Trenching Through Mature Conifer Root Zones for Sewer Installation

A driveway row of 8–10" diameter western red cedar (Thuja plicata) and Douglas fir (Pseudotsuga menziesii), spaced 12–18" apart, 5–12 ft from house on a 45° slope, requires a 4–5 ft deep sewer trench.

Root System Facts

• Roots: Top 12–18" of soil
• Spread: 1.5–2× branch width
• Cedar: 25 ft wide at maturity (prunable narrower)
• Fir: 20 ft wide, 60–80 ft tall (poor pruning response)
• Fir may have deep taproot under trunk

Trench Impact

• Cutting roots kills severed portions
• Up to ⅓ root loss tolerable without severe decline
• Stability minimally affected (anchoring roots deeper)
• Health impact significant: stress, dieback, pest susceptibility

Critical Problem: Overcrowding

12–18" spacing for species needing 20–25 ft width is unsustainable. Trees are mutually suppressing. Wrong species for space.

Recommended Path

1. Install sewer line with minimal root cutting (directional bore if possible)
2. Remove firs (poor pruning response, too large)
3. Retain cedars as managed hedge with professional pruning
4. Or: Remove all, install sewer, plant appropriate hedge species (Taxus, Ilex, Thuja 'Green Giant')

Consult ISA Certified Arborist on-site before trenching.