Beams & Timbers Sizing
Comprehensive reference for structural beams and timbers — dimensions, cross sections, weight, span tables, and the difference between hand-hewn and sawn timbers.
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Beam & Timber Dimensions
Timbers are defined as lumber with a minimum dimension of 5 inches or more in both thickness and width. Beams are rectangular timbers where the width is at least 2 inches greater than the thickness. The table below covers all common sizes from 4x4 posts through 12x12 heavy timbers.
| Nominal | Actual (Dry) | Cross Section | Weight / LF | BF / LF |
|---|---|---|---|---|
| 4 x 4 | 3-1/2" x 3-1/2" | 12.25 sq in | 3.4 lbs | 1.33 |
| 4 x 6 | 3-1/2" x 5-1/2" | 19.25 sq in | 5.4 lbs | 2.00 |
| 4 x 8 | 3-1/2" x 7-1/4" | 25.38 sq in | 7.1 lbs | 2.67 |
| 4 x 10 | 3-1/2" x 9-1/4" | 32.38 sq in | 9.0 lbs | 3.33 |
| 4 x 12 | 3-1/2" x 11-1/4" | 39.38 sq in | 11.0 lbs | 4.00 |
| 6 x 6 | 5-1/2" x 5-1/2" | 30.25 sq in | 8.4 lbs | 3.00 |
| 6 x 8 | 5-1/2" x 7-1/2" | 41.25 sq in | 11.5 lbs | 4.00 |
| 6 x 10 | 5-1/2" x 9-1/2" | 52.25 sq in | 14.6 lbs | 5.00 |
| 6 x 12 | 5-1/2" x 11-1/2" | 63.25 sq in | 17.6 lbs | 6.00 |
| 8 x 8 | 7-1/2" x 7-1/2" | 56.25 sq in | 15.6 lbs | 5.33 |
| 8 x 10 | 7-1/2" x 9-1/2" | 71.25 sq in | 19.8 lbs | 6.67 |
| 8 x 12 | 7-1/2" x 11-1/2" | 86.25 sq in | 24.0 lbs | 8.00 |
| 10 x 10 | 9-1/2" x 9-1/2" | 90.25 sq in | 25.1 lbs | 8.33 |
| 10 x 12 | 9-1/2" x 11-1/2" | 109.25 sq in | 30.4 lbs | 10.00 |
| 12 x 12 | 11-1/2" x 11-1/2" | 132.25 sq in | 36.8 lbs | 12.00 |
Weights are approximate for kiln-dried Douglas Fir at ~34 lbs/ft3. Species, moisture content, and presence of defects will affect actual weight.
Load Capacity Considerations
Factors Affecting Capacity
- 1.Species: Douglas Fir and Southern Pine have among the highest bending strengths. White Pine and Cedar are significantly weaker.
- 2.Grade: Select Structural timber has the highest allowable stresses. No. 1 is roughly 80-90% of Select. No. 2 is 60-75% of Select.
- 3.Moisture: Dry timber (MC below 19%) is stronger than green timber. Kiln-dried beams carry approximately 10-15% more load than green equivalents.
- 4.Load Duration: Short-term loads (wind, seismic) allow higher stresses than permanent loads (dead weight, snow storage).
Allowable Bending Stress (Fb)
Reference values for select common beam species (Select Structural grade, dry conditions):
| Species | Fb (PSI) |
|---|---|
| Douglas Fir-Larch | 1,500 |
| Southern Pine | 1,500 |
| Hem-Fir | 1,100 |
| SPF | 1,150 |
| Red Oak | 1,150 |
| White Oak | 1,100 |
Span Tables
The following tables provide general span guidance for common beam applications. These are conservative estimates based on Douglas Fir #1 grade. Always consult a structural engineer for final design decisions — especially with reclaimed timber, where hidden defects may reduce capacity.
Floor Beam Maximum Spans
Based on 40 PSF live load + 10 PSF dead load, L/360 deflection limit, Douglas Fir #1 grade.
| Beam Size | Species / Grade | Span @ 16" OC | Span @ 24" OC |
|---|---|---|---|
| 4 x 8 | Douglas Fir #1 | 8 ft 2 in | 6 ft 8 in |
| 4 x 10 | Douglas Fir #1 | 10 ft 5 in | 8 ft 6 in |
| 4 x 12 | Douglas Fir #1 | 12 ft 8 in | 10 ft 4 in |
| 6 x 8 | Douglas Fir #1 | 10 ft 10 in | 9 ft 6 in |
| 6 x 10 | Douglas Fir #1 | 13 ft 9 in | 12 ft 1 in |
| 6 x 12 | Douglas Fir #1 | 16 ft 8 in | 14 ft 7 in |
Ridge Beam Approximate Spans
Approximate maximum spans for ridge beams supporting rafters on both sides. Values assume Douglas Fir Select Structural or better.
| Beam Size | Max Roof Span | Load Condition |
|---|---|---|
| 4 x 6 | Up to 10 ft | Light (20 PSF snow) |
| 4 x 8 | Up to 14 ft | Light (20 PSF snow) |
| 6 x 8 | Up to 18 ft | Moderate (30 PSF snow) |
| 6 x 10 | Up to 22 ft | Moderate (30 PSF snow) |
| 6 x 12 | Up to 26 ft | Moderate (30 PSF snow) |
| 8 x 10 | Up to 28 ft | Heavy (40 PSF snow) |
| 8 x 12 | Up to 32 ft | Heavy (40 PSF snow) |
| 10 x 12 | Up to 36 ft | Heavy (40 PSF snow) |
Important: These span tables are for general reference only. Actual allowable spans depend on specific loading conditions, support configurations, connection details, and local building codes. A licensed structural engineer should review all beam sizing for load-bearing applications.
