A code-compliant residential deck is governed by five tables in 2021 IRC Section R507: joist span (R507.6), beam span (R507.5), post height (R507.4), footing size (R507.3.1), and ledger fastener spacing (R507.9.1.3). For a typical 16 ft Γ 12 ft pressure-treated Southern Pine deck at 16" o.c.: 2x10 joists span 14 ft, a 2-2x10 beam holds about 6 ft between posts, a 6x6 post supports up to 14 ft of height at 60 sq ft tributary, and a 23" round footing Γ 9" thick covers the load at 1,500 psf soil. Plus 2 Γ Simpson DTT2Z lateral hold-downs are mandatory on every ledgered deck per R507.9.2.
Essential Tools
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View Product (paid link)π 1. The Five Documents That Govern Every Residential Deck
Building a deck is one of the most code-driven projects a homeowner can take on. Almost every dimension β joist size, beam span, post height, footing diameter, ledger fastener spacing β comes from a published table somewhere. Knowing which document governs which decision saves hours of guesswork at the lumber yard.
Primary deck construction references
| Document | What it covers | Status |
|---|---|---|
| 2021 IRC Section R507 | Joist, beam, post, footing, ledger, lateral connection β five prescriptive tables | Most-adopted current code |
| AWC DCA 6 (2018 reprint) | Prescriptive companion guide β ledgered, single-level decks β€ 200 sq ft | Free PDF from American Wood Council |
| AWPA U1 | Pressure-treatment retention by Use Category (UC3B / UC4A / UC4B) | Required by IRC R317.1 |
| ASTM A153 / A653 | Hot-dip galvanized hardware (G185 sheet steel for connectors) | Required by IRC R317.3 |
| ICC-ES Reports | Per-product joist spacing for Trex (ESR-3168), TimberTech (CCRR-0101), Fiberon (ESR-4947), Deckorators (CCRR-0195) | Manufacturer-specific |
β οΈ Where DCA 6 and 2021 IRC differ, IRC governs
DCA 6 was last reprinted in 2018 and reflects the 2015 IRC. The biggest visible difference is post size: DCA 6 still requires 6x6 minimum, but 2021 IRC R507.4 first permits 4x4 posts prescriptively at small tributary areas. If your inspector cites DCA 6, that's fine β but where the two conflict, the adopted IRC edition wins.
βοΈ 2. The Load Assumptions Behind Every Span Table
Every R507 prescriptive table assumes a specific load case. Match that case and the tables work. Step outside it and you need engineered design.
Live load β 40 psf
Uniform across the deck surface. The 40 psf base assumes ordinary residential use β people, furniture, light snow. Snow loads of 50, 60, or 70 psf each step into a separate IRC sub-table that bumps the joist or beam up one nominal size.
Stair live load is the same 40 psf or a 300 lb point load on a 4 sq in area, whichever produces the greater stress.
Dead load β 10 psf
The deck's own framing, decking, and built-in fixtures. 10 psf comfortably covers PT framing + composite deck boards + a railing system. Stone pavers, concrete tile, or a hot tub push past this and require engineered design.
Concentrated loads above 100 psf β most hot tubs qualify β are explicitly excluded from DCA 6 prescriptive scope.
Guard top-rail load β 200 lb
200 lb concentrated at any point along the top rail. The 2021 IRC relaxed the prior requirement that the rail resist load in any direction β guards no longer need to handle inward or upward force.
Infill (balusters, panels) carries 50 lb on 1 sq ft, not concurrent with the top-rail load.
Soil bearing β 1,500 psf default
IRC R507.3.1 footing tables assume 1,500 psf, the conservative national default. Stronger soils (2,000 / 2,500 / 3,000 psf) permit smaller footings β but only with documentation.
Concrete strength assumed = 2,500 psi (DCA 6 footnote 4). Standard ready-mix and bagged products easily exceed this.
π 3. Joist Span β IRC Table R507.6
Joists run from the ledger to the outer beam. Their span is the distance between those two supports. IRC Table R507.6 sizes joists by species, nominal size, and on-center spacing.
