Deck Expansion:

Extending An Existing Deck

In This Article:

The perimeter (rim joists) of the new structure is set up level and square, then holes are dug for supporting posts, and load-carrying beams are fabricated. Deck joists are installed and secured to prevent twisting.

Related Articles:
Skill Level: 3 (Intermediate) Time Taken: About 16 Hours

By , Editor



Tight Squeeze:

This 3-year old house had a decent deck when it was built, but after the homeowners installed a hot tub, there wasn't much deck space left.


Another view. There is lots of room to expand the original deck.


Notice how the deck joists are cantilevered over a double 2x8 "beam". This beam is just a treated 2x8 lag-bolted to each side of the row of support posts. This beam/post assembly had 5 posts, spaced 6 feet apart, for a total length of almost 24 feet.


The first task was to tear apart the old stairs. Since these steps were built with deck screws it was easy to disassemble them. My Makita impact driver made the task easier, since many of the deck screws were quite tight.

One trick is to use a very small flat screwdriver to clear the dirt from the screw heads, so the driver bit doesn't slip.


The next step was to establish the perimeter of the deck extension. I clamped a 6 foot long piece of 2x8 in place, using a block of 4x4 to hold up one end.


...and the other end was held in place with a metal angle bracket.


In a few minutes I was able to establish the perimeter of the deck extension... but these rim joists are not yet level and square.


Ensuring Level Framing:

I used this inexpensive torpedo laser level to make the rim joists level.

I used some shims to carefully adjust the device until it was perfectly level.


Just in front of the laser level I placed the end of the tape measure on the top surface of the original joist. This dimension (about 1") is the target number.

You can just barely see the laser dot in front of the red arrow.


At the diagonally opposite corner I placed the tape measure in front of the laser beam. Of course the dot is much wider, but I simply measure at the center of the dot.

Luckily this corner was close to my target dimension, so I didn't adjust the height of the rim joist.

In fact, I want the deck to have a slight slope away from the house, so this dimension, about 1/8" lower than the old deck, is just fine.

At the other corner the dot landed on 2 inches, so I loosened the clamp and raised the rim joist by 3/8 inch.


Ensuring A Square Layout:

I measured across the diagonals. This diagonal was 24'-5-3/4", while the other was 24'-4-3/8", which is 1-3/8" less. That's too far out-of-square for me.

I needed to adjust the frame so this longer dimension would shrink by more than one-half inch. I just picked up the frame and shoved it over a bit, then re-measured the diagonals. They were within 1/8" of being the same, which is good enough for something like a deck.

I used a can of lawn marking paint to lay out the post positions.


I made this grid layout on the grass. There will be a 4x4 post wherever the white lines intersect, plus posts where the long lines meet the short rim joists.

Except that this was WRONG! I decided to deviate from the original plan and put the support posts 6 feet apart, like the old deck, instead of 8 feet apart, as originally planned. But this would cause problems later, so I reverted back to the original plan.


I dug holes for the posts using a flat-blade shovel to cut the grass and a post-hole digger to reach deep.


Post hole depth:

Normally the bottom of deck posts need to be below the maximum frost depth. But here in Northern Michigan we are lucky to have soil that is mostly sand. This soil is so well-drained that the local codes do not always require that footings reach below the frost depth. If there was clay in the soil then the footings would need to be 48" deep to ensure that frozen soil does not heave the posts upward.



I used an 8x8 inch tamper to pack down the soil in the bottom of the hole. This is critical. If the soil is not properly compacted then the footing will settle over time and the deck will sag.


The bottom of the hole needs to be level so the footing block will be level. I just placed a torpedo level against the handle of the tamper, and this worked pretty good to ensure that the bottom was flat and level.

This entire process was kinda tedious. If the tamper handle was not close to plumb, I had to use a flat-blade shovel to scrape off some dirt on the high side and shift it to the low side, then tamp it again and re-check with a level. In two directions. It took upwards of 10 or 15 minutes per hole to make these adjustments.

If the footing block is not level then the treated 4x4 post will only have a small amount of wood that is actually bearing upon the pad, and that might cause those wood fibers to crush and settle over time. It's a pretty trivial concern, I'll admit.