Post / Column Capacity Reference
Axial load capacity for timber posts varies with height, species, and end conditions. The following table shows approximate allowable axial loads for common post sizes at typical heights.
| Post Size | 8 ft Height | 10 ft Height | 12 ft Height |
|---|---|---|---|
| 4 x 4 | 12,400 lbs | 9,600 lbs | 7,400 lbs |
| 4 x 6 | 18,600 lbs | 14,900 lbs | 11,700 lbs |
| 6 x 6 | 33,800 lbs | 29,100 lbs | 24,200 lbs |
| 6 x 8 | 45,100 lbs | 38,800 lbs | 32,300 lbs |
| 8 x 8 | 63,500 lbs | 57,200 lbs | 50,200 lbs |
| 10 x 10 | 103,000 lbs | 96,000 lbs | 88,000 lbs |
| 12 x 12 | 152,000 lbs | 145,000 lbs | 136,000 lbs |
Approximate values for Douglas Fir #1, pin-pin end conditions. Actual capacity depends on grade, species, bracing, and connection details.
Hand-Hewn vs. Sawn Beams
Reclaimed beams come in two primary forms: hand-hewn timbers shaped with axes and adzes, and sawn timbers processed through a sawmill. Each type has distinct characteristics that affect sizing, appearance, and structural properties.
Hand-Hewn Timbers
Dimensions: Hand-hewn timbers are never perfectly uniform. A beam labeled as 8x8 may vary from 7-1/2" to 8-3/4" along its length. Each face has subtle undulations from the axe work, and corners may be slightly rounded or irregular.
Cross Section: Typically slightly tapered from one end to the other, following the natural taper of the log. Cross sections may be slightly trapezoidal rather than perfectly rectangular.
Surface: Shows distinctive axe marks (scalloping) that are highly prized for exposed applications. The texture adds visual depth and character.
Age: Most hand-hewn timbers date from the 1700s through the 1890s. Species are typically White Oak, White Pine, American Chestnut, or Eastern Hemlock.
Best For: Exposed mantels, decorative ceiling beams, architectural accents, rustic post-and-beam construction where visual character is paramount.
Sawn Timbers
Dimensions: Sawn timbers are more consistent than hand-hewn, but reclaimed sawn timbers still tend to be closer to true nominal dimension than modern lumber. A reclaimed sawn 8x8 often measures 7-3/4" to 8" — larger than a modern 7-1/2" x 7-1/2".
Cross Section: More uniform and rectangular. May show circular saw marks (pre-1900) or band saw marks (post-1900) on surfaces.
Surface: Smoother than hand-hewn but still shows mill marks and patina. Circular saw marks appear as curved striations; band saw marks appear as straight, closely spaced lines.
Age: Sawn timbers span a wider date range, from the 1840s (when circular saws became common) through the mid-1900s. Species vary widely by region and era.
Best For: Structural applications, timber framing, exposed beams where a cleaner profile is desired, or any application requiring more consistent dimensions.
| Characteristic | Hand-Hewn | Sawn (Reclaimed) | Modern (New) |
|---|---|---|---|
| Dimensional Consistency | Low (varies 1/4" - 1") | Medium (varies 1/8" - 1/2") | High (within 1/8") |
| Typical Size vs. Nominal | At or above nominal | Near true nominal | Below nominal |
| Surface Texture | Axe marks, scalloped | Saw marks, patina | Smooth, surfaced |
| Structural Grading | Visual inspection only | Visual or machine graded | Machine stress rated |
| Typical Species | Oak, Pine, Chestnut | Mixed (region-dependent) | DF, SYP, SPF |
| Price Range | $$$$ (premium) | $$$ (moderate-high) | $$ (standard) |
| Best Application | Decorative / character | Structural / exposed | Structural / hidden |
Beam Weight for Transport Planning
Large timbers are heavy. Knowing approximate weights helps you plan equipment, transport, and installation. The table below shows weights for common beam lengths in Douglas Fir.
| Size | 8 ft | 10 ft | 12 ft | 16 ft | 20 ft |
|---|---|---|---|---|---|
| 4 x 4 | 27 lbs | 34 lbs | 41 lbs | 54 lbs | 68 lbs |
| 6 x 6 | 67 lbs | 84 lbs | 101 lbs | 134 lbs | 168 lbs |
| 6 x 8 | 92 lbs | 115 lbs | 138 lbs | 184 lbs | 230 lbs |
| 8 x 8 | 125 lbs | 156 lbs | 187 lbs | 250 lbs | 312 lbs |
| 8 x 10 | 158 lbs | 198 lbs | 238 lbs | 317 lbs | 396 lbs |
| 8 x 12 | 192 lbs | 240 lbs | 288 lbs | 384 lbs | 480 lbs |
| 10 x 10 | 201 lbs | 251 lbs | 301 lbs | 402 lbs | 502 lbs |
| 10 x 12 | 243 lbs | 304 lbs | 365 lbs | 486 lbs | 608 lbs |
| 12 x 12 | 294 lbs | 368 lbs | 442 lbs | 589 lbs | 736 lbs |
Douglas Fir at ~34 lbs/ft3 kiln dried. Oak beams weigh approximately 35-40% more. Green timber may weigh 50-80% more than these values.