Calculate total roof site coverage area with overlaps in Archicad

[#1]

This article describes how to calculate the total area covered by multiple roof (or slab) elements on a site, including the percentage of rainfall or site coverage, while correctly accounting for overlapping regions in Archicad. It is useful when an accurate, verifiable roof or site coverage value is needed for documentation or regulatory requirements, and when overlaps between elements would otherwise distort the result. The workflow combines Solid Element Operations with a dedicated slab, custom Properties, and Interactive Schedules to produce a transparent and easy-to-check calculation based on a community workflow shared by experienced Archicad users.

Issue description

When several roof or slab elements overlap in plan, simple summation of their individual surface areas leads to an incorrect total coverage value. This affects tasks such as calculating roof rainfall area, site coverage percentage, or differentiating permeable and impermeable areas. A method is required that produces a single, non‑overlapping coverage area and related percentages, while still being easy to validate with additional drafting checks.

Solution 

Prepare the rainfall slab

1. Place a slab that covers the whole site or the relevant roof catchment area in plan, using a distinguishable surface and a semi‑transparent material to keep it visually separate from other elements.

2. Assign this slab to a dedicated Layer (and optionally to a Layer Combination) that will later be used in Solid Element Operations and scheduling.

3. Ensure that the slab’s top surface area can be listed in schedules by using the standard slab listing parameters for gross and conditional top surface.

A clear plan screenshot of the rainfall slab placed over the project should be inserted here, with the slab visually differentiated and key edges highlighted.

Use Solid Element Operations on the slab

1. Open the Solid Element Operations dialog and set the rainfall slab as the Target.

2. Select the elements that define holes or permeable areas (for example roofs, openings, or other elements in a dedicated Layer Combination) and set them as Operators.

3. Apply a Subtraction type operation so that the Operators cut the slab volume upward, reducing the slab’s top surface in the overlapping areas.

4. Rebuild the affected views if necessary so the result of the SEO is correctly reflected in 3D and in listings.

5. Confirm in 3D and in plan that the slab’s apparent footprint now corresponds to the desired rainfall or coverage area, including all subtractions.

At this point, the slab’s net top surface area (considering SEOs) can be listed separately from its gross area, which will be used in the subsequent property expressions.

Define Properties for rainfall calculations

1. Open Property Manager from the Options menu to create a new Property Group dedicated to coverage or Tip‑related calculations.

2. Create a Boolean Property (for example “Schedules”) that will be used to flag the rainfall slab for scheduling. This Property should be set to True only for the dedicated slab.

3. Create a Property “Rainfall area” with data type Area and define it as an expression that subtracts the conditional top surface area from the gross top surface area of the slab, or vice versa, according to the desired definition.

{Property:Slab/Surface Area of the Slab Top (Gross)} - {Property:Slab/Surface Area of the Slab Top (Conditional)}​​

4. Create a Property “Rainfall percentage” with data type Number and define it as an expression that divides the rainfall area by the gross top surface area of the slab and multiplies the result by 100 to obtain a percentage:

( {Property:Tip of the Month/Rainfall area} / {Property:Slab/Surface Area of the Slab Top (Gross)} ) * 100​

5. Ensure that the classification of the slab and materials allows these Properties to be available for that element, since Property visibility depends on Classification assignments.

Create and configure the schedule

1. Create a new Element Schedule that targets slabs, or duplicate an existing one that already includes slab area fields.

2. In Scheme Settings, add the “Schedules” Property as a Criterion and set it to True so only the dedicated rainfall slab is listed.

3. Add the “Rainfall area” and “Rainfall percentage” Properties as Fields in the schedule, along with any identification fields (such as element ID or name) that support documentation.

4. Optionally include the slab’s gross and conditional top surface area fields as additional columns to make the underlying values visible and to support manual checks.

5. Update the schedule and confirm that a single row representing the rainfall slab now displays the total rainfall area and the corresponding percentage.

Verify results with fills

1. Trace the effective rainfall or coverage area with a Fill that matches the slab’s projection in plan after SEOs have been applied.

2. Read the Fill area in plan and compare it with the rainfall area reported by the schedule to confirm that they align within acceptable tolerances.Use this comparison whenever changes are made to the model to maintain confidence that the automated schedule still reflects the intended coverage area.

3. Use this comparison whenever changes are made to the model to maintain confidence that the automated schedule still reflects the intended coverage area.

---

These techniques were originally shared by Community member @Steve Jepson. If you’d like to explore the original discussion, click here to read the full thread!

This article is part of the ‘Tip of the Month’ series on Graphisoft Insights, highlighting valuable community-driven solutions for modeling challenges. Try these methods in your next project and experience the difference in model quality and efficiency.