Joist span β 40 psf live + 10 psf dead, No. 2 grade, wet service
| Species | Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|---|
| Southern Pine | 2x6 | 9'-11" | 9'-0" | 7'-7" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" | |
| 2x10 | 16'-2" | 14'-0" | 11'-5" | |
| 2x12 | 18'-0" | 16'-6" | 13'-6" | |
| DF-L / HF / SPF (incised) | 2x6 | 9'-6" | 8'-4" | 6'-10" |
| 2x8 | 12'-6" | 11'-1" | 9'-1" | |
| 2x10 | 15'-8" | 13'-7" | 11'-1" | |
| 2x12 | 18'-0" | 15'-9" | 12'-10" | |
| Redwood / Cedars | 2x6 | 8'-10" | 8'-0" | 6'-10" |
| 2x8 | 11'-8" | 10'-7" | 8'-8" | |
| 2x10 | 14'-11" | 13'-0" | 10'-7" | |
| 2x12 | 17'-5" | 15'-1" | 12'-4" |
π The cantilever rule (DCA 6 / 2018+ IRC)
Maximum joist cantilever past the outer beam is the lesser of the table value or L/4 of the actual main span. So a 12 ft main span limits cantilever to 3 ft regardless of what the table allows. Cantilever assumes L/180 deflection with a 220 lb point load.
β οΈ Joist spacing is set by the decking, not the framer
Almost every wood-plastic composite (Trex, TimberTech, Fiberon, most Deckorators lines) requires 16" o.c. perpendicular and 12" o.c. diagonal. Pressure-treated 5/4Γ6 also defaults to 16" o.c. PT 2x6 and Deckorators Surestone (mineral-based composite, CCRR-0195) are the two exceptions that allow 24" o.c. β and Surestone alone permits 24" o.c. on a residential composite deck.
ποΈ 4. Beam Span β IRC Table R507.5
The beam carries one end of every joist. IRC Table R507.5 sizes the beam by the joist span it supports: a wider deck means more load per linear foot of beam, so beams span shorter distances between posts.
Southern Pine multi-ply beam β feet between posts
| Beam | Joist span supported β | 6 ft | 10 ft | 14 ft | 18 ft |
|---|---|---|---|---|---|
| 2-2x8 | 8'-6" | 6'-6" | 5'-6" | 4'-9" | |
| 2-2x10 | 10'-1" | 7'-9" | 6'-6" | 5'-9" | |
| 2-2x12 | 11'-11" | 9'-2" | 7'-9" | 6'-9" | |
| 3-2x10 | 12'-9" | 9'-9" | 8'-3" | 7'-3" | |
| 3-2x12 | 15'-0" | 11'-7" | 9'-9" | 8'-7" |
For a 16 ft Γ 12 ft Southern Pine deck (joists 16 ft long), a 2-2x10 beam holds about 6'-1" between posts β meaning a 12 ft outer beam needs 3 posts (two corners plus one in the middle).
β οΈ DF-L / Hem-Fir / SPF beams take a much bigger hit than joists
The incising adjustment on beams is brutal: a 2-2x10 DF-L supporting a 6 ft joist span allows 7'-9" vs. SYP's 10'-1" β a 23% reduction. On joists the reduction is only ~3%. The actual ratio varies from 77% at the short end to 88% at the long end of the table, so don't apply a flat percentage β pull the value from the species-specific row.
πͺ΅ 5. Post Size β IRC Table R507.4 (new in 2021)
The 2021 IRC was the first to permit 4x4 posts prescriptively. DCA 6 historically required 6x6 minimum on every deck regardless of size. Many local jurisdictions still enforce that as habit even when their adopted code permits 4x4.
