Built-Up Posts vs. Solid: What You Need to Know
The question we are asking here is whether or not built up post or stronger than solid posts. The simple answer is no. Solid post are stronger than built up post, in most cases. We mentioned that that condition or response qualifier “in most cases” because it’s not a compketely a cut and dry issue.
Like so many of the case studies and topics that we analyze here on our website, this is another one where there’s some black and some white but lots of gray. We’re going to look at pictures today of a exterior rear deck, built on the back of a freestanding home, where the deck is supported with vertical post made from compiled assemblies of three individual 2×6 boards. In this particular case, the posts could have simply been made instead with 6×6 lumber. That’s a more common and typical methodology and it would have been much stronger more effective here as well.
In the picture below, you can see the long run of the girder which supports the majority of the frame of the deck. The joist attach to a ledger, back at the exterior face of the building and perpendicular joists run out from that point slightly beyond the top of the girder. Another section in a trapezoidal shape extends further out, cantilevered yet further beyond the girder but additionally supported at its outer edge.
Judging solely by the post and girder layout shown below, it looks like the deck was built in multiple sections. Like many if the root floor plans of homes here in Washington DC built in the early 20th century, this deck has had sections added on and expanded throughout the years. The section shown below to the left was probably added more recently.
Typical joist spacing for deck floor systems is most often at or around 16 inches. Sixteen inches is a good increment for splitting most standard mill length boards. In other words, a typical 8 foot board, for example, has to have joist that are coincident with both ends of the board. The first and where the board starts, is always generally coincident with the outside edge of the deck. In most cases that’s the side rim joist. That deck board then runs perpendicular to the joist and at an 8-ft increment would terminate or end at the 7th joist.
Seven joist bays or seven sections of the joist themselves and the intermediary space between them, at 16” center spacing of the deck joists, would be equivalent to 112 in. However an 8-ft board length, for example, would only be 96 in and the way it actually works is that with seven joists, there’s only six entire bays between them. So the total width of the joist bays spacings would be 96 inches in six bays and would use seven joists. However, the 7th and final joist in this assembly would be split between the end of the first 8-foot deck board and the next one in the same linear run.
The girder is spaced near the end of the deck, but it’s actually carrying more than half of the dead load, more than half of the net weight of the deck.
You may notice that at the end of the deck, at the outside corner which runs in line with the rear side corner of the house, there is a angled chamfer or “dog ear” that is intentionally built into the deck. This essentially rounds off what would otherwise be a square or rectilinear shape at the outside corner of the deck. Instead of a 90° corner there, it’s an assembly of two 45° Corners that create an eased or somewhat near rounded type of edge. This type of corner could also be referred to as a mitered corner.
Here you can see a closer view of the deck post. At the top, you can notice that there is a gap where tgectgree laminated or assembled and sandwiched boards are separating or pulling away from one another. In some cases, the building code allows for built up assemblies of multiple boards joined together to create a collective load path that is almost equivalent, but not normally as quite as strong as a single individual larger timber or board.
In those cases like this though, the building code requires that multiple fasteners be installed to laminate or adhere these boards together. Particular types of rated fasteners are required such as bolting or structural screws and exterior grade adhesive. In this case it looks like only minimal fasteners have been applied and an insufficient or possibly omitted adhesive between the individual boards.
We generally recommend that when multiple boards are laminated together to form a unified built-up structural member, they be liberally covered, with almost an extremely excessive amount of adhesive. Adhesives are expensive, especially high quality exterior grade adhesives, but we consider it money well spent because the entirety of the surface between the board should really be coated and covered with adhesive so all areas of contact are firmly glued together.
In this coming week, we’re going to look at additional pictures of this deck and discuss additional reasons why some of the structural capacity requirements are important for maintaining the overall load path and why particular phasers are required for laminating and why individual boards generally have greater strength, when sized correspondingly to laminated smaller individual lumber elements.
We recommend building and homeowners here in Washington DC use a contractor, like us, who understands and cares about quality, deck and patio longevity, snd simply doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios, you can give us a can or just drop us a line. We are happy to help with almost all steps of the deck and patio 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
