How to Build a Deck Foundation: Step-by-Step Guide

“Can’t we just lay some decking already?!” That line nods to a kitchen post from last December about all the small prep steps that had to happen before painting cabinets — and I feel like we’re at the same stage with the deck. We’re making steady progress, but getting impatient. After setting all the joists we were eager to lay decking… until I realized there were a handful of details to finish first. Womp womp. Don’t worry — you do get to see decking being installed at the end of this post (and that process is ongoing), so we’re much closer to the finish line than the start. Whew.

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The first task was an inspection — a voluntary framing inspection, actually. Our next required county inspection was a final one after everything was complete, but the idea of finishing the whole deck only to discover a problem made us nervous. Sherry called the county and learned we could get an optional framing inspection to confirm we were on the right track. Yes, please.

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Spoiler: we passed. I did make a small mistake by notching my 4×4 posts — the county allows notching only for 6×6 posts, and 4x4s are supposed to attach with a special bracket. The inspector was sympathetic, though. Because our deck is low to the ground and not large, he wasn’t concerned. He even joked that “this thing’ll hold up five elephants and about 500 of those” (pointing to Clara through the window). In short: unless we’re planning a circus or inviting 499 toddlers over, he gave the go-ahead. Maybe the next inspector will bring cookies.

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With the inspection behind us, I completed the framing around the joists by adding two band boards to finish the exposed edge of the deck where a few joists were uneven and hanging over.

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This is the angled portion of the deck where the stairs will descend, so it required extra attention and several miter cuts. I started by trimming the excess from two joists so I could attach a 2×8 band board to the end. I used a nylon string to mark the 45-degree angle and a pivot ruler to draw the cut lines.

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The circular saw was set for angle cuts and I sliced the joists in place. (Full disclosure: the photo is a reenactment — it’s not actually how I hold the saw while cutting.)

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Here are the two angled joist ends after cutting.

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I cut the 2×8 band board to size, including an angled cut where it would meet the house.

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After fitting it in place — balancing it and temporarily nailing it before final screws — I secured the board with screws.

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That finished one edge, but the open end still needed a board. I cut another 2×8 with a miter so it would meet the previously attached band board and rested it loosely in place.

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Before attaching it permanently I trimmed the girders that were sticking out. I’d left them long intentionally during framing so they could be fine-tuned later. I used the circular saw to cut most of the way through, then finished with a reciprocating saw to get through both 2x10s.

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With the girders trimmed I set the board on the girder edges and screwed it into the rim board against the brick and into the angled band board. After trimming excess on the angled band board, the deck edges looked much cleaner.

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Next I added counter flashing. One condition from my earlier failed inspection was to add flashing over the tops of the ledger boards to better protect them from moisture.

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On the siding side I slipped the flashing under the bottom siding piece and tacked it in place with a few nails. On the brick side I folded a small section of flashing and tucked it into a groove cut in the mortar — the same groove the original flashing occupied. I folded a drip lip at the bottom to make sure water runs away from the brick.

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I cut vinyl to go around the joists and ran a bead of clear silicone into the mortar crevice to help hold the flashing and keep moisture out. The flashing was tedious but straightforward, and it’s one more step toward installing the deck boards.

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Before starting the deck boards I also planned the stairs and railing. Rather than wait, I wanted to add any reinforcements or attachments now while the framing was exposed. I used two spare 2x10s as stair placeholders to find where stair posts would attach and where guard rail posts should go.

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I chose to run the stair railing inside the stringers so the steps wrap around the posts a bit. Once the post locations were marked, I figured out how they’d attach. The county instructions weren’t very clear, and we had to hunt for the right hardware.

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On our third Home Depot trip we found Deck Tension Ties, which — along with a helpful magazine article — clarified how to handle the angled connections. The older method of bolting guard posts to the outside of the deck isn’t considered as secure, so these tension ties add rigidity and help meet inspection standards. I screwed a tension tie to the rim board against the house, drilled matching holes in the 4×4 post (set to the county-required 36″ railing height), and bolted the post to the deck with 1/2″ bolts.

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For posts not adjacent to the rim board, I added blocking between joists and attached tension ties on either side. I left some posts uninstalled so they wouldn’t get in the way when I lay the deck boards, but I placed the hardware now while it was easy to access.

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By the end of the prep work I’d also added some joist hangers where the joists met the angled band board. Maybe now the deck can hold six elephants?

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This guard-rail prep took two full days. Small spaces meant many tools wouldn’t fit, so I did a lot of screwing and bolting by hand. Weather didn’t help, either — pop-up thunderstorms cut short several afternoons. Plus, Sherry has book deadlines and Clara needs watching, so our available work windows are limited.

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Despite delays, progress continued — I finally started installing actual deck boards. We’re using mitered corners to create a framed perimeter and adding a few “zipper” seams for visual interest, an idea we mentioned earlier. So far it’s looking great, and we can’t wait to install more boards and share updated photos.

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Now that things are moving forward, the effort is starting to pay off — about time, right?

What nitty-gritty or unglamorous tasks have you tackled lately? Finished paint touch-ups? Replaced smoke detector batteries? Weeded your patio? Replaced filters? This is starting to sound a lot like my to-do list…