Post height (Southern Pine, 40 psf live load) by tributary area
| Tributary area | 4x4 | 4x6 | 6x6 | 8x8 |
|---|---|---|---|---|
| 20 sq ft | 14'-0" | 14'-0" | 14'-0" | 14'-0" |
| 40 | 11'-1" | 14'-0" | 14'-0" | 14'-0" |
| 60 | 8'-11" | 11'-4" | 14'-0" | 14'-0" |
| 100 | 6'-7" | 8'-7" | 14'-0" | 14'-0" |
| 160 | 4'-6" | 6'-6" | 11'-2" | 14'-0" |
Tributary area = (joist span Γ· 2) Γ (post spacing). For a 16 ft Γ 12 ft deck with 3 posts at 6 ft o.c., interior posts carry (16 Γ· 2) Γ 6 = 48 sq ft, which rounds up to the 60 sq ft bracket.
π Posts must be UC4A ground-contact treated
AWPA U1 Use Category UC4A β the same retention required for fence posts and pole barns. UC3B above-ground retention is not sufficient for deck posts even when the post sits on a Simpson ABU66Z standoff base. The end tag on every piece must say UC4A.
π³οΈ 6. Footings β IRC Table R507.3.1
Footings carry the deck's entire vertical load into the soil. IRC Table R507.3.1 sizes them at 1,500 psf default soil bearing β and the footing must extend below the local frost line to prevent winter heaving.
Footing size at 1,500 psf soil, 40 psf live load
| Tributary area | Round dia. | Square | Thickness |
|---|---|---|---|
| 5 sq ft | 8" | β | 6" |
| 20 | 14" | 12" Γ 12" | 6" |
| 40 | 19" | 17" Γ 17" | 7" |
| 60 | 23" | 21" Γ 21" | 9" |
| 100 | 30" | 27" Γ 27" | 12" |
| 160 | 38" | 34" Γ 34" | 15" |
Frost depth by region
The footing extends below the frost line β never use a national map; pull the value from your local building department's adopted IRC Table R301.2(1).
12" minimum (often less in Miami-Dade)
24"β30" (Northern VA usually 30")
36"β42" (Chicago = 42")
48"
60"β72"+
60"+ (adfreeze becomes a design issue)
π Concrete volume formula
V (cu ft) = Ο Γ (D/2)Β² Γ Depth, with D and Depth in feet. Multiply by post count, then add 10% for spillage.
Bag yields: 80 lb = 0.6 cu ft; 60 lb = 0.45; 50 lb = 0.375. So 1 cu yard (27 cu ft) = 45 Γ 80 lb bags.
A 12" Γ 48" tube footing: Ο Γ 0.5Β² Γ 4 = 3.14 cu ft β 6 Γ 80 lb bags. A 23" Γ 48" tube: Ο Γ (23/24)Β² Γ 4 = 11.5 cu ft β 20 Γ 80 lb bags.
π© 7. Ledger Fasteners β IRC Table R507.9.1.3(1)
The ledger is the deck's most safety-critical connection. Get this wrong and the deck pulls itself off the house in shear. IRC R507.9.1.3 governs spacing, edge distances, and minimum fastener types.
Ledger fastener spacing (in.) β 50 psf total load, β€1/2" sheathing
| Fastener | β€6 ft | 8 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|
| 1/2" lag screw | 30" | 23" | 15" | 13" | 11" | 10" |
| 1/2" through-bolt | 36" | 36" | 29" | 24" | 21" | 19" |
Edge distances per Table R507.9.1.3(2)
- β’ 2" from top edge of ledger; 3/4" from bottom edge
- β’ 3/4" from top of band joist; 2" from bottom
- β’ 2" from ledger ends
- β’ 5" max vertical separation between rows
- β’ Fasteners staggered top-to-bottom in two rows; must penetrate fully through band joist
β οΈ What you cannot attach a ledger to
Per IRC R507.9.1.1: brick or masonry veneer (not structural), cantilevered floor framing, and any band joist smaller than the ledger. Ledger itself must be 2x8 nominal minimum, must be PT Southern Pine / incised PT Hem-Fir / or naturally durable No. 2 or better, and must equal joist depth (no shorter than joists).
