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

SECTION 1

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.

1963 • Sutherland's Sketchpad

Ivan Sutherland demonstrates parametric constraints in a graphical interface, planting the conceptual seed for all later BIM tools that link geometric rules to design intent.

1982 • AutoCAD 1.0

Autodesk launches AutoCAD, replacing drafting boards with digital lines and arcs—but floors and roofs remain simple hatched rectangles with no embedded intelligence.

2000 • Revit 1.0 Released

Charles River Software ships Revit with a parametric change engine, introducing system families for walls, floors, and roofs that update all views in real time.

2004 • Revit Adds Extrusion Roofs

Autodesk extends the roof tool with an extrusion workflow, allowing complex barrel-vault and shed-roof profiles that footprint sketches cannot easily generate.

2020s • Generative Design & Interop

Modern Revit integrates Dynamo scripting and cloud collaboration, enabling algorithmic floor/roof generation and real-time multi-discipline coordination.

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.

SECTION 2

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.

System Families

Floors and roofs are system families—built-in categories whose types are defined inside the project, not loaded from external .rfa files. Each type specifies a layered assembly (structure, substrate, finish) and host behavior.

Sketch-Based Creation

Both tools require a sketch mode in which you draw closed boundary loops (footprint) or open profiles (extrusion). The green check mark finalizes the sketch; the red X cancels it.

Footprint vs. Extrusion

Footprint sketches the plan outline and projects the element vertically; extrusion sketches a cross-section profile and sweeps it along a horizontal axis. The choice depends on the geometry you need.

Slope & Offset Parameters

Roof edges can carry individual slope arrows or 'Defines Slope' checkboxes, while floors support sub-region shape editing for drainage. Height offsets position the element relative to the level datum.

Edit Mode Re-entry

After creation, selecting the element and clicking Edit Footprint / Edit Profile reopens sketch mode. All lines, constraints, and slope definitions remain editable, preserving the parametric chain.

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KEY TAKEAWAY

Think of a footprint sketch the way a cookie cutter works: you press a closed shape down into dough and the thickness projects straight through. An extrusion sketch is more like a pasta machine: you draw the cross-section of the noodle and push it sideways through the roller. In both cases the profile you define is the DNA of the resulting 3-D form—change the profile, and every view updates instantly.

SECTION 3

Visual Explanation — Footprint vs. Extrusion

Left: the footprint method draws a closed plan-view loop and projects it vertically to create thickness (t). Right: the extrusion method draws an open cross-section profile and sweeps it along a horizontal axis to generate complex roof forms such as barrel vaults.

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.

SECTION 4

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.

LEVEL OFFSET

Element Elevation = Level Datum + Height Offset from Level

The Level Datum is the absolute height of the associated level (e.g., Level 2 = 3200 mm). The Height Offset is a positive or negative displacement that shifts the top face of a floor or the base of a roof relative to that datum.

ROOF SLOPE

Rise / Run = tan(θ) → θ = arctan(Rise / Run)

Revit expresses slope as a ratio (e.g., 1 : 12 or 9 : 12) or in degrees. A slope of 6 : 12 means 6 units of rise for every 12 units of horizontal run, corresponding to θ ≈ 26.57°. This value is assigned per sketch line via the Defines Slope checkbox and the slope parameter in the Options Bar.

SLOPE ARROW ELEVATIONS

Slope = (Head Height − Tail Height) / Projected Length

A slope arrow is a sketch-mode annotation with a tail (low point) and head (high point). You set the head and tail heights; Revit computes the slope from the difference divided by the arrow's horizontal length. This is the preferred method for precise drainage on floors.

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Extrusion Depth vs. Footprint Thickness

Do not confuse extrusion start / end values with slab thickness. In an extrusion roof, the start and end parameters define how far the profile sweeps along the extrusion axis (measured from the reference plane), while the thickness comes from the type's layer structure—just as it does for footprint elements. Misunderstanding this distinction is the single most common source of modeling errors for new users.

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.

SECTION 5

Detailed Workflows — Creating & Editing

Creating a Floor (Footprint)

Navigate to a plan view at the desired level. Go to Architecture → Build → Floor. Revit enters sketch mode and dims the surrounding model.
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).
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.
Optionally add a slope arrow for drainage—draw from the high point to the low point, then set tail and head heights in Properties.
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)

Flowchart: if the roof can be described entirely by planar slopes from a plan outline, choose Footprint; if the cross-section requires curves or complex geometry, choose Extrusion. Both paths converge when editing—selecting the element and clicking Edit Footprint / Edit Profile reopens sketch mode.

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.

SECTION 6

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.

Gable Roof via Footprint Sketch

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Step 1 — Open the Correct View and Tool

Navigate to the floor plan view associated with the roof level (e.g., 'Level 2 – Roof'). From the ribbon, go to Architecture → Build → Roof → Roof by Footprint. Revit enters sketch mode and highlights the available walls.

