How to Extend an Elevated Deck

Co-authored by Glenn Carreau

Last Updated: December 5, 2022 References

Do you wish you had more deck space to kick back and relax on? Crafting your own deck extension is well worth the effort, and you’ll be surprised by how much you can accomplish all by yourself! Once you're done, you'll have a beautiful and spacious patio that you can furnish however you like and enjoy for many years. Read on for a complete DIY guide to extending an elevated deck at home.

Things You Should Know

Make a blueprint for your extension and ensure it's up to code, double-checking the local frost line and the existing deck's substructure before finalizing plans.
Create a layout of the extension by removing some (or all) of the existing floorboards and setting up a ledger board based on the type of extension you want.
Outline the extension perimeter with wood stakes and twine before anchoring in corner posts with concrete.
Build out the extension frame, connecting it to the existing deck and the new posts, before laying down floorboards and finishing the deck with stain or sealant.

Part 1

Part 1 of 3:

Planning the Deck Extension

1
Research local building codes to ensure your deck is permitted. Local building codes often regulate the size and weight-bearing capacity of elevated decks. Check with your local code office and ensure that your deck extension is designed completely according to code. This is crucial because homeowner's insurance doesn't cover injuries that happen on a deck that isn't built to code.^{[1]
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- Call your local code office to speak to a worker there, or go directly to your state’s website for the necessary information. Alternatively, sites like BuildingsGuide have a directory of every state website in the US.
- Get a building permit for the extension if your local town or homeowner's association requires one. Bring in a schematic for the extension with dimensions so they can get a clear picture of your plans.
2
Determine the frost line in your area. The frost line is the average depth to which the ground freezes when it's cold. If you have cold winters in your area, look up your location online by zip code to find the frost line on a site like The National Weather Service. When building an elevated deck, its support posts will typically need to be buried at least 6 inches (15 cm) below the frost line to be secure.^{[2]
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- Some building codes may even require your support posts to be buried 12 inches (30 cm) below the frost line. Check local regulations to be sure!
- If a deck's support posts aren't below the frost line, the annual frost-thaw cycle can cause those posts to buckle and deteriorate over time.
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3
Inspect the substructure of your existing deck. Before extending a deck, it's vital to confirm the existing substructure is physically stable. Inspect the deck’s frame for rot or deterioration, and ensure the ledger board is firmly anchored to the side of your house. In addition, the support posts holding up your deck shouldn't show any signs of heaving and thawing.^{[3]
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- A frame is a structure that holds your deck's floorboards in place. Look underneath your elevated deck for spongy, discolored wood (rot), and consider hiring a professional inspector to ensure the deck is examined properly.
- Decks attached to your home have a ledger board: a plank of wood connecting the house and deck. The ledger board should be correctly and securely fastened to the house before you continue with the extension.
- Heaving and thawing cause wood to buckle after cold weather. Ice gets in the cracks of the wood and expands; when the ice thaws in the spring, the wood splinters and breaks.
4
Plan the design and structure of your new extended deck. Rather than guessing what you'll need and how you think the deck should look, design your new deck, so you know exactly how you'll need to build it. Determine a layout for the extension and the material you want to use when it's time to start building.^{[4]
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- To make planning easier, consult an architect or use an online deck-designer tool to plan your extension.
5
Pick out decking materials for the extension. For an extension, it's most aesthetically pleasing to match whatever material the existing deck's floorboards are made from. However, you have more freedom if you plan to replace the entire floor or refinish it with a uniform color. Choose whatever materials you'll need, including floorboards, wood posts, and planks for the deck frame.^{[5]
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- Lumber for the floorboards should be pressure-treated, and you can choose between natural wood like cedar or redwood, composite boards (which mix wood and plastic), or plastic PVC decking.

Part 2

Part 2 of 3:

Creating a Layout of the Extension

1
Strip the surface boards from your existing deck. The easiest way to create a uniform floor for your extended deck is to replace the entire thing. That way, the deck's floorboards are the same color and age—there's no wear and tear to separate the old boards from the new ones. Back screws out of each board by setting your impact driver to reverse, and then easily remove the boards.^{[6]
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- If you have a painted deck, use a handheld screwdriver to remove the paint from the top of the screw before removing it.
- Similarly, clear wood away from the top of a screw using a Dremel tool if the wood is blocking your screwdriver.
- Using an impact driver (a type of drill) is more effective than a cordless driver because it has more torque, which is the rotational force that helps you remove screws from the wood.
2
Remove the fascia board from the side of the deck, alternatively. Understandably, you might not want to remove every floorboard from your deck—especially if the existing wood isn't very old or worn down. In that case, prep the deck by removing the fascia board from whichever side is getting extended. If that side has a railing, remove that too.^{[7]
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- To remove the fascia, use a hammer and a small pry bar to remove the nails connecting the fascia and pull it away from the wood deck.
- A fascia board, also called a "deck skirt," is the trim that makes your deck look polished. It covers the deck's wood perimeter and serves as added protection against the elements.
3
Decide whether you want a same-level, step-down, or step-up extension. There's more than one way to extend a deck! The easiest way is to simply create an extension on the same level as the existing deck. However, you could also choose to make a step-down extension lower than the original deck or a higher step-up extension.
- If you choose a same-level extension, use one of the existing substructure support beams as the ledger board. Whatever edge of the deck you extend, anchor the new frame to that sideboard—no need to add a ledger board.
- Add joists to the bottom of the beam if you want a step-down extension. You can still use the sideboard of the existing deck, but all joists (boards that form the deck's structural base) should be attached to its bottom rather than its top.
- Attach a new ledger board for a step-up extension. This extension does require a new ledger board; fix the new sideboard on top of the existing substructure's edge and connect the extension frame to that board.
4
Measure out the extension perimeter with wood stakes and twine. Based on your blueprints, map out the extension's edges by hammering wood stakes into the ground where each new corner will be. Then, connect the stakes with mason's twine. This way, you have a physical outline of the extension to work with.^{[8]
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- Ensure the twine is level with the top of the existing deck's rim joist (a technical term for the boards that make up the sides of your deck's structural base).
5
Mark post positions along the string and dig holes in the ground. Next, it's time to figure out where each of the extension's support posts will go! Measure the twine connecting your corners; then, every 8 feet (2.4 m), use a piece of colored tape to indicate where to place a post. Dig holes under each marker (including corners) that are 12 inches (30 cm) wide and 24 inches (61 cm) deep.^{[9]
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- Using a tape measure is the simplest way to calculate each supporting post position.