π¨ Lateral hold-downs are mandatory β R507.9.2
Every ledgered deck β regardless of size β needs lateral-load connection. Standard solution: 2 Γ Simpson DTT2Z hold-downs (1,500 lb each) within 24" of each end of the deck, with 1/2" Γ 6" SDS rods through the band joist into a floor joist below. The four-location alternative is 4 Γ DTT1Z at 750 lb each. This is the single most-skipped requirement on DIY decks and the most common cause of catastrophic failure.
πͺ΅ 8. Decking β Wood, Composite, PVC
Decking is where the deck's look and the project budget meet. Span ratings come from each manufacturer's ICC-ES Evaluation Report β there is no "global" composite rule.
Pressure-treated wood
- β’ PT 5/4Γ6 (1" Γ 5-1/2"): 16" o.c.
- β’ PT 2Γ6 (1-1/2" Γ 5-1/2"): 24" o.c.
- β’ Janka hardness varies; SYP ~690 lbf
- β’ ACQ / CA-B / MCA chemistry; UC3B for boards, UC4A for framing
- β’ Cheapest material on a deck
Cedar / Redwood
- β’ Western Red Cedar: 350 Janka, 23 lb/ftΒ³
- β’ Redwood (Construction Heart): 450 Janka, 26 lb/ftΒ³
- β’ Naturally durable β only heartwood grades qualify
- β’ Stainless fasteners mandatory (HDG can stain cedar)
- β’ 5/4Γ6 board at 16" o.c. perpendicular
Tropical hardwoods
- β’ Ipe: 3,510 Janka, 69 lb/ftΒ³ (sinks in water)
- β’ Cumaru: 3,330 Janka, 68 lb/ftΒ³
- β’ Garapa, Tigerwood β lighter alternatives
- β’ Pre-drilling mandatory; carbide tools required
- β’ Hidden-fastener systems strongly preferred
Composite & PVC
- β’ Trex Enhance / Transcend: 16" o.c. (ESR-3168)
- β’ TimberTech / AZEK PRO: 16" o.c. (CCRR-0101)
- β’ Fiberon Concordia: 16" o.c. (ESR-4947)
- β’ Deckorators Surestone: 24" o.c. (CCRR-0195)
- β’ 25β50-yr warranties; thermal expansion ~3/16" per 20 ft
π Deckorators Surestone β the only 24" o.c. composite
Most wood-plastic composites require 16" o.c. perpendicular. Deckorators Voyage and Vault β both Surestone mineral-based composite per CCRR-0195 β permit 24" o.c. residential and 16" o.c. diagonal. That drops the joist count roughly 33% on the same deck. Surestone is also ~35% lighter than wood-plastic composite and warranted for ground/water contact.
Linear footage and waste factors
LF = (Length Γ Width Γ 12) Γ· (face_width + side_gap). Wood gap is typically 1/8"; composite gap is 3/16".
- β’ Straight perpendicular: 10% waste
- β’ Diagonal 45Β°: 15% waste
- β’ Picture-frame border: 15% waste
- β’ Herringbone / chevron: 20% waste
Worked example: a 12 Γ 16 ft deck (192 sq ft) with Trex Transcend 5.5" face + 3/16" gap = 5.6875" effective board width. 192 Γ 12 Γ· 5.6875 = 405 LF. With 16 ft boards: 405 Γ· 16 = 25.3 β 26 boards before waste, 29 boards with 10% waste.
π§ 9. Hardware β Joist Hangers, Post Bases, Hold-Downs
Per IRC R317.3 / DCA 6 Min. Req., all metal in contact with PT lumber must be hot-dip galvanized (ASTM A153 Class D for β€3/8", Class C for >3/8"), mechanically deposited zinc Class 55, or stainless. Sheet-steel connectors (joist hangers, hold-downs) need ASTM A653 G185 minimum β Simpson's ZMAX finish meets this.