My Preferred Method:

I would prefer to pour concrete into the hole to fill the space below the post, using post anchors like this. This type of anchor connects the post to the gob of concrete. If there are any uplifting forces (such as heaving, frozen soil that bonds to the post), the concrete footing should prevent the post from being lifted out of the hole.


I carefully dropped this 12-inch octagonal post footing block into the hole. I pounded on the block with a 4x4 to settle it into the soil.


I set a 4x4 treated post into the hole and clamped it to the rim joist. The outer edge of this post was 16 inches from the outside of the long rim joist (14" from the inside). I don't have a reason why I used this dimension for the cantilever distance; I usually see decks cantilever over the beam by about 24 inches.


I drove 4 deck screws through the rim joist into the post. But before doing this I gave the top of the post a couple of whacks with a hammer to make sure it was seated well.


The deck so far.

There are 2 short rim joists with 2 posts each. The long rim joist is not yet fully secured.


I installed an intermediate joist, one that will sit adjacent to a pair of supporting posts.

I didn't need to make a joist line up with these posts, but this makes it easier to establish a location for the posts, and also to hold them in place while I build up the beam that will support all the joists.

At this point the long rim joist was temporarily held up by two short blocks of 4x4 (red arrows, above) that rested on the lawn. The 24 foot long rim joist was made of an 8 foot board plus a 16 footer. I made these boards meet at this intermediate joist.

At the deck end I used a joist anchor to position the joist.


At the cantilevered end I screwed the rim joists to the new joist. I purposely made this new joist land on the joint between the 16 foot and the 8 foot 2x8's that comprise the outer rim joist.


For the second intermediate joist, I screwed a joist hanger to the old deck rim joist.


... and another joist hanger to the new rim joist. This hanger will just be temporary.

I couldn't easily do this on the first intermediate joist because the hanger would have to cross the gap between two boards.


I placed the joist in the joist hanger.


I drove some deck screws through the rim joist into the end of the new joist.

At this point the 4 posts in the middle of the deck are just sitting on cement pads, I haven't yet backfilled the holes. These posts will need to be aligned with the corresponding posts on the outside joists, so the beam will be straight.

If a deck project required a permit and inspection of the footing depth, this would be the time to call the building department to have the inspector come out. This can be a real nuisance for people who work regular jobs during the week and work on home projects on weekends, because you can't cover up the footings until they are inspected. Not all areas require an inspection of the footing depth, so call your local building department for more info.


I ran two stringlines from one end of the deck to the other. I find that the easiest way to fasten the string is to simply wrap it under the corners of the temporary spacer block and pull tight.

Why the spacer block?  This is necessary to keep the string above the joists that I am trying to align. By using a block of wood at each end of the string, I can tell if one joist is higher than the end joists. If I didn't use spacer blocks I would not be able to tell if one joist was too high because it would be hitting the string.

I can use these stringlines to make sure all four joists are in the same 2-dimensional plane, and to set the posts so each row lies in a straight line.


One of the intermediate joists was too low by over 3/8". The spacer blocks were 11/16" thick, so the distance from the joist to the string should be same.

To fix this problem I just unclamped the temporary support blocks and raised up the frame.

Note that it's important to use spacer blocks of exactly the same thickness. The best way is to cut a piece of wood into small pieces so they all came from the same stock.

With a clamp loosely holding the post, I adjusted the position of each post until it was directly in line with the string. I used a level to make sure the post was plumb, then I beat on the top of the post with a hammer to make sure it was seated against the concrete pad in the hole.

I also had to shift the concrete footing pad around the bottom of the hole to ensure that the post was reasonably close to being in the center of the pad. I used a shovel and a 4-foot-long pry bar to push against the side of the pad. Reaching into the hole was way too difficult.

At this point I backfilled the post holes, filling in only about 6 inches of dirt at a time and tamping the loose soil by walking on it or pounding it with a piece of 4x4 post. Where possible I used my 8x8 tamper to compact the soil.

The structure so far. All the temporary supports have been removed.


The Beam:

I clamped a 16 foot long 2x8 to the deck posts and used a prybar to push them up against the underside of the intermediate joists. It's critical to make this beam tight against the initial joists.

