Splice in a Deck Rim joist

How to Splice a Deck Rim Joist: Strengthening Your Deck’s Structure

Today we are looking at a particular case study that defies structural logic and is simply an incorrect method of deck carpentry and building principles. The picture below shows a deck built onto an existing row home. Like most decks in this type of configuration and like most buildings in Washington DC, this deck is built off the kitchen at the rear of the ground floor of the building. The arrow in the picture below points to a seam where a section was added on to extend the deck farther to the left.

When you look closely, you can see an incongruence and a subsidence at the joint between this added on section and the original deck. The original girder runs from right to left and almost reaches this new part of the deck, but the newer extension is not supported by the girder at all.  The girder itself is not installed properly as well. The gutter like this in most cases should be built from a doubled or joined 2x material.  Essentially, at the side, the main deck had a rim joist that ran perpendicular from the ledger, passed the existing girder and the new portion of the deck, the smaller added on section, also has a new separate rim joist and these two rim joists join together without a proper structural connection.

However, even at this connection, the sistering or joining of these two separate joists, under each of their individual systems of decking, do not join properly. They’re not connected with significant structural hardware or connectors. At least hurricane clips or similar type of joining hardware should have been used at the girder, for example, but that hardware has also been omitted.  If you look at the outer rim joist, parallel to the ledger, but at the outer side instead of the inner side, there is a single galvanized carriage bolt at the left side which supports the intermediary guardrail post.  No significant hardware has been installed at the other post, that’s insufficient.

The next picture below shows a closer view of the joint between the original outer rim joist and the new section of framing. Among a list of other defects in the construction, it also looks like the carriage bolt here is electroplated galvanized steel instead of hot dipped galvanized steel.  Hot dipped galvanized steel has improved performance characteristics for resisting deterioration in exterior installations.

Proper deck construction, here in Washington DC, requires specific fastener types, installation methods, and connecting hardware to ensure structural integrity and connectivity of the various elements in the load path of the deck. 

The building code requires several provisions for deck connections which have been omitted or under-applied in this case:

1. Ledger board connections attach the deck to the house, and must use at least 1/2-inch lag screws or through-bolts, installed in a specific pattern based on the deck’s dimensions. These fasteners must penetrate the building’s rim joist and be properly flashed to prevent water infiltration. Hot-dipped galvanized, stainless steel, or other corrosion-resistant materials are required for all exterior fasteners and hardware.  The electroplated galvanization is not sufficient.
2. Beam-to-post connections must utilize approved post caps, shouldering, or sufficiently rated connecting hardware. Simple toe-nailing is not acceptable. 
3. Post bases must be anchored to concrete footings using approved anchors or cast-in-place J-bolts. Posts should be secured to the beam using galvanized through-bolts or specialized connecting hardware, not just nails. We’ll look closer at this in an upcoming article, but here they installed the post into the underground footing which technically does not reach or break the particular rule as stated yet has other downsides which we will describe next week.
4. Joist hangers are required wherever joists meet beams or ledgers. These must be rated for exterior use and sized appropriately for the lumber dimensions. Each hanger must be filled with the correct number and size of fasteners as specified by the manufacturer. When joists are spliced, they must occur over a beam or bearing point and use appropriate connecting hardware.
5. Guard rail posts need to be securely fastened to resist the required loads (typically 200 pounds in any direction). This usually requires multiple through-bolts or specialized mounting brackets. The connection must be engineered to transfer loads into the deck’s main structure, not just the deck boards or rim joist.  Here, it looks like that connection is missing.
6. For lateral stability, diagonal bracing or structural screws are required to prevent rack and sway. Modern deck construction often requires lateral load connectors at the house connection to prevent the deck from pulling away from the building. These typically consist of hold-down tension devices or similar approved hardware installed at prescribed intervals.
7. All fasteners and hardware must maintain appropriate edge distances and spacing to prevent splitting of lumber, and pre-drilling is often required for through-bolts and lag screws. When connecting different sections of a deck, such as in additions or extensions, proper sister joisting techniques with multiple fasteners and connecting plates are necessary to ensure structural continuity.

In the next picture below, looking alongside the outer rim joists, you can see a deflection at the joint between the original section of the deck in the new section of the deck.

In this upcoming week’s blog, we’re going to take a look at another case study of a deck intentionally built with an off-centered intermediary post underneath the main girder. In that particular condition though, unlike the case shown here in today’s blog, the off-centering meets the requirements of the building code and is used for a beneficial configuration of the deck.

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