Are you wondering if you should use a double rim joist for your deck? In most cases, a double rim joist is only needed if your deck will carry heavy weight, has multiple levels, or spans a long distance. For small or simple decks, a single rim joist is often enough. In this guide I will explain what a rim joist is, when using two is better, the problems, and some simple tips. Let’s talk like neighbors about what will work best for your backyard. We’ll keep it easy and walk through the steps together.
What is a Rim Joist on a Deck?
Picture your deck like a big wooden frame. The rim joist is like the edge piece that holds everything together. It’s a board that runs around the outside of the deck, perpendicular to the main floor joists. Those floor joists are the long pieces that support the deck boards you walk on.
A rim joist, sometimes called a deck band joist or deck rim board. It connects the ends of the floor joists. It keeps them straight and strong. For example, If your deck is attached to your house, one rim joist will sit against the wall. The others make the outer edges. Without it, the joists could bend or sink over time.
Now, a double rim joist? That’s just two rim joists nailed together side by side. It adds extra thickness and strength. It’s kind of like wearing two pairs of socks on a cold day. You don’t always need it, but sometimes the extra layer helps. The same goes for rim joist framing, where everything has to line up the right way.
When to Use a Double Rim Joist on a Deck
So, when do you actually need a double rim joist on your deck? It really depends on the project. Let’s look at a few everyday examples, and I’ll explain each one in plain terms so you can tell if it makes sense for your deck.
Support Large or Overhanging Decks
If your deck stretches far from the house or has sections hanging out with no posts under them, a double rim joist can really help. It spreads the weight more evenly so the deck doesn’t droop in the middle. A deck built over a slope without the extra support, it could start to feel shaky after a few years. Adding a double rim joist helps keep overhanging decks strong and steady.
Handle Heavy Loads
A double rim joist can carry extra weight without trouble. It works like a stronger beam along the edges. For example, if you’re having a big family barbecue and lots of people gather on one side, it helps keep the deck steady. It’s also handy if you use two layers of decking or add heavy features.It’s especially useful with double decking or when adding heavy features.
Make Railings More Stable
Railings are there to keep everyone safe, but they need a strong base. A double rim joist makes it easier and sturdier to attach the posts. The extra thickness gives more grip for screws or bolts. Imagine leaning on the railing while watching the sunset if it shakes, it ruins the moment. Doubling the rim joist helps keep railings solid, especially for posts on the outside edge.
Add Support for Long Spaces Between Posts
Sometimes you want fewer posts so the view stays open. A double rim joist makes that possible because you can space the posts farther apart without losing strength. It helps cover the gaps along the deck edge. For example, if your deck is 20 feet long, one rim joist might bend a little, but doubling it gives extra support where the posts connect.
Improve the Look of Your Deck
A double rim joist makes the deck edge thicker and more polished. It covers the ends of the joists neatly and gives the deck a pro look. If you’re adding trim or decorative edging, it creates a clean line that improves curb appeal.
Prevent Moisture and Rot
Wood and water never get along. A double rim joist adds an extra layer that helps slow down rot, especially if you seal it properly. But remember it’s not a perfect fix. In rainy areas, this extra board helps protect the inside of the deck edge. Just be sure there’s enough airflow so the wood can dry out.
Compatibility with Modern Deck Designs
Modern decks often have extras like steps or built-in lights. A double rim joist works great with these features and fits well with composite boards or hidden clips. If your deck has more than one level, it helps connect everything neatly.
Meet Building Codes and Regulations
Rules vary by town, but some codes suggest or require a double rim joist for bigger decks or heavy loads. For guards and posts, joists should be at least 2×8. Check your local office they might want it for safety. It’s smart to follow these to avoid fixes later.
