Adding Slope to a Flat Roof: Establishing Ridge Lines and Low Points

Now that we have established our flat roof geometry, we'll enhance it by adding ridge lines and low points to create the proper drainage slope. This critical step transforms a basic flat roof into a functional design that effectively manages water runoff. Begin by selecting the roof element using the Tab method for quick selection, though you can also utilize the crossing selection tool combined with the Filter option to isolate only roof elements—particularly useful in complex models with multiple overlapping components.

With the roof selected and currently set to flat (no slope assigned to edges), the shape editing tools become available in the Modify Roofs contextual tab. These specialized tools are essential for creating sophisticated roof geometries. Start by adding ridge lines at grid line B and grid line 3 using the Add Split Line option. This tool allows precise placement of structural elements that will define the roof's primary slopes. Locate the midpoint intersection and extend the split line completely across the roof until it reaches the opposite edge, indicated by the blue dashed reference line. This establishes your first critical ridge line that will serve as the high point for water drainage.

Repeat this process to establish the second ridge line, ensuring both are properly aligned with your structural grid. The precision of these ridge lines directly impacts the roof's structural integrity and water management performance, making careful placement essential for professional results.

Next, define the ridge line elevation at eight inches above the zero datum point—typically located at drain positions. This height differential is crucial for ensuring adequate water flow velocity toward drainage points. Access the Modify Sub Elements tool and use window selection to capture all ridge line points simultaneously. This batch selection method significantly improves workflow efficiency compared to individual point selection. In the Options bar, input the eight-inch elevation adjustment, which will uniformly raise all selected points to create the desired crown.

The final step involves strategically placing low points to complete the drainage pattern. Select the Add Point option to insert drainage low points at calculated positions across the roof surface. Position these points at midpoint intersections to ensure balanced water collection and optimal flow patterns. Use the precise midpoint snap to guarantee accurate placement—proper positioning here prevents water pooling and ensures code compliance for commercial drainage requirements.

Apply the Modify Sub Elements tool again to set these drainage points to zero elevation, creating the valleys necessary for water collection. Notice how the software automatically generates valley lines connecting high and low points, creating a comprehensive slope pattern. To visualize the completed geometry, switch to the 3D view where you can verify that water will flow naturally from ridge lines down the created slopes to the drainage points. This flat roof methodology provides superior control over complex drainage patterns while maintaining the flexibility to modify slopes as project requirements evolve.