Sketch mode active; modify panel displays Pick Walls, Line, and Slope tools.
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Step 2 — Pick Walls to Define the Boundary

With Pick Walls active, click the four perimeter walls. Revit creates magenta sketch lines aligned to the wall cores. In the Options Bar, set the overhang to 600 mm so the roof extends past the wall face. Ensure the flip arrows point outward if necessary.

Four boundary lines form a closed 13.2 m × 9.2 m loop (12 m + 2 × 0.6 m by 8 m + 2 × 0.6 m).
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Step 3 — Configure Slope per Edge

For a gable roof, select the two 13.2 m edges (running along the building length). Uncheck Defines Slope in their properties—these will become the gable ends (vertical planes). The two 9.2 m edges retain the Defines Slope checkbox. In the Options Bar, set the slope to 6.93 : 12 (which equals tan 30° ≈ 0.577, so rise = 6.93 for every 12 of run), or simply type 30° if your project is set to display slope in degrees.

Two sloped edges at 30° and two non-sloped gable edges define a symmetrical gable profile.
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Step 4 — Verify Ridge Height

The half-span of the short dimension is (9.2 m ÷ 2) = 4.6 m. At 30°, the rise = 4.6 × tan 30° ≈ 4.6 × 0.577 ≈ 2.66 m. Added to the base level at 3.5 m, the ridge sits at approximately 6.16 m above ground. Confirm this is acceptable for zoning height limits.

Ridge height ≈ 6.16 m above Level 1 (3.5 m wall + 2.66 m rise).
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Step 5 — Finish and Inspect

Click the green check mark (✓) to finalize. Switch to a 3-D view ({3D} default or a camera perspective) to inspect the roof. If walls need to extend up to the underside of the roof, select the walls, enable Attach Top/Base → Top → Roof so they follow the gable slope automatically.

A fully parametric gable roof with 600 mm overhangs, attached walls, and correct slope appears in all views.

SECTION 7

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.

Criterion	Footprint	Extrusion
Sketch view	Plan view (closed loop)	Elevation or section (open profile)
Applies to	Floors and roofs	Roofs only
Slope control	Per-edge Defines Slope + slope arrows	Implicit in the profile shape
Curved sections	Cannot create barrel vaults or curved ridges	Full support for arcs and splines
Ease of editing	Very intuitive; slope parameter changes instantly	Requires switching to profile view to reshape
Best for	Hip, gable, flat roofs; all floor slabs	Barrel vaults, sawtooth, shed, custom forms
Wall attachment	Walls attach to underside of footprint roofs natively	Walls can attach, but geometry may require manual adjustment

Comparison of footprint and extrusion methods for floors and roofs in Revit.

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KEY TAKEAWAY

Think of your tool choice the way a sculptor chooses between carving and casting. Footprint is like carving: you define the boundary and subtract material along simple slope rules, which is fast and clean for standard forms. Extrusion is like casting a mold: you define the exact cross-section and push it through space, giving you total freedom over the profile at the cost of a slightly more complex setup. Neither is universally superior; the design intent dictates the method.

SECTION 8

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.

Introductory Concept	Advanced Extension
Footprint floor with slope arrow	Shape Editing — use Add Point / Add Split Line to create complex drainage patterns with multiple low points on a single slab
Footprint roof with Defines Slope	Roof by Face — apply a roof system to a mass surface for free-form geometries generated from conceptual mass families
Single-layer floor type	Compound layer editing — define structural deck, insulation, membrane, and finish layers with variable thicknesses, material assignments, and wrapping conditions
Manual sketch boundaries	Dynamo scripting — algorithmically generate floor and roof boundaries from design parameters, spatial analysis, or imported GIS data
Extrusion roof (simple barrel vault)	In-place mass + Roof by Face — model double-curved shells and NURBS-derived surfaces that extrusion alone cannot achieve

How introductory floor and roof concepts scale into advanced Revit workflows.

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.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain the fundamental difference between creating a roof by footprint and creating a roof by extrusion. In what type of view does each method require you to sketch, and why does that distinction matter for the resulting 3-D geometry?

Show answer

A footprint roof is sketched in a plan view as a closed boundary loop; Revit generates the 3-D form by projecting slopes inward from designated edges, making it ideal for hip, gable, and flat roofs whose geometry is fully determined by their plan outline and slope values. An extrusion roof is sketched in an elevation or section view as an open profile; Revit sweeps that profile along a horizontal axis for a specified distance, which is necessary for curved or irregular cross-sections (barrel vaults, sawtooth roofs) that cannot be described by planar slope parameters alone. The sketch view matters because it determines whether Revit interprets your lines as a plan boundary to be sloped or a section profile to be extruded.

Question 2: BASIC CALCULATION

A gable roof has a slope of 8 : 12 and spans 10 m from eave to eave (no overhang). Calculate the ridge height above the eave line and the slope angle in degrees.

Show answer

The half-span (run) is 10 m ÷ 2 = 5 m. With a slope of 8 : 12, the rise per unit of run is 8/12 ≈ 0.6667. Therefore the ridge height above the eave = 5 m × 0.6667 ≈ 3.33 m. The slope angle θ = arctan(8/12) = arctan(0.6667) ≈ 33.69°. In Revit, you would enter either '8 : 12' or '33.69°' in the slope field of the Defines Slope edges, and Revit would generate a ridge at 3.33 m above the reference level.

