Source: https://guthlabs.ai/docs/instruments/deck-board-joist-calculator/implementation/

instrument implementation / implementation-contract

# Deck Board & Joist Calculator implementation

A source-bound implementation contract for Deck Board & Joist Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_WITH_ERRATA**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-with-binding-errata

**Evidence ID**: t4-079

**Effect**: Computation or source lookup only

## Input and validation requirements

Zero joist spacing produces an undefined joist count, and the specification provides no formula for the required screw estimate. Nonzero waste also produces fractional board counts unless an additional rounding rule is defined.

## Required corrections

````
[
  {
    "vector": 4,
    "field": "board_count_as_written / purchasable",
    "wrong": "251.85 / 252",
    "correct": "25.3 / 26",
    "cause": "expected outputs contradict her own working (which correctly gets 22×1.15=25.3); 10× board-length slip when writing outputs"
  }
]
````

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "deck_width": 192,
    "deck_length": 144,
    "joist_spacing": 16,
    "board_width": 5.5,
    "board_length": 192,
    "waste_percent": 10,
    "units": "in"
  },
  "expected_outputs": {
    "joist_count": 10,
    "board_count_as_written": 29.7,
    "purchasable_board_count_if_finally_ceiled": 30,
    "screw_estimate": "not computable: no screw formula"
  },
  "working": "Joists: 192 ÷ 16 = 12; ceil(12) = 12; 12 + 1 = 13. Correction: using the stated deck_width of 192 gives 13 joists, not 10. Boards: deck area = 192 × 144 = 27,648 in². One board area = 5.5 × 192 = 1,056 in². Raw boards = 27,648 ÷ 1,056 = 26.181818...; ceil = 27. Waste multiplier = 1 + 10/100 = 1.10. Written formula result = 27 × 1.10 = 29.7; final purchasable count, if ceiled, = 30."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/deck-board-joist-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/deck-board-joist-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/deck-board-joist-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `tranche errata record`