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Calcimator

Green Roof Waterproofing Calculator

Root barrier and membrane for vegetated roof systems.

About this calculator

Saturated Weight (PSF) depends on exactly one input: Green Roof Type (which sets growing-medium depth and density). It's a per-square-foot constant -- Roof Length, Roof Width, Number of Roof Drains, Penetrations, and Parapet Perimeter have NO effect on it whatsoever, no matter how far they're moved across their full declared ranges. Total Saturated Weight is different: it's Saturated Weight (PSF) multiplied by Roof Area, so it does scale with Roof Length and Roof Width even though the per-square-foot figure doesn't.

Green Roof Type is a coded 1/2/3 switch (extensive/semi-intensive/intensive) that changes both growing-medium depth (2-6 in / 6-12 in / 12-24 in average) and density (55/60/65 lb per cu ft dry), and its effect on Saturated Weight (PSF) is large and non-linear: going from extensive (1) to intensive (3) more than quintuples the saturated dead load per square foot, since both the depth and the density increase together. Roof Length and Roof Width move Estimated Cost, Membrane Area, and Total Saturated Weight with identical, symmetric influence, since Roof Area is their simple product -- but they leave Saturated Weight (PSF) completely unchanged. Parapet Perimeter and Number of Penetrations move Estimated Cost only modestly -- through membrane turn-up and flashing detail area -- compared to the effect of roof size or green roof type.

Inputs

ft
ft
ft

Results

Membrane area (sq ft)

2,003

Root barrier (sq ft)1,620
Drainage layer (sq ft)1,575
Growing medium (cu yd)18.5
Saturated weight (PSF)36.7
Total saturated weight (lbs)55,000
Estimated material cost ($)$16,878.00
How to Use This Calculator
  1. Enter Roof Length (ft) and Roof Width (ft) — the vegetated roof area to be waterproofed.
  2. Select Green Roof Type (1=extensive/sedum, 2=semi-intensive/perennials, 3=intensive/shrubs) to set growing medium depth.
  3. Input Number of Roof Drains, Penetrations, and Parapet Perimeter (ft) requiring special flashing treatment.
  4. Review Membrane Area (sq ft), Root Barrier (sq ft), and Growing Medium (cu yd) to develop material quantities.
  5. Check Saturated Weight (PSF) and Total Saturated Weight (lbs) to verify the structural system can support the assembly.
  6. Use Estimated Material Cost for budgeting — note that labor and structural upgrades are additional.

How the result changes with Roof length (ft)

Roof length (ft)Membrane area (sq ft)
251,163
381,599
752,843
1254,523

What each input means

Roof length (ft)
Length of the green roof area.
Roof width (ft)
Width of the green roof area.
Type (1=extensive, 2=semi, 3=intensive)
1 = Extensive (sedum, 2-6 in. media). 2 = Semi-intensive (perennials, 6-12 in.). 3 = Intensive (shrubs/trees, 12+ in.).
Roof drains
Number of roof drain assemblies requiring special flashing.
Penetrations
Pipes, vents, and other penetrations through the membrane.
Parapet perimeter (ft)
Linear feet of parapet walls requiring membrane turn-up.

What each result means

Membrane area (sq ft)
Total waterproofing membrane including flashings and 12% waste.
Root barrier (sq ft)
HDPE or copper-laminated root barrier area.
Drainage layer (sq ft)
Dimpled drainage mat or geocomposite area.
Growing medium (cu yd)
Volume of lightweight growing medium needed.
Saturated weight (PSF)
Fully saturated dead load in pounds per square foot for structural design.
Total saturated weight (lbs)
Total weight of saturated green roof assembly.
Estimated material cost ($)
Materials only — membrane, root barrier, drainage, filter fabric, and growing medium.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Roof length (ft) = 50, Roof width (ft) = 30, Type (1=extensive, 2=semi, 3=intensive) = 1, Roof drains = 4 = 6 input(s) provided
  2. Calculate Membrane area
    Membrane area = totalMembraneArea * 1.12
    2003 = 2003
  3. Calculate Root barrier
    Root barrier = roofArea * 1.08
    1620 = 1620
  4. Calculate Drainage layer
    Drainage layer = roofArea * 1.05
    1575 = 1575

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Do more roof drains or penetrations increase Saturated Weight?

No -- Number of Roof Drains, Penetrations, and Parapet Perimeter have zero effect on Saturated Weight (PSF) or Total Saturated Weight, across their entire input ranges. Saturated Weight (PSF) depends only on Green Roof Type (which sets growing-medium depth and density) -- it's a fixed per-square-foot figure that doesn't move with roof dimensions at all. Total Saturated Weight then scales that fixed PSF figure by Roof Area. Drains, penetrations, and parapets only affect Membrane Area and Estimated Material Cost, through added flashing.

How much heavier is an intensive green roof than an extensive one?

Substantially heavier -- going from extensive (Green Roof Type 1, sedum with 2-6 in. of media) to intensive (Type 3, shrubs/trees with 12-24 in. of media) more than quintuples Saturated Weight (PSF), because both the growing-medium depth and its density increase together. This is the single biggest structural consideration when choosing a green roof type, well beyond the cost difference in materials.

What matters more for Estimated Cost: roof size or roof type?

Both matter, but through different outputs. Roof Length and Roof Width scale essentially everything -- membrane, root barrier, drainage layer, and growing medium volume -- symmetrically. Green Roof Type changes growing-medium depth and therefore volume and cost per square foot, but does not change the membrane, root barrier, or drainage-layer area calculations, which depend only on roof area.

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