Floors & Roofs — Create and edit floors and roofs (footprint/extrusion) (intro)

Historical Context & Motivation

For centuries, architects communicated floor slabs and roof assemblies through hand-drafted section drawings, relying on conventions of line weight and hatch patterns to convey material layering. The shift from two-dimensional drafting to three-dimensional digital modeling—beginning with early solid-modeling research in the 1960s and accelerating through the CAD revolution of the 1980s—gradually exposed a critical limitation: individual drawings could not automatically coordinate with one another, leading to conflicts between floor plans, sections, and structural documents that only surfaced during construction. Building Information Modeling (BIM) emerged precisely to resolve this disconnect, embedding geometric, material, and analytical data inside parametric objects rather than dumb lines. Autodesk Revit, first released in 2000 by the startup Charles River Software (later Revit Technology Corporation, acquired by Autodesk in 2002), was built from the ground up on a parametric change engine that propagates edits across every view simultaneously. Within this framework, floors and roofs are not mere surface representations; they are system families governed by type properties, layer structures, and host-boundary relationships that make the digital model behave more like an actual building than a collection of geometry.

The core question this lesson addresses is deceptively straightforward: how does Revit translate a designer's intention—a flat concrete slab here, a sloped copper roof there—into a fully parametric 3-D element that participates in schedules, sections, and energy analyses? The answer lies in understanding two creation methods, footprint and extrusion, and the editing workflows that refine them after initial placement.

Core Principles & Definitions

Before diving into tool mechanics, it helps to anchor a few foundational ideas that govern every floor and roof you will ever model in Revit. These principles recur whether you are sketching a simple rectangular slab or a complex folded-plate roof, and understanding them will save considerable troubleshooting time later.

Visual Explanation — Footprint vs. Extrusion

The diagram above captures the essential difference between the two workflows. On the left side, the footprint method begins in a plan (top-down) view where you sketch a closed boundary of lines and arcs—think of it as tracing the outline of the room below. When you finish the sketch, Revit extrudes that shape perpendicular to the work plane (usually straight down for floors, or vertically for flat roofs) by the thickness defined in the floor or roof type's layer structure. This approach is ideal for horizontal slabs, conventional hip or gable roofs whose ridges and valleys can be inferred from slope-defining edges, and any situation where the plan outline is the primary design driver.

On the right side, the extrusion method flips the logic: you work in an elevation or section view and draw an open profile—a line or series of arcs that represents the cross-sectional shape of the roof. Revit then sweeps that profile along a specified axis for a defined start and end distance, generating the three-dimensional surface. This workflow excels for barrel vaults, sawtooth factory roofs, and any roof whose section profile is more complex than what slope parameters alone can describe. Note that extrusion roofs are available only for roofs, not for floors; floor geometry is always defined by a footprint sketch.

How It Works — Parameters & Slope Logic

Although Revit is not a math-forward application in the way that structural analysis software is, understanding the numerical parameters behind floors and roofs is essential for controlling geometry precisely. Three key parameter families govern vertical positioning, slope angle, and layer composition, and they interact in ways that a purely visual approach may not reveal.

A useful mental model is to think of every floor and roof as a sandwich: the type properties define the sandwich's layers (bread, filling, toppings), while the instance properties (level, offset, slope) position and tilt that sandwich in space. The sketch boundary merely outlines the sandwich's plan shape or section profile. Keeping these three dimensions of control separate—layers, position, and outline—will help you troubleshoot any floor or roof that does not look as expected.

Detailed Workflows — Creating & Editing

Creating a Floor (Footprint)

1. Navigate to a plan view at the desired level. Go to Architecture → Build → Floor. Revit enters sketch mode and dims the surrounding model.
2. Use Line, Rectangle, Circle, or Pick Walls to draw a closed loop. Pick Walls is the fastest method—select the bounding walls and Revit offsets the boundary to the wall core or finish face (adjustable via the Options Bar).
3. Set floor type in the Properties palette (e.g., Generic - 300mm) and adjust the Height Offset from Level if the top of slab should not coincide with the level line.
4. Optionally add a slope arrow for drainage—draw from the high point to the low point, then set tail and head heights in Properties.
5. Click the green check mark (✓) to finish the sketch. If Revit detects overlapping or open loops, it will alert you before committing.

Creating a Roof (Footprint vs. Extrusion)

The flowchart makes the decision-making process explicit. For a conventional hip roof on a rectangular plan, the footprint method is straightforward: you pick the four bounding walls, check Defines Slope on all four edges, set a uniform slope value (say 6 : 12), and Revit computes the hip ridges automatically. If you uncheck 'Defines Slope' on two opposing edges, you get a gable roof instead. For a butterfly roof, sawtooth profile, or any form whose cross-section includes curves, switch to the extrusion method: pick or create a reference plane as the work plane, open an elevation view aligned with that plane, sketch the profile, and set the extrusion start and end distances to define how far the profile sweeps. In both cases, editing later simply reopens the same sketch environment.

Worked Example — Creating a Sloped Roof by Footprint

Imagine you are designing a small gallery pavilion with a simple gable roof. The building footprint is 12 m × 8 m, the walls are 3.5 m tall, and the client wants a 30° roof pitch with a ridge running along the 12 m dimension. Walk through the following steps to create this roof using the footprint method.

Footprint vs. Extrusion — Strengths & Limitations

Choosing between the two methods is not an abstract academic exercise; it has direct implications for how efficiently you can model, edit, and document a building. The table below distills the practical trade-offs.

Connection to Advanced Techniques

The introductory workflows covered in this lesson form the foundation on which a wide range of advanced Revit techniques are built. As you move deeper into modeling fundamentals, you will encounter scenarios where footprint and extrusion methods alone are insufficient, and familiarity with the concepts below will prepare you for those challenges.

As a visual-arts student, you may find that the expressive limit of footprint and extrusion methods arrives sooner than it does for students working on conventional residential or commercial projects. Sculptural canopies, parametrically tessellated roof panels, and undulating floor plates all push beyond what these two core tools handle natively. However, every one of those advanced workflows ultimately resolves back into the same system-family logic—layers, offsets, and sketch boundaries—so the conceptual vocabulary you are building now remains fully relevant.

Practice Problems

Lesson Summary