TuffBlock Deck Calculator
Calculate how many TuffBlocks you need based on your deck dimensions and joist size. Preview the support layout and get a basic timber cut list.
- 150,000+ builds
- 25-year warranty
- No email required
Based on 2024 IRC Table R507.6, 40 psf live load, No. 2 lumber. This calculator assumes a freestanding rectangular deck with joists perpendicular to the deck boards and TuffBlocks positioned at the calculated support rows.
Planning estimate only. Overhang and support spacing are checked against the selected joist-span and cantilever limits. Snow loads, post sizing and local code requirements are not assessed.
Example support layout only. Deck-board lines indicate direction only. Support rows are positioning guides, not calculated beams or bearers. Blocking, rim joists, joist hangers, fixings and connections are not shown or calculated. Quantities may change with alternative framing or overhang arrangements, concentrated loads, site conditions or local requirements. Do not use this diagram as a construction plan. Verify the final layout and quantities before ordering materials or building.
Your framing design determines your TuffBlock layout.
This calculator provides a preliminary TuffBlock quantity and joist estimate for one common rectangular, single-level freestanding deck layout. It is a planning tool only, not a structural design, engineering assessment or permit-ready construction plan.
This calculator does not calculate or verify:
- Deck-board direction, material, thickness and manufacturer requirements
- Joist direction, lumber species, grade, treatment and actual dimensions
- Beams, bearers, joist hangers, blocking, doubled members and concealed supports
- Stairs, guards, openings, roofs and concentrated loads such as spas or planters
- Soil, concrete, gravel or paver conditions, slope, compaction, settlement and drainage
- Wind, snow, frost, seismic, flood, termite and other environmental conditions
- Uplift, lateral restraint, anchoring, connections and the complete structural load path
- Local codes, amendments, approvals, permits and inspection requirements
Confirm your final framing layout, materials, quantities, site conditions and regulatory requirements with the relevant manufacturers, your local building authority, a licensed contractor or a qualified design professional before ordering materials or beginning construction.
Your lumber sets the spacing, not the block
This calculator demonstrates one common approach using joists supported directly at calculated intervals. It is not the only way to build with TuffBlock.
Decks can be framed in different ways depending on joist direction, beams, joist hangers, overhangs, deck height, site conditions and the finished look you're trying to achieve. For example, some designs position the supporting structure further beneath the deck edge to create a more concealed, floating appearance.
Always design your framing around appropriate span requirements and local building regulations rather than relying solely on the calculator result.
If you're between two layouts, go with more support. It's the difference between a deck that feels solid and one that doesn't.
Common deck sizes and estimated quantities
These examples use Redwood / Cedars / Pines joists (No. 2 grade) at 16-inch on-centre spacing, boards along the deck width, no overhang — the same assumptions as the calculator above with its defaults. Selecting a different species, spacing, board direction or overhang in the tool above may return a different block count.
| Deck size | Deck area | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|---|
| 8 x 8 ft | 64 sq ft | ||||
| 10 x 10 ft | 100 sq ft | ||||
| 12 x 10 ft | 120 sq ft | ||||
| 12 x 12 ft | 144 sq ft | ||||
| 16 x 12 ft | 192 sq ft | ||||
| 20 x 12 ft | 240 sq ft |
Quantities are planning estimates, not structural engineering advice. Final requirements depend on site conditions, framing details and local codes. Select any quantity to load it into the calculator.
How your estimate is calculated
The calculator turns deck dimensions into joist runs, then places supports along each run without exceeding the allowable span for your selected lumber species and joist size. Worked example: a 20 × 12 ft floating deck, with 2x8 Redwood / Cedars / Pines joists at 16-inch on-centre, boards along the deck width and no overhang.
Set joist spacing
Select 12, 16 or 24-inch on-centre joist spacing across the deck width — 16-inch is the default and a common choice for residential decks; 12 or 24-inch may suit your decking material and framing.
Example: 16 in OC across 12 ft gives 10 joists.
Select species and joist size
Each lumber species and nominal joist size combination has its own conservative maximum support span.
Example: 2x8 Redwood / Cedars / Pines at 16 in OC spans up to 10 ft 7 in.
Calculate support positions
End TuffBlocks sit flush with each joist end by default (or inset by your selected overhang). Additional blocks are distributed so no gap exceeds the allowable span.
Example: 3 support rows create two 10 ft spans.
Generate the estimate
Blocks per joist run are multiplied by the total number of joists required.
Example: 3 blocks × 10 joists = 30 TuffBlocks (1 × 24 Pack + 1 × 8 Pack).
Important build considerations
The result is a starting point for standard low-profile layouts. These conditions need additional design judgement.
Ground-level decks
Allow for drainage, ventilation and adequate ground preparation beneath the frame.
Joist span
Do not exceed the displayed support span for the selected lumber species and nominal joist size.
Cantilevers
Concealed edges and perpendicular blocking alter the standard support layout.
Sloped sites
This calculator assumes a level, ultra-low-profile deck with joists directly on blocks.
Local requirements
Confirm footing, span, permit and attachment requirements with your local authority.
TuffBlock calculator FAQs
Enter the deck length, width, lumber species, joist size and joist spacing above. The calculator estimates the number of joists for your selected spacing, calculates blocks required along each run for the allowable span, and multiplies the two figures.
TuffBlock support spacing is calculated using the allowable joist span for your selected lumber species, joist size and joist spacing. Actual support requirements depend on your framing design, site conditions, applicable loads and local building codes.
The result illustrates one possible direct-joist foundation layout, not a complete structural design. Always verify your final support spacing and construction method before building.
By default, the outer support rows sit flush with the joist ends — they are not automatically inset. If you select an equal overhang at both ends, those outer rows move inward by that amount, up to whichever is smaller: BuildTuff's 12-inch maximum end overhang, or the selected lumber species' IRC cantilever allowance for the resulting back span.
Yes. TuffBlocks are designed as an above-ground alternative to traditional concrete deck blocks and pier footings for ground-level floating decks and small structures. They eliminate the need for digging and concrete while providing strong, stable support when properly spaced.
Yes. TuffBlocks are commonly used for shed foundations, small backyard structures and platforms. The same support principles apply, but check your shed manufacturer's base requirements before relying on the estimate.
Permit requirements vary by location. Many small, freestanding ground-level decks do not require permits, but always check local building regulations before starting construction.
Related deck-building resources
Move from quantity planning to product comparison and installation guidance.
How many deck blocks do you need?
A detailed guide explaining deck block spacing, joist layout, and how to estimate the right number of TuffBlocks for your project.
Read the guide → COMPARE 02TuffBlock vs concrete deck blocks
Compare TuffBlocks with traditional concrete deck blocks and pier footings to see which foundation system is right for your build.
Compare systems → INSTALL 03How to install TuffBlocks step-by-step
Get detailed instructions for placing, spacing and levelling TuffBlocks for a strong, ground-level deck foundation.
View installation →Got your TuffBlock number?
Everything you need to move from estimate to build.