FirstRig
Backfocus Solver
Menu
firstrig.com

Observatory build Chapter 4 of 10

Framing

Skirtboard on posts, joists in hangers with no ledger, and blocking that has to follow a circle rather than a grid.

7 min read

Read this

This is one person’s build, not engineering guidance. Local codes, soils and loads vary.

On this page
  1. What changed from NexDome's drawing
  2. The members
  3. The pier fits between the joists
  4. Ring blocking must follow the circle
  5. Bay blocking, installed early

The structural scheme comes from NexDome's own sample deck drawing: a perimeter skirtboard bolted to posts, joists hung inside it, and 2×4 decking. Two things were adapted.

What changed from NexDome's drawing

Joist spacing. Their sheet specifies 24″ o.c. over a 93¾″ span. This deck spans 117″, where 2×8 at 24″ o.c. no longer passes. 16″ o.c. is the correct adaptation. Do not carry their 24″ figure across to a longer span — it is the one number on that sheet that does not transfer.

The ledger. Their detail uses a 2×4 ledger carrying the joist hangers. Here the hangers fasten directly to the inside face of the 2×10 rim. It works fine for a 2×8 hanger, and it removes a piece of framing that would otherwise have sat about ¼″ above the soil.

The members

Skirtboard — 2×10 PT on all four sides, top edge at +10¾″, bolted to each post with two ⅜″ × 6″ hot-dip galvanized carriage bolts.

A corner where two pressure-treated boards meet, above a black plastic CAMO deck block set in crushed gravel.
A corner of the skirtboard on its post, the post seated in a CAMO block on a gravel pad.
Deck edge section with skirt and decking overhang Section through the deck edge. A CAMO block carries a 4x4 post, which carries the 2x10 structural skirtboard. A fascia skirt is applied to the outside face and stops two inches above grade for ventilation. The decking runs over the top and overhangs the fascia by one inch to shed water. Deck edge — skirt and overhang grade 1″ 2″ min 5/4 decking, 1/8″ gapscrown up, 2 screws per joist 1″ overhangsheds water clear of the skirt Fascia skirtgoes on BEFORE the decking 2x10 structural skirtboard 2x8 joist CAMO block + 4x4 post Leave the 2″ bottom gap open all around — it is the only ventilation under a 12″ deck

Joists — 2×8 PT, nine of them, in face-mount hangers, tops flush at +10¾″, clear span 117″. Positions along Y:

2.25 · 12 · 28 · 44 · 60 · 76 · 92 · 108 · 117.75

The 16″ spacings through the middle are what create the pier bay. The narrower gaps at each end are extra support where the skirtboard wants it.

Deck plan with off-center dome Plan of a 10 by 10 foot deck. Nine joists run across. The 88 inch dome wall ring is centered at 52 by 52 inches rather than the deck middle, leaving 8 inches behind, a 24 inch door apron on one side, and a 22 inch future bay strip on another. Eight CAMO blocks sit at the corners and quarter points of the two long sides. Deck plan — 10′ × 10′, dome off-center door apron future bay 10′-0″ (120″) 52″ to center 10′-0″ (120″) Pier / dome center(52, 52) — not the middle 24″ door apronstep on, open door, step in 22″ bay stripblocking pre-installed 8 CAMO blocks4 per long side 8″ behind the dome · 88″ ring · 24″ apron · the deck is fully committed
Parallel floor joists inside a rectangular perimeter frame seen from above, with a round hole dug in the soil between two of them and deck blocks under the edges.
The joists hung inside the perimeter, before any blocking, with the pier hole between two of them. A 12″ pier fits between two joists with 1¼″ clear, so no joist gets cut.

Decking — 5/4 × 6 PT running the Y direction, ⅛″ gaps, 21 boards at 120″.

All hardware hot-dip galvanized or stainless. Modern PT chemistry destroys electroplated fasteners.

The pier fits between the joists

A 12″ pier fits between two joists at 16″ o.c. with 1¼″ clear all around. No joist gets cut. No trimmers, no headers — just two pieces of blocking between the joists at Y 44 and Y 60, centered at X 44 and X 60, giving a 14½″ × 14½″ clear opening.

The 16″ pier in the original plan needed a fully framed opening with doubled trimmers and headers. Dropping to 12″ removed all of it. That is the single biggest simplification in the build.

Ring blocking must follow the circle

Ten pieces of 2×8, placed where the 88″ wall-ring circle crosses each joist bay, tops flush at +10¾″.

This is the part that is easy to get wrong, and it was in fact gotten wrong on the first pass — the blocking was originally placed on convenient grid lines, which looked reasonable in a solid view and was obviously useless in a framing plan.

The dome's 250 lb arrives as a line load around an 88″ circle, not spread across the floor. Blocking on the wrong line carries nothing.

Joist bay (Y)X positionsLength
12 – 2821.8 · 82.214.5″
28 – 4411.0 · 93.014.5″
44 – 608.0 · 96.014.5″
60 – 7611.0 · 93.014.5″
76 – 9221.8 · 82.214.5″

Nothing in the first or last bay — the circle does not reach that far.

Deck framing with short blocking pieces between the joists at staggered positions, and a cardboard concrete form standing in the pier opening.
Blocking between the joists, stepping from bay to bay rather than running along one grid line. The dome's load arrives as a line around a circle, so that is where the blocking goes.

Bay blocking, installed early

Four more pieces at X 34 and X 70 in the bay strip, even though the bay is hypothetical. A few offcuts today; awkward to retrofit under a finished deck.