Common Simpson Strong-Tie joist hangers
| Hanger | Joist size | Min. capacity | DCA 6 minimum |
|---|---|---|---|
| LUS26 | 2x6 | 410 lb | β₯ 400 lb |
| LUS28 / LUS28Z | 2x8 | 600 lb | β₯ 500 lb |
| LUS210 / LUS210Z | 2x10 | 740 lb | β₯ 600 lb |
| LUS210-2 | Double 2x10 | 1,830 lb | β |
π¨ Use Simpson N10 nails β deck screws and roofing nails are NOT permitted
Simpson's installation instructions explicitly require 10d Γ 1-1/2" HDG nails (N10) in all face holes plus all triangle holes β typically 8β10 nails for 2x8, 10β12 for 2x10. Substituting deck screws voids the load rating. This is the #1 inspector flag on DIY-built decks.
Coastal / saltwater hardware
IRC R507.2.3 mandates Type 304 or 316 stainless steel for every fastener and connector within 300 ft of saltwater shoreline. 316 (with molybdenum) is preferred for direct splash, salt fog, or pool decks. Galvanized hardware in salt fog corrodes through within 5β10 years and pulls itself out of the lumber. Cost is typically 4β6Γ HDG β and this is non-negotiable code.
Structural screw alternatives to lag screws
- β’ FastenMaster LedgerLOK β ICC-ES ESR-1078, replaces 1/2" lag screws per IRC R104.11. No pre-drill required.
- β’ FastenMaster ThruLOK β true through-bolt replacement; passes through ledger, sheathing, and band joist with cap nut.
- β’ Simpson SDWS Timber screw β Strong-Drive structural fastener.
- β’ GRK RSS β Rugged Structural Screw, 1/4" primary diameter.
πͺ 10. Stairs β IRC R311.7
Stair geometry has the tightest tolerances of any deck element because uneven risers cause falls. The IRC sets ranges, and the Blondel comfort formula sets the sweet spot.
2021 IRC R311.7 stair geometry limits
| Parameter | Value |
|---|---|
| Maximum riser height | 7-3/4" (R311.7.5.1) |
| Minimum riser height | 4" |
| Minimum tread depth | 10" with nosing OR 11" without |
| Riser / tread variation in a flight | 3/8" max |
| Minimum stair width | 36" clear above handrail |
| Headroom | 6'-8" minimum |
| Max vertical rise per flight | 12'-7" before landing |
| Handrail required | 4 or more risers (R311.7.8) |
| Handrail height | 34"β38" above tread nosing |
π Calculating risers and treads
- β’ Risers = β Total_rise Γ· 7.75 β
- β’ Riser_height = Total_rise Γ· Risers (must be β€ 7.75")
- β’ Treads = Risers β 1
- β’ Comfort rule (Blondel): 2R + T = 24" to 25"
Worked example: 48" total rise β β48Γ·7.75β = 7 risers @ 6.86", 6 treads @ 11", total run 66". Blondel check: 2Γ6.86 + 11 = 24.7 β comfortable.
Stringer rules
- β’ Solid 2x12 stringer: max 13'-3" horizontal span with stringers β€ 18" o.c.
- β’ Cut stringer: residual material under tread cut must be β₯ 5" wide (AWC commentary)
- β’ Stringer spacing: 16" o.c. for wood treads; 12" o.c. for composite (IRC R507.2.2 + manufacturer reports require this for nearly all composite stair treads regardless of flat-deck spacing)
- β’ Number of stringers = β stair_width Γ· 16 β + 1 for cut wood; β stair_width Γ· 12 β + 1 for composite
π‘οΈ 11. Guards & Railings β IRC R312
Guards are required where the walking surface is more than 30" above grade (within 36" horizontally of the edge). They protect against falls and are tested for both top-rail and infill loads.