Question 3: INTERMEDIATE

You are modeling a concrete floor slab that must drain to a central floor drain. The slab is 15 m × 10 m, and the drain is at the center. The minimum drainage slope required by code is 1.5%. Describe the Revit workflow to achieve this, and calculate the elevation difference between the slab perimeter and the drain point.

Show answer

In Revit, create a floor by footprint with the 15 m × 10 m boundary. After finishing the initial flat sketch, select the floor and use Shape Editing tools (Add Point at the drain location, Add Split Lines from corners to center if desired) or, more simply, add a slope arrow from the midpoint of each edge to the center point. For a slope arrow approach, the longest run is from the midpoint of the 15 m edge to the center: √((7.5)² + (5)²) ≈ 9.01 m (diagonal from a corner to center would be even longer, but a per-edge slope arrow covers the perpendicular distance). The perpendicular run from the 15 m edge midpoint is 5 m, and from the 10 m edge midpoint is 7.5 m. At 1.5% slope, the elevation drop over 7.5 m = 7.5 × 0.015 = 0.1125 m (112.5 mm), and over 5 m = 5 × 0.015 = 0.075 m (75 mm). The perimeter would sit 75–112.5 mm above the drain, depending on direction; you would set the tail height at 0 and head heights accordingly for each slope arrow.

Question 4: APPLIED

An art museum requires a barrel-vault roof over a 20 m × 40 m gallery hall. The vault has a semicircular cross-section with a radius of 10 m (matching the 20 m span). Which Revit roof method would you use? Describe the sketch, the key parameters you would set, and explain why the alternative method would fail.

Show answer

You would use Roof by Extrusion. First, establish a reference plane perpendicular to the 40 m axis at one end of the building. Open an elevation view aligned with that plane. Using the arc tool, sketch a semicircular profile with a radius of 10 m—its endpoints would sit at the springing line (top of wall) and the apex would be 10 m above that line. In the Properties palette, set Extrusion Start to 0 and Extrusion End to 40,000 mm (or −20,000 to +20,000 if centered on the reference plane). Set the Reference Level and offset to align the profile base with the wall top. Finish the sketch. The footprint method would fail because it can only generate planar (straight-line) slopes between plan-view edges; it has no mechanism to describe a curved cross-section. Checking 'Defines Slope' on all four edges of a rectangle produces a hip roof—never a semicircular vault. The extrusion method is the only built-in tool that allows arcs in the roof section profile.

Question 5: CRITICAL THINKING

A colleague argues that since Revit offers Roof by Face (applied to a conceptual mass), the footprint and extrusion methods are obsolete for complex architectural forms. Construct a nuanced counter-argument that addresses modeling efficiency, parametric editability, documentation, and the role of system families in BIM coordination.

Show answer

While Roof by Face offers unmatched formal freedom—applying roof panels to doubly curved, NURBS-derived mass surfaces—it does not render footprint and extrusion methods obsolete for several reasons. First, footprint and extrusion roofs are native system families with fully editable compound layer structures; Roof by Face panels inherit limited layer data and may require manual overrides for accurate construction documentation. Second, parametric editability is more direct: changing a footprint roof's slope is a single property edit, whereas modifying a mass surface requires reopening the conceptual mass environment, adjusting the form, and regenerating the roof-by-face instance—a multi-step process that complicates iterative design. Third, BIM coordination depends on accurate scheduling, tagging, and interference detection; system-family roofs participate in these workflows seamlessly, while mass-hosted roofs sometimes require additional workarounds for structural engineers and MEP coordinators to read correctly. Finally, for the vast majority of building projects—even visually ambitious ones—90% of the roof area is planar or singly curved, making footprint and extrusion the appropriate and efficient tools. Roof by Face should be understood as a complementary capability for the remaining 10%, not a wholesale replacement.

SUMMARY

Lesson Summary

Summary

Floors and roofs in Autodesk Revit are system families created through sketch-based workflows that come in two forms. The footprint method draws a closed plan-view boundary and projects it vertically—ideal for flat slabs, hip roofs, and gable roofs controlled by Defines Slope checkboxes and slope arrows. The extrusion method draws an open cross-section profile in elevation and sweeps it along an axis, enabling barrel vaults, sawtooth forms, and other complex geometries that planar slope parameters cannot describe. Floors are always footprint-based, while roofs support both methods.

Key parameters include Height Offset from Level for vertical positioning, slope ratio or angle for roof pitch (calculated as rise ÷ run = tan θ), and extrusion start / end distances for sweep length. After creation, selecting the element and clicking Edit Footprint or Edit Profile re-enters sketch mode, preserving full parametric editability. These introductory tools form the foundation for advanced workflows including Shape Editing, Roof by Face, compound layer structures, and Dynamo-driven algorithmic generation.