Elevated greenhouse deck — tributary load & load path

The problem: a simplified teaching case for the concept behind MM's planned greenhouse/carport addition. The real build (25×25 ft, elevated on masonry piers, an 18" fieldstone retaining wall doing double duty as both gravity support and cut-slope retainer, occupied second-story greenhouse intended as a flagship feature) is squarely engineer-stamp territory — an occupied multi-story addition on a wall that's also fighting lateral soil pressure is not a DIY sign-off case. This chapter builds the load-path vocabulary to work with that engineer as a peer, not a spec to build from directly.

Simplified case: a basic 12×12 ft elevated greenhouse deck on 4 symmetric corner piers, no retaining wall involved. Concepts pulled in — new, first multi-support load path: tributary area — how total floor load divides among multiple supports.

Loads

• Framing dead load: ~12 psf
• Growing medium (8" wet soil, ~100 pcf): ~67 psf
• Occupant/tending live load: ~20 psf
• Total: ~100 psf — note: raised-bed soil is far heavier than people expect; wet growing medium alone can approach or exceed a normal floor's entire design load budget

Calculation

• Total load on floor: 100 psf × 144 sq ft = 14,400 lb
• With 4 symmetric corner piers, each takes roughly ¼ of the tributary area: 14,400 ÷ 4 ≈ 3,600 lb per pier

Answer / method

Total load ÷ tributary area per support = the load that support has to carry. This is the method that scales to any pier count, footprint, or the real flagship greenhouse once it's fully dimensioned — at which point it moves to a licensed structural engineer given the retaining-wall/gravity-load combination and occupied multi-story scope.