Part 3

Part 3 of 3:

Building the Deck Extension

1
Anchor deck posts in each hole. Use a circular saw to cut your wood deck posts slightly longer than needed. Place a concrete form tube into each hole and fill them with dry concrete, then mix in water according to the provided instructions and sink an anchor bolt into the center of the wet concrete until only the threaded top is visible. Let the concrete dry and fasten post bases to each anchor bolt, then attach the posts.^{[10]
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- Concrete typically needs about 24 hours to dry properly, so stop working and return to your deck the next day once the anchor bolts are in place. Use roughly half a bag to fill each hole before mixing in water.
- Concrete forms are plastic or cardboard tubes that can hold the concrete, and once hardened, the concrete and anchor bolt form the footing on your deck extension.
- Position post bases over the exposed anchor bolts and fasten them by tightening a washer and nut by hand.
- Then, attach wood posts by sliding them into the bases and drilling them in place with structural screws (which should be included with the bases).
2
Cut a new joist and clamp it to the original joist with C clamps. When you cut it, ensure the new joist is 3 inches (7.6 cm) shorter than the deck's outside joist. Then, use C clamps to hold the new joist in place against the old one while you work on attaching them properly. Drill ⁵⁄₁₆ inch (0.79 cm) bolt holes through both joists, spacing the holes 4 feet (1.2 m) apart.^{[11]
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- Attach the joists by inserting 3.5 inches (8.9 cm) by ⁵⁄₁₆ inch (0.79 cm) bolts into each hole and fastening them with a washer and nut.
- Once the washers and nuts secure each bolt, you can tighten them with a socket wrench to ensure the joists are firmly attached.
- As joists are the boards that form your structural base, this new joist makes up the side of the extension attached to the existing deck.
3
Measure and cut rim joists to create the outer edge of your deck. Complete the external base by cutting rim joists from 2x8 or 2x10 lumber to fit the desired length of your deck. Then, attach the rim joists to the exterior face of your corner posts, so they're level with your original deck's joists. Use metal corner brackets to connect the new beams with the old ones and the new joists at the far end of the frame.^{[12]
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- Ensure the rim joists can extend to the outside face of your corner posts, matching up with the older joists on your original deck.
4
Attach internal joists to the connecting joist with metal joist hangers. Use your circular saw to cut internal joists that run perpendicular to the existing deck flooring and fit between the rim joists. Space out your internal joists 16 inches (41 cm) apart and connect them to the rim with metal joist hanger brackets, 1.625 inches (4.13 cm) treated wood screws, and a drill.^{[13]
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- Internal joists are the support beams holding up floorboards inside the deck's base.
5
Install deck boards running parallel to the current deck. Essentially, your new deck boards should match existing ones—or they should all run in the same direction if you're replacing them. Cut 2x4 or 2x6 planks to the planned length of your deck and fasten them to the base using 2 1.625 inches (4.13 cm) treated deck screws per joint between the deck boards and joists.^{[14]
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- In other words, there should be 2 screws affixed to the deck board every time it meets an inner joist and on either end where the board meets the rim joists.
- Once again, use your circular saw to cut the planks to your desired length.
6
Smooth the deck boards to fit the joists and prevent splinters. Once every floorboard is screwed into place with the impact driver, completing the basic extension, examine the ends of the floorboards for rough or sharp edges. Use a rotary tool with a round-over bit to smooth out the ends of the new deck so that you (and anyone else using it) are less likely to get a splinter from unfinished wood.
7
Resurface the new deck to match the original if necessary. Clean your deck thoroughly with a biodegradable deck cleaner. Then, apply a layer of wood stain or wood sealant (your choice) to the entire deck and let it dry for a full day. Refinishing the deck ensures that the whole thing matches, even if one section of the floor is older than the other.^{[15]
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- If you're staining the deck (rather than using a sealant), sand the deck's surface with an orbital sander and 60-grit sandpaper first to help the wood absorb the stain better. Vacuum the deck after you sand it to pick up the dust.
- Don't worry about the underside of the deck; it won't need any stain or sealant, especially if it's made from pressure-treated wood.
- Of course, you can add stain or sealant to the deck even if you took the time to cover it with entirely new floorboards.