most any product can be toxic when misused. It is our self-education in the wise use of what we have that determines our true environmental nature.
Commonly used "green" pressure treated lumber indeed uses a toxic chemical preservative, CCA. Countless studies have been done on this product, and none have convincingly shown that the lumber itself has any toxic effects. This is because the chemical does not leach in any great quantity from the wood. I have written about this in an article on pressure treated wood at the site, https://www.naturalhandyman.com/iip/infxtra/infpre.html. You will find links to other sites on this topic there, also.
In brief, most of the concerns and dangers surrounding PT wood arise in the process of cutting it, when sawdust is released into the air and onto the ground... not in its use while installed. Also, the use of pressure treated wood is limited to outside applications with only a few exceptions. It should never under any circumstances be ingested, made into toys, used for food preparation surfaces or burned in a stove or fireplace! There are some limited uses allowed for interior construction, such as for permanent wood foundations, floor joists, subfloors and in the framing of high moisture areas. Local building codes and requirements apply, of course. There are guidelines on its use available at various websites in our Links Library as well as in the aforementioned article on pressure treated wood.
With the spread of the extremely desructive underground Formosan termite after WWII in the South, many states are requiring increased usage of treated woods in new construction. Louisiana, for example, has suffered millions of dollars in damage from this wood-hungry invader. Unlike the subterranean termite, the Formosan termite can make nests above ground making control much more difficult.
Concerning stains, I am unaware of any commonly available consumer-applied EXTERIOR stains or preservatives that are truly non-toxic. There is, however, a second type of pressure-treated wood that uses a copper-based preservative, ACQ. I am currently gathering information on this product and should have an article on the topic available before the summer.
There are a few INTERIOR stains available that are of low toxicity. One downside to them is that they are much less durable than the more common solvent-based products. Some paint stores do carry them, but if you can't find anything locally visit the Oikos website and use their search engine to find the products you want.
If you happen to discover a product that I should know about, please drop me a line… I'll will pass on the information!