2021 IRC R312 guard requirements
| Parameter | Value |
|---|---|
| Required where | Walking surface > 30" above grade |
| Minimum height (residential) | 36" |
| CA / WA / IBC commercial | 42" |
| Stair guard height | 34" min from nosing |
| Sphere rule (general) | 4" sphere shall not pass |
| Sphere rule (stair triangle) | 6" sphere shall not pass |
| Sphere rule (stair guard openings) | 4-3/8" sphere shall not pass |
| Top-rail load | 200 lb concentrated, any point |
| Infill load | 50 lb / sq ft on 1 sq ft |
π Baluster math
- β’ 1-1/2" square baluster + 4" max gap = 5.5" o.c. β 2.18 / LF
- β’ 3/4" round baluster + 4" max gap = 4.75" o.c. β 2.53 / LF
A 16 Γ 12 ft attached deck has 16 + 2(12) = 40 LF of exposed perimeter. With 1-1/2" square balusters: 40 Γ 2.18 β 87 balusters.
π 12. Regional & Jurisdictional Variations
The same prescriptive deck is not legal everywhere. Snow load, frost depth, salt proximity, and wildfire zones all push the design.
California β WUI fire zones, 42" guards
CRC 2022 (based on 2021 IRC) adds Wildland-Urban Interface (WUI) amendments. WUI requires ignition-resistant decking β ASTM E84 Class B or A. Trex products meet Class B; Fiberon ArmorGuard Pro FR is Class A. Standard PT wood is generally not permitted in WUI without additional fire-treatment certification. Many SF Bay Area, LA, and SF jurisdictions also amend guard height to 42".
Florida HVHZ β Notice of Acceptance required
In Miami-Dade and Broward HVHZ counties, every railing, decking product, fastener, and connector needs Florida Product Approval or NOA. 180 mph wind zones often require lateral connection at 4 locations instead of 2. Stainless mandatory throughout the Florida Keys.
Pacific Northwest β 50β70 psf snow, cedar tradition
Cascade counties and Oregon foothills default to 50β70 psf ground snow load. PT Douglas Fir-Larch (incised) is the framing default; Western Red Cedar deck boards remain popular on the wet side. R507.6(2)/(3)/(4) sub-tables required β joists are typically 2x10 minimum at 16" o.c. even for short spans.
Northeast snow belt β deepest footings
Vermont, New Hampshire, and Northern Maine combine 60β70 psf snow with 60β72" frost depth. A 200 sq ft deck here can require 40+ bags of concrete β at the upper end of homeowner-feasible without ready-mix delivery. 6x6 posts mandatory at all but the smallest tributary areas.
Coastal β within 300 ft of saltwater = stainless
IRC R507.2.3 makes Type 304 / 316 stainless mandatory on every connector and fastener. Hawaii and parts of California extend the requirement to 500β1,500 ft inland because of trade-wind salt drift. Hardware cost goes from ~$300 in HDG to $1,500β$1,800 in 316 stainless on a typical 200 sq ft ledgered deck.
π 13. Worked Example β Complete Materials Take-off
Scenario: 16 ft Γ 12 ft attached deck, 4 ft above grade, Southern Pine PT framing, 5/4Γ6 PT decking perpendicular layout, 36" railing, 48" stair total rise, 36" frost depth, non-coastal site.