Also Read: 8 Best Deck Joist Tape Alternatives
Cons of Using a Double Rim Joist on Decks
Building rules can change from place to place. Around Richmond, Midlothian, and nearby areas, some codes may call for a double rim joist on bigger decks or ones that carry more weight. For railings and posts, joists usually need to be at least 2×8. It’s always smart to check with your local office if they may require it for safety. Following the rules from the start can save you from costly fixes later.
Might Trap Water
If water gets between the two boards, it can stay there and cause mold or rot. Rain in hidden spots can slowly make the wood weak. Sealing helps, but in rainy places, it’s still a risk.
Raise Material Costs
More wood means more cost. A double rim joist can raise your lumber bill by 20–30%. For a small deck, that extra money may not be worth it. Think about whether you really need the extra strength or if it’s just extra spending.
Make Installation More Complex
Putting two boards together takes more time. They have to line up right, or the deck won’t look straight. For beginners, this can be hard, especially when adding posts to the rim joist. That’s where LHC Services can help. Our team has years of experience building strong, safe decks in Richmond and nearby areas, so you can trust the job will be done right.
Limited Use in Some Designs
Not every deck needs a double rim joist. For a small patio deck, one rim joist is usually enough. Adding another one might just make it look bulky and add extra work you don’t need.
Potential for Over-Engineering
Sometimes we add more than we need. If your deck is small and light, extra strength is like wearing a helmet to bed. It’s safe, but not useful. It can just make the job harder without giving real benefits.
Weight Considerations
Using two layers makes the deck heavier. This can push down on the posts and base, especially on tall decks. If the ground is soft, you may need changes to keep it strong.
Best Practices for Installing a Double Rim Joist
- Measure carefully – Use a tape measure and a level. Always measure twice so your cuts are correct and your deck stays straight.
- Choose the right fasteners – Use screws or nails that won’t rust, since your deck will be outside in the rain and sun.
- Attach the inner rim joist first – Fix the first rim joist along the edge of the deck and make sure it’s straight.
- Add the outer rim joist – Place the second rim joist right against the first one. Nail or screw it into the floor joists at an angle so it holds tight.
- Protect from water – Put flashing tape (a waterproof tape) on top of the rim joist to stop water from soaking into the wood.
- Join long boards the right way – If you have to connect two boards, do it over a floor joist so the joint stays strong.
- Use joist hangers – These small metal brackets make the connection between joists and beams stronger and safer.
- Leave small gaps for airflow – If you add trim or banding to cover the edges, leave tiny spaces so air can move and the wood can dry after rain.
- Know when to call a pro – Building a deck takes skill, and mistakes can make it unsafe. If you’re a beginner, LHC Services can help. Our team builds strong, safe decks for homeowners in Richmond, Midlothian, and nearby areas.
Alternatives to Double Rim Joists
If you don’t need a double rim joist? You still have options. You can add extra blocking between joists to cut down on bounce without making the deck bulky. Metal brackets are another quick way to add strength where it’s needed. You could also use one thicker rim board, like engineered wood, which is strong but easier to work with. These choices can save time and money on smaller decks, but always make sure they’re strong enough for the weight your deck will carry.
Final Remarks
In the end, a double rim joist can make your deck stronger, nicer, and safer—especially for big decks or ones that carry a lot of weight. But remember, it also means more cost, more work, and the chance of water getting trapped. Think about your space, budget, and what you want from your deck. Sometimes a single rim joist is all you need. If you’re not sure, LHC Services is here to help. We build strong, safe decks in Richmond, Midlothian, and nearby areas. Reach out to us for advice or a free quote we’d love to help bring your deck ideas to life.
Frequently Asked Questions
A rim joist and a band joist are really just two names for the same thing. Both are the boards that run around the outer edge of the deck frame and tie the joists together.
Yes, you can use a double rim joist with composite decking. It works well for extra strength. Just make sure to follow the instructions from the decking manufacturer so everything is installed the right way.
A double rim joist deck usually costs more because it uses extra lumber. You can expect to pay about 20–30% more for the wood, and it also takes more time to build, which can raise labor costs.