Why this page exists
A deck estimator that will not show its work is asking for trust it has not earned. Below are the exact tables DeckFoundry reads, how values between table rows are handled, what is calculated versus assumed, and — importantly — what DeckFoundry deliberately does not do.
If a configuration falls outside a published table, DeckFoundry says so and flags it for engineering review rather than extrapolating a number that looks authoritative.
1. Joist spans — IRC R507.6
Maximum joist span by lumber size and spacing, for Southern Pine #2 at 40 psf live load and 10 psf dead load.
| Joist | 12" O.C. | 16" O.C. | 24" O.C. |
|---|---|---|---|
| 2×8 | 13'-1" | 11'-10" | 9'-8" |
| 2×10 | 16'-2" | 14'-0" | 11'-5" |
| 2×12 | 18'-0" | 16'-6" | 13'-6" |
The check is a direct comparison, not an approximation:
→ PASS, and the achieved span is shown next to the limit
2. Cantilever — IRC R507.6
Maximum joist overhang past the beam. This is indexed by the actual backspan — the supported distance behind the cantilever — read directly from the published table.
A common shortcut is to treat the allowable cantilever as one quarter of the backspan. DeckFoundry does not use that rule. It reads the table, because the table and the rule of thumb disagree in the ranges that matter.
Where the table marks a combination as not permitted, DeckFoundry reports it as not permitted rather than substituting a nearby value.
3. Beam spans — IRC R507.5
Maximum beam span from Tables R507.5(1) through R507.5(4), indexed by number of plies, beam size and tributary width, for Southern Pine.
Each of the four load tables has its own column layout. DeckFoundry reads each table separately rather than flattening them into one grid, because the tables are not interchangeable.
4. Footing size — IRC R507.3.1
Round footing diameter by soil bearing capacity, tributary area and load.
Values between published rows
Real projects rarely land exactly on a table row. Where the code's footnotes permit interpolation, DeckFoundry interpolates. Where they do not, it does not.
For load specifically, DeckFoundry rounds up to the next published table at or above the actual load rather than blending two tables. A higher load's values are always equal or more restrictive, so this is strictly more conservative than interpolating — and it is a direct reading of a real published table rather than a value invented between two of them.
Where it does not: round up to the next published table.
Never blend tables the code does not authorize blending.
If the required footing exceeds the largest published diameter, or exceeds commonly stocked form sizes, DeckFoundry flags it rather than printing a number nobody can buy.
5. Post height — IRC R507.4
Maximum deck post height by post size and tributary area, per Table R507.4, Southern Pine.
6. Soil bearing
Soil bearing capacity defaults to a presumptive value. It is an assumption, not a measurement.
7. Materials and cost
Structural results drive the takeoff — joist count, beam plies, post and footing counts all come from the checks above, not from a square-foot rule of thumb.
- Decking — board count from actual board width and the selected pattern, with the pattern's waste factor applied
- Waste — varies by pattern, because a herringbone deck does not waste what a straight deck wastes
- Labor — by phase, from deck area and complexity
- Overhead and profit — applied as percentages you set, shown as separate lines rather than buried in unit prices
Prices are editable starting points, not market quotes. Lumber pricing moves constantly and varies by yard. Verify against your supplier before you buy.
8. What DeckFoundry does not do
Limits, stated plainly
- It does not replace a stamped engineered plan where your local code requires one.
- It does not certify code compliance, permits or engineering adequacy.
- It does not cover species other than Southern Pine, or load cases outside the published tables. Those configurations are flagged, not estimated.
- It does not account for local amendments to the IRC. Jurisdictions modify the code; yours may.
- It does not guarantee pricing. Quantities and rates are starting points for your market.
- Legal document templates are not legal advice, and lien waiver wording is prescribed by statute in some states.
Confirm local code requirements before construction.
9. Determinism
DeckFoundry's calculations are deterministic and run in your browser. The same inputs produce the same result every time. No language model generates spans, quantities, footing diameters or costs.
The engine is regression-tested against fixed baselines. Three reference projects must produce identical totals on every build; if any of them moves, the build is treated as broken until the change is explained.
10. English and Spanish
Both languages run the same engine. There is no separate Spanish calculation path — only the labels differ.
Lumber dimensions, grades and code references stay in English in the Spanish version, because that is what is printed on the material, on the plan and on the permit. Translating them would make the output harder to use, not easier.
Spanish structural terminology is undergoing native review.