Materials list
| Item | Qty | Source |
|---|---|---|
| Joists β 2x10 SP @ 16" o.c. | 10 Γ | IRC R507.6 (max 14'-0" span) |
| Beam β 2-2x10 SP | 2-ply, 12 ft long | IRC R507.5 (6'-1" max post spacing) |
| Posts β 6x6 SP UC4A | 3 Γ | IRC R507.4 (60 sq ft tributary) |
| Footings β 23" round Γ 36" deep | 3 Γ β 36 cu ft | IRC R507.3.1 + frost depth |
| Concrete β 80 lb bags | 60 bags | 0.6 cu ft each + 10% spillage |
| Decking β PT 5/4Γ6 Γ 16 ft | 29 boards | 405 LF + 10% perpendicular waste |
| Joist hangers β Simpson LUS210Z | 10 Γ | ZMAX G185 for ACQ / MCA PT |
| Hanger nails β Simpson N10 (1 lb box) | 1 box (~100 nails) | 10d Γ 1-1/2" HDG |
| Ledger fasteners β 1/2" lag @ 11" o.c. | ~14 lag | IRC R507.9.1.3(1), 16 ft joist span column |
| Lateral hold-downs β Simpson DTT2Z | 2 Γ | IRC R507.9.2 β MANDATORY |
| Post bases β Simpson ABU66Z | 3 Γ | 1" standoff per R317.1.4 |
| Deck-board face screws β #8 Γ 2-1/2" | ~750 screws | 2 per joist crossing per board |
| Stairs β 7 risers @ 6.86", 6 treads @ 11" | 3 Γ 2x12 stringers | IRC R311.7 |
| Railing β balusters @ 5.5" o.c. | ~88 balusters | IRC R312, 40 LF perimeter |
All numbers above come straight out of the deck calculator, which runs the same five IRC R507 lookups for you and adjusts for snow load, frost depth, coastal stainless, and decking-product spacing rules.
π§ 14. When to Step Out of Prescriptive Tables
DCA 6 and IRC R507 are wonderful when they apply β but their prescriptive scope has hard edges. Past these limits, an engineered design is required.
- β’ Deck area > 200 sq ft β past DCA 6 prescriptive scope (IRC R507 still applies for joist/beam/post sizing but engineer review recommended).
- β’ Joist span > 18 ft or beam span > 18 ft β past every prescriptive table.
- β’ Deck length > deck width (long-narrow) β out of DCA 6 scope; still permitted under IRC R507 with proper ledger sizing.
- β’ Hot tub or any > 100 psf concentrated load β DCA 6 Β§Min. Req. item 8 explicitly excludes.
- β’ Snow load > 70 psf or dead load > 10 psf (stone pavers, concrete tile decking) β past every R507 table.
- β’ Free-standing deck > 30" above grade with seismic / high-wind exposure β engineered lateral system required.
- β’ Multi-level deck β DCA 6 covers only single-level; IRC R507 silent on stacked decks.
β 15. The 10 Most Common DIY Deck Mistakes
- Skipping lateral hold-downs. R507.9.2 requires 2 Γ DTT2Z on every ledgered deck. The most-common cause of catastrophic ledger failure.
- Using deck screws in joist hangers. Simpson explicitly forbids it. Use Simpson N10 (10d Γ 1-1/2" HDG) in every face and triangle hole.
- UC3B framing instead of UC4A. Joists and beams must be ground-contact rated even when nowhere near the ground β AWPA U1 says so.
- Wrong joist spacing for the decking. Most composites need 16" o.c. perpendicular; only Deckorators Surestone allows 24". Don't default to 24".
- Galvanized hardware in salt zones. Within 300 ft of saltwater, every connector must be Type 304 or 316 stainless per R507.2.3 β no exceptions.
- Ignoring frost depth. A 12" footing in a 48" frost zone heaves 1"β4" each winter, twisting the deck off the ledger over 5β10 years.
- Attaching ledger to brick veneer. R507.9.1.1 prohibits it β veneer is non-structural. Ledger must hit the band joist behind the veneer.
- Stair risers varying more than 3/8". R311.7.5.1 β small variances cause falls. Cut all risers at once, not one at a time.
- Composite stair treads at 16" o.c. Almost every composite product caps stair-tread span at 12" o.c. regardless of flat-deck rating. ASTM D7032 label is on every piece.
- Skipping the 1" post standoff. R317.1.4 requires at least 1" between PT post and concrete. Use Simpson ABU- series, not direct-bury or post-on-pad.
Ready to size your deck?
The free deck calculator runs all five IRC R507 table lookups for you β joist span, beam span, post height, footing size, and ledger fastener spacing β plus stairs, railing, and concrete bag count. Inputs include snow load, frost depth, framing species, and the per-product joist spacing for Trex / TimberTech / AZEK / Fiberon / Deckorators.
Open the Deck Calculator β