TuffBlock Deck Calculator
Calculate how many TuffBlocks you need based on your deck dimensions, board direction, timber and framing clearance. Preview the support layout and get an indicative timber list.
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Planning estimate only. This calculator does not establish compliance with AS 1684.2, the NCC or local building requirements. Assumes all timber framing is more than 150mm above ground and a finished deck height of no more than 915mm. Blocking, fixings, joist restraint, stock lengths and joints are not calculated. Check that your decking manufacturer permits joist centres up to 450mm, confirm timber treatment and actual fit, and verify the final framing design for your site.
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 approval-ready construction documentation.
This calculator does not calculate or verify:
- Deck-board material, thickness and manufacturer requirements
- The actual timber species, stress grade, treatment or supplied dimensions
- Bearers, joist hangers, rim members, fixings or complete connection design
- Stairs, guards, openings, roofs or concentrated loads such as spas or planters
- Soil, concrete, gravel or paver conditions, slope, compaction, settlement or drainage
- Wind, flood, termite and other site-specific environmental conditions
- Uplift, lateral restraint, anchoring or the complete structural load path
- NCC requirements, state or territory variations, local building requirements, approvals and inspections
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 timber 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, bearers, overhang, framing clearance, site conditions and the finished look you're trying to achieve. For example, some designs position the supporting structure further beneath the deck edge for 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 are unsure between layouts, use the more conservative supported arrangement and verify the final framing design before building.
Common deck sizes and estimated quantities
These examples use Treated Pine F7 joists, boards along the deck width, no overhang — the same assumptions as the calculator above with its defaults. Selecting a different species, board direction or overhang in the tool above may return a different block count.
| Deck size | Deck area | 90x45mm | 120x45mm |
|---|---|---|---|
| 2.4 x 2.4m | 5.8 sqm | ||
| 3 x 3m | 9.0 sqm | ||
| 3.6 x 3m | 10.8 sqm | ||
| 3.6 x 3.6m | 13.0 sqm | ||
| 4.8 x 3.6m | 17.3 sqm | ||
| 6 x 3.6m | 21.6 sqm | ||
| 6 x 4.8m | 28.8 sqm |
| Deck size | Deck area | 90x45mm | 120x45mm | 140x45mm | 190x45mm | 240x45mm |
|---|---|---|---|---|---|---|
| 2.4 x 2.4m | 5.8 sqm | |||||
| 3 x 3m | 9.0 sqm | |||||
| 3.6 x 3m | 10.8 sqm | |||||
| 3.6 x 3.6m | 13.0 sqm | |||||
| 4.8 x 3.6m | 17.3 sqm | |||||
| 6 x 3.6m | 21.6 sqm | |||||
| 6 x 4.8m | 28.8 sqm |
“Under 400mm” means the underside of the framing is more than 150mm and less than 400mm above ground; framing at or below 150mm above ground is not covered by this tier. For Treated Pine F7 at this clearance, stress grade alone is not enough: the timber's treatment and durability must suit your ground clearance and exposure, so confirm the timber you are supplied is appropriate for your application.
For Treated Pine F7 in this tier, the 1,500mm TuffBlock spacing cap is the governing constraint, so the different joist sizes can legitimately return the same block total.
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 TuffBlock support rows along each run without exceeding the allowable span for your selected timber and framing clearance. Worked example: a 4.8m x 3.6m freestanding deck, 140x45mm Treated Pine F7 joists, 400mm or above framing clearance, boards along the deck width and no overhang.
Enter deck size and board direction
Enter your deck length and width in metres, and choose which way the deck boards run. Joists always run perpendicular to the boards.
Example: 4.8 x 3.6m, boards along width → joists run along the 4.8m length.
Select timber and framing clearance
Choose your timber species/grade and your framing clearance — the underside of the framing is either more than 150mm and under 400mm above ground, or 400mm or above. This determines which published span table applies.
Example: Treated Pine F7, 400mm or above → 140x45mm rated to 2.6m continuous span.
Calculate support positions
The calculator finds the fewest support rows that keep each span within the allowable span for your timber and within TuffBlock's 1,500mm maximum spacing, whichever is smaller. For TDS4 and the Hyne T3 Green+ MGP10 table, two support rows use the published single-span value and three or more use the continuous-span value. For TDS13 low-clearance combinations, the same TDS13-derived allowable span applies either way, as TDS13 gives no separate continuous-span value.
Example: 5 support rows create four 1,200mm spans along the 4.8m run.
Generate the estimate
Support rows per joist run are multiplied by the total number of joists required (spaced automatically at up to 450mm centres).
Example: 5 support rows × 9 joists = 45 TuffBlocks (4 × 12-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 timber species, grade and joist size.
Overhang
Selecting an overhang moves support rows inward by the smaller of BuildTuff's 300mm maximum, 25% of the single-span figure, or 50% of the actual backspan.
Sloped sites
This calculator assumes a level, ground-level deck with joists directly on TuffBlocks.
Local requirements
Confirm footing, span and approval requirements with your local council or building certifier, and against the NCC/AS 1684 where applicable.
TuffBlock calculator FAQs
Enter your deck dimensions, board direction, timber species and framing clearance above. The calculator works out how many joists you need, how many support rows each joist run needs, and multiplies the two.
For TDS4 and the Hyne T3 Green+ MGP10 table, the calculator uses the published single-span value for a two-support-row layout and the continuous-span value for layouts with three or more support rows. For TDS13 low-clearance combinations, the calculator applies the same TDS13-derived allowable span in either case because TDS13 does not provide a separate continuous-span value for these selections.
Every interval is also capped at TuffBlock's 1,500mm maximum spacing, whichever is smaller. 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, flush with each joist end. Selecting an equal overhang moves the outer rows inward, up to whichever is smaller: BuildTuff's 300mm maximum, 25% of the timber's single-span figure, or 50% of the actual backspan.
TuffBlock may be used as an above-ground foundation option for suitable freestanding projects, subject to the product's installation requirements, site conditions, structural design and applicable local requirements. The calculator does not determine whether TuffBlock is suitable as a replacement for a specific engineered or code-required footing.
TuffBlocks may be suitable for certain shed foundations, but this deck calculator does not design or validate shed foundations — check your shed manufacturer's base requirements and relevant local approvals separately.
Requirements vary by project, structure height, location and local authority. Confirm applicable NCC requirements, Australian Standards, planning rules and engineering requirements with your council, private building certifier or a suitably qualified building professional before construction.
Related deck-building resources
Move from quantity planning to product comparison and installation guidance.
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Everything you need to move from estimate to build.