Deck Post Lumber Selection and Construction: Ensuring Durability and Safety
Here on our website, we’ve looked at several different types of deck installations that have shortcomings or failures in the layout and construction, as examples to learn from to show proper building technique versus improper construction. In recent weeks, we’ve looked at some of the framing elements of the deck structure joist systems and today we’re going to also take a look at wood post species, wood preservative treatments and post bases.
Like many rear decks here in Washington DC, these buildings are laid out in a row home type configuration, built side-by-side. The deck at the rear of the building is connected back to the rear facade of the building where a ledger board, part of the deck joist system, connects to the building. These Ledger boards are generally bolted through and into the rear facade.
At the rear part of the deck, since it is farther away from the building itself, it cannot connect to the building directly and needs another method to support the weight of the deck at the rearmost end. In this case, three posts support a girder that runs perpendicular to the remainder of the deck joists. That girder is built, with a vertical flat edge that runs upward and connects to the bottom of those deck joist.
The deck boards themselves, then run perpendicular to the deck joist. The weight of those deck joists and the live load of the deck, the weight of the people and furniture who use the deck, is transferred down through the girder to the three posts below.
You may notice that the girder is not built with just one single joist, but instead built with a double (sandwiched ) laminated pair of joists which together have a much greater strength than a single independent framing member.
These deck posts don’t actually connect to the ground itself, normally, they connect to footings installed and are largely buried underground. If you look at the right side of the deck, you can see, in the picture above, and then more closely in the next photo below, the post on the right hand far side is significantly out of plumb. It is out of kilter at an approximately 9% degree angle.
The post should have simply been anchored, with a base plate that is bolted to the substrate footing. This problem, here, which now could lead to the collapse of the deck, could have been easily avoided if a proper base plate would have been installed.
These posts are made from 6×6 PT SYP framing lumber. PT SYP is a common industry abbreviation for pressure treated Southern Yellow Pine. Untreated Lumber can be harvested and milled for use, from a variety of different types of tree species. Some of the most common species include Douglas Fir, Hem Fir, and Spruce-Pine-Fir (often referred to as SPF). Pressure treated wood, as needed for deck posts like this, are generally made from Southern yellow pine.
Southern Yellow Pine (SYP) is commonly used in the milling and treatment processes for pressure-treated lumber and then used accordingly in deck posts because of its natural properties which have a tendency for a degree of rot resistance and treatability. Its structure of interconnected cells with tiny pores allows for deep penetration of preservatives. This relatively deeply penetrating preservative makes this species wood more resistant to decay and insect damage.
While the porous structure of SYP wood allows for relatively deep penetration of preservatives in the pressure-treating process, it also makes wood more susceptible to water absorption. When exposed to moisture, especially at the cut ends where there is less chemical absorption, water can eventually seep into the wood fiber through these pores. This moisture can eventually also lead to expansion of the wood fiber matrix which leads to continued exposure or increasing exposure through successive rain and precipitation events and in turn cause the growth of fungi and bacteria, which cause decay and rot.
To help to prevent accelerated rot and deterioration it’s helpful to treat the cut ends of lumber with a wood preservative. This type of treatment has to be applied in the field, because even when treatment is applied in the manufacturing process, it can’t be applied deep into the center of the boards , especially where cuts are made in the field because only at that point, in the field can the cut areas actually be accessed and treated.
This will help seal those newly exposed pores which will help to reduce the amount of moisture the wood can absorb later when exposed to the elements. Additionally, using pressure-treated lumber for components that will be in direct contact with the ground can further protect against decay and insect infestation.
Here though in this specific case, the post bases should have been used, both to connect the post to the footing below and to separate the post from the ground where it will otherwise wick up more moisture than it should. That direct contact, which should be separated or broken with a spacer in the base plate, could be prevented. We’ll look at more photographs of similar issues and talk more about these issues in this coming week, here on our website.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities. You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
