| Scenario | Deck size | Material | Planters | Water feature | Live load | Est. service total |
|---|---|---|---|---|---|---|
| Family seating | 4 m × 3 m | Timber | 4 × 35 kg | None | 2.4 kPa | ≈ 3.1 kPa |
| Entertaining | 5 m × 4 m | Composite | 6 × 45 kg | 300 L + 40 kg | 3.0 kPa | ≈ 4.0 kPa |
| Heavy finishes | 3 m × 3 m | Pavers | 8 × 50 kg | None | 2.4 kPa | ≈ 5.0 kPa |
The calculator combines dead load (self-weight and permanent items) with live load (people and movable items). Garden features are converted into an average distributed load over the deck area.
- Area: A = L × W
- Total dead load: D = Duniform + Dplanters + Dwater
- Service total: T = D + Llive
- Factored total: Tf = T × SF
- Joist line load: w = T × s (spacing s as length units)
- Load per joist: F = w × span
- Simple reaction (two supports): R = F ÷ 2
The “limit total load” check is a quick comparison only. Deck design also depends on spans, lumber grade, connections, posts, footings, and lateral bracing.
- Select your unit system and enter deck length and width.
- Enter joist spacing and span to estimate joist line loads.
- Pick a decking material, then add any extra permanent weight.
- Add planters and water features to include garden-specific loads.
- Set a safety factor and a limit load for a quick check.
- Click Calculate deck load to view results above the form.
- Use Download CSV or Download PDF for records.
If you plan heavy concentrated items (large planters, masonry, hot tubs, ponds), consult a qualified professional before building.
1) Why deck loads matter
A garden deck is a small structure with big consequences. Typical design live loads range from 2.0–4.0 kPa (about 40–80 psf), covering people, furniture, and movement. When loads exceed the intended capacity, you risk excessive deflection, squeaks, connection loosening, and long‑term damage at posts and footings.
2) Dead load is more than decking
Dead load includes boards, joists, fasteners, rails, benches, pergola framing, and finishes. Heavier surfaces can add meaningful weight: composite boards often run higher than timber, and pavers can be several times heavier. Use “extra dead load” when you add permanent elements such as planter boxes, privacy screens, or masonry trims.
3) Garden features add hidden weight
Planters and water features are common load surprises. Wet soil and saturated media increase mass, and water is heavy (about 1 kg per liter or 8.34 lb per gallon). This calculator converts those items into an average area load, helping you compare a few small pots versus multiple large containers or a pond with pumps and stones.
4) From surface load to joist line load
Surface loads (kPa or psf) become joist line loads when multiplied by joist spacing. For example, increasing spacing from 400 mm to 600 mm raises the load per joist by 50% at the same surface pressure. The line load estimate supports quick checks of support reactions and overall framing demand before you choose lumber sizes and connection details.
5) Use the safety factor wisely
A safety factor adds buffer for uncertainty, placement errors, and variable use. It does not replace code checks for span tables, lateral bracing, ledger attachment, and footing capacity. If the “service total” is near your limit, consider reducing concentrated garden loads, adding posts, decreasing joist spacing, or engaging a qualified professional for design verification.
1) What does “service total load” mean?
It is the combined dead load and live load used for everyday use. The calculator also shows a factored load by multiplying the service total by your chosen safety factor.
2) Why are planters and water averaged over the deck area?
Averaging gives a quick overall comparison for planning. If a heavy item sits in one spot, you should also check localized framing, blocking, and bearing near that location.
3) How do I choose a live load value?
Start with local code guidance for decks and balconies. Use higher values for entertaining, dense seating, or movable heavy items. When unsure, choose a conservative number.
4) Is the limit load a building code requirement?
The limit here is a user-defined comparison target. Codes vary by region, occupancy, and exposure. Always verify required design loads and details with your local authority.
5) What joist spacing should I use for stronger performance?
Smaller spacing reduces the load carried by each joist and improves stiffness. Common spacing is 400 mm (16 in). Heavier finishes or longer spans often benefit from tighter spacing.
6) Does this calculator size joists or beams?
No. It estimates loads and a simple joist line load. Final sizing depends on lumber grade, species, span tables, beam design, connections, and lateral bracing requirements.
7) When should I consult a professional?
Consult an engineer or qualified designer for heavy concentrated loads, hot tubs, ponds, masonry finishes, tall posts, unusual geometry, or any deck attached to a home.