Inputs
Example data table
| Depth (mm) | Slope (%) | Distance (m) | Drainage | Fire | Traffic | Containment | Sample score | Sample level |
|---|---|---|---|---|---|---|---|---|
| 60 | 5 | 1.5 | Good | Low | Low | Yes | 28 | Low |
| 90 | 8 | 0.8 | Average | Medium | Medium | Yes | 72 | High |
| 120 | 12 | 0.4 | Poor | High | High | No | 121 | Severe |
| 45 | 18 | 2.0 | Average | Low | Medium | No | 67 | Moderate |
| 75 | 3 | 1.0 | Good | Medium | Low | Yes | 45 | Moderate |
Sample scores are illustrative. Your result depends on all selected factors.
Formula used
This calculator uses a weighted risk score capped at 150:
Total Risk = Depth + Drainage + Slope + Fire + Type + Proximity + Traffic + Containment + Pests + Irrigation
- Depth score rises sharply above 75 mm to reflect moisture and hidden hazards.
- Drainage and irrigation add penalties for persistent dampness and decay.
- Slope and containment capture washout, migration, and trip risk.
- Fire exposure and proximity increase risk near structures and ignition sources.
- Traffic and pest history represent operational wear and inspection needs.
How to use this calculator
- Measure average mulch depth at several points.
- Estimate slope and nearest structure distance.
- Select material type and drainage condition honestly.
- Choose fire exposure and typical foot traffic.
- Click Calculate Risk to view results above.
- Download CSV or PDF for records and approvals.
Notes for practical use
- Re-check depth after storms, wind, or heavy foot traffic.
- Keep mulch away from drainage inlets and walking edges.
- Use clear borders to prevent migration onto pavements.
- In high exposure zones, prefer non-combustible buffers.
Depth selection and performance targets
Mulch depth affects weed suppression, moisture buffering, and surface stability on landscaped works. Shallow layers dry quickly, expose soil, and allow wind erosion. Overly deep layers trap moisture, conceal edges, and increase slip risk at transitions. Specify a target band and tolerances, then verify thickness after raking and settlement. For many beds, 50–75 mm is a practical baseline, adjusted for material and exposure. When depth exceeds the recommended band, consider removing and regrading rather than topping up, because hidden voids and uneven surfaces remain.
Moisture retention, drainage, and biological activity
Risk rises when depth combines with poor drainage, clay subgrades, or frequent irrigation. Saturated mulch can create anaerobic pockets, produce odor, and accelerate timber decay near cladding. Fine grades also crust and shed water, sending runoff toward pavements. Coordinate grading, subbase permeability, and overflow routes so wet zones drain predictably.
Slope behavior and migration control
On sloped verges, mulch behaves like a granular layer and can migrate downslope under rainfall, foot traffic, and blower cleaning. Migration exposes soil, blocks inlets, and creates trip lines at borders. Use edging, terrace breaks, and fiber tackifiers where slopes exceed common maintenance limits. Confirm that down slope hardscape has a clean edge to capture drift.
Fire exposure and structure setbacks
In fire prone areas, combustible mulch can support ember ignition and transmit heat to vulnerable details. Maintain non combustible buffers adjacent to walls, decks, timber services, and vents. Favor conservative depths near structures, keep moisture balanced, and remove dry debris routinely. Stockpiles should be separated from ignition sources and monitored during hot, windy periods.
Quality control and lifecycle maintenance
Include depth checks in handover and routine maintenance plans. Measure several points per bed, record deviations, and correct low spots after storms. Track pest activity, compaction, and surface level changes at paths and ramps. Using a scored risk output supports design decisions, contractor documentation, and safer ongoing operation across the site. Document changes so crews repeat the safe setup.
FAQs
What depth range is usually safer for most beds?
Many projects perform well around 50–75 mm, adjusted for material, drainage, and exposure. Keep depth consistent, avoid burying plant crowns, and reduce thickness near paths, vents, and timber details.
How does distance to structures affect risk?
Closer mulch increases moisture contact with walls and can worsen ember exposure in fire-prone areas. Maintain a non-combustible strip and keep mulch below cladding clearances so drainage and inspections remain easy.
What should I do if drainage is poor?
Reduce depth, improve grading, and consider a coarser layer over a permeable base. Verify downpipes and surface falls, and avoid daily irrigation. Persistent wet spots should be remediated before adding more material.
Does slope change the recommended depth?
Yes. On slopes, thinner, well-contained layers reduce migration and trip lines. Use edging, breaks, or tackifiers to hold material. After heavy rain, recheck depth at the toe and along borders.
Is rubber mulch treated differently in the score?
Rubber can elevate heat and smoke concerns and may behave differently under fire exposure. Use manufacturer guidance, confirm drainage, and keep clear setbacks. The calculator adds a modest penalty to reflect these factors.
How often should I reassess mulch depth risk?
Reassess after storms, seasonal maintenance, or any change in irrigation and traffic. A quick monthly check is common for busy sites. Record observations so repeated issues, like washout or pests, are addressed early.