Before putting this board in position, I sighted down the edge of the board to see which way the board curved. Long lumber almost always has a curve to it, and you should always examine the board to determine which edge has the "crown", or outward-curving shape.

The "crown" edge should always be placed up, so that if the framing sags over time the board will tend to straighten out.

I drove in a pair of lag screws at each post.

These are GRK 5/16" x 4" long lag screws that have a Torx #30 drive. (That 6-pointed star pattern.) These screws do not require any pre-drilling.

Here I'm using a Makita 12 volt impact driver, which is quite capable of driving these lags. It's also much faster than using conventional lag bolts, pre-drilling the holes and using a mechanic's ratchet wrench to tighten them.


This is the deck frame after the beams had been bolted to the posts. There are two of these beams, and each is made from a pair of 2x8's.


The connections at the post.

I used 16 foot long boards and 8 foot long boards to to create the beam that is just under 24 feet long.


A Small Problem:

By sighting down the top of the long rim joist I realized that the wood was kinda bowed, as Southern Yellow Pine often is. I had installed the 16 foot long board with the crown upwards (like you're supposed to), so I snapped a chalk line between the upper corners of the long rim joist, to give me a more accurate picture of how warped the structure was.

I used a portable power planer to remove some wood in the highest part of the rim joist. In some places the wood was almost 1/2 inch higher than the chalk line.


I laid out the joist locations along the old rim joist and the new rim joist. These are 16 inches on center, except the first joist spacing, which is 3/4" less.


I made a mark like this to help position the joists.

There are all sorts of ways to mark joist layout. The most common is to draw one line to indicate one edge of the joist, and an X to indicate which side of the line the joist goes on.


Another Problem:

When I set the 6 foot long joists in place, I was not able to get the ends of the joists aligned with the top of the old deck (you can that see in the background) and the top of the new rim joist.

This is caused by the curvature of the 2x8's used to make the beam. I installed those boards with the crown (curvature) upwards, but they were so badly curved that they created problems in other areas..

I slipped a 3x5 file card under each of the 4 points where the joist crossed over the beam, to see which areas were in contact. This is easier than crawling around in the dirt to look for the gaps.

The red mark indicates an area that was in contact. I'll need to plane down these spots.


The first group of five joists sitting in place after I spent 20 minutes planing the tops of the beams.


I tacked the joist hangers in place by hammering in the built-in prongs.


I used 8d hot-dipped-galvanized joist hanger nails to fasten the hanger to the old rim joist...


...and 16d spiral deck nails in the angled holes of the hanger.


The Easy Way:

This is the Senco palm nailer. I call this tool the Poor Man's Air Nailer. It works with any nail that will fit inside the tip.


At the cantilevered end, I drove three 3" deck screws through the rim joist into the end of each joist.

I never use nails here because nails pull out easily when driven into the end-grain of wood.


Extra Stuff:

To keep the joists from twisting,  I fastened these scraps of 4x4 between the double 2x8 beam and the joists. This is a supplement to toe-nailing, which is not a very strong method of joining wood.

This was simple: A couple of deck screws to hold the block in place, and then two 16d nails to fasten each of the three boards to the 4x4 block.

I only did this at the outer beam, figuring that the joist hangers will keep the joists from twisting near the inner beam.

I toe-nailed each joist (red arrows) on both sides.


The deck framing completed. Now the handrail posts need to be attached and then the decking installed.


Later I installed this composite decking material, but that is topic for a separate article. Click here to read about the process of installing composite deck boards.


After the new deck boards were completed the deck looked like this. Now there is room for the hot tub and some people on the deck.


Continue to the next step:  Installing Composite Deck Boards.



Tools Used:

  • Cordless Drill/Driver
  • Basic Carpentry Tools
  • Level, 4'
  • Miter Saw
  • Table Saw (or Circular Saw)
  • Jig Saw
  • Shovels
  • Post Hole Digger
  • Tamper, 8"x8"

Materials Used:

  • Treated Lumber, 2x8x16'
  • Treated Lumber, 2x8x12'
  • Treated Posts, 4x4x8'
  • Concrete Post Pads, 12" Wide
  • Galvanized Nails, 16d
  • Deck Screws, 3"
  • GRK Lag Screws, 5/16 x 4"

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Copyright 2005

Written May 15, 2005