Reading Simple Roof Plans for Rafters and Trusses

What a Simple Roof Plan Is Trying to Tell You

A basic roof drawing is a compact set of instructions: it communicates the roof’s geometry (where edges, ridges, hips, and valleys land) and the framing intent (rafters vs. trusses, spacing, bearing points, and key details). Your job when reading it is to translate lines, symbols, and notes into three-dimensional framing decisions—without guessing. When something is not explicitly shown, you flag it for verification before cutting or setting members.

Plan View vs. Elevation: Two Different Questions

Plan view (roof plan)

A roof plan is a top-down view. It answers: “Where is everything located in footprint terms?” It typically shows:

• Roof outline and overhangs
• Ridge line location
• Hips and valleys as diagonal lines
• Changes in roof area (additions, bump-outs, dormers)
• Notes for pitch, framing type, spacing, and sheathing
• Reference bubbles to sections/details

Elevation view

An elevation is a side view. It answers: “How tall and how steep is the roof?” It typically shows:

• Roof slope/pitch visually
• Ridge height relative to walls
• Eave and rake conditions
• Overhang appearance (but not always dimensioned precisely)

Practical reading tip: Use the plan to locate lines and bearings; use elevations/sections to confirm slopes, heights, and edge conditions.

Roof Pitch Notation: How Slope Is Communicated

Pitch is commonly written as “X:12” or “X/12,” meaning X inches of rise for every 12 inches of horizontal run. Examples: 4:12, 6/12, 12:12. Sometimes pitch is shown with an arrow and text near a roof plane, or in a general note that applies to the whole roof unless otherwise called out.

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Step-by-step: confirm what the pitch note applies to

• Find the pitch note on the roof plan (often near the center of a roof plane) or in general notes.
• Look for exceptions (e.g., “PORCH ROOF 3:12” while main roof is 6:12).
• Check section/elevation references if multiple slopes meet—plans can show geometry but not always the vertical relationships clearly.

Overhang Dimensions: What’s Measured and From Where

Overhangs are often dimensioned at eaves and rakes. The critical reading task is identifying the reference line: is the dimension taken from the outside face of wall framing, from sheathing, or from finish? Plans may show a roof outline offset from the wall line, with a dimension like 1'-6" O.H. (overhang).

Common overhang callouts

• 1'-0" EAVE O.H. TYP. (typical eave overhang)
• 1'-6" RAKE O.H. (gable/rake overhang)
• SOFFIT ALIGN or similar notes indicating alignment requirements

Field verification trigger: If the plan doesn’t state the reference (framing vs. finish), confirm with the designer or compare to wall sections and fascia/soffit details.

Span, Bearing Lines, and Ridge Location

Span and bearing lines

Roof framing needs support. Plans often show bearing lines (walls, beams, or headers) that carry roof loads. These may be indicated by heavier lines, labeled beams, or notes like BEARING WALL, GLB (glulam beam), or LVL.

Ridge location

On a simple gable roof, the ridge is typically centered between two exterior bearing walls (but not always—offset ridges happen). On plans, the ridge is a line running along the peak, often labeled RIDGE or shown as a long line where two roof planes meet.

Step-by-step: locate ridge using plan information

• Identify the main roof footprint (outer roof edge line).
• Find the ridge line (center line of the roof planes meeting).
• Check dimensions from ridge to exterior walls or to centerlines; if not dimensioned, look for symmetry cues or section references.
• Confirm bearing points under the ridge: ridge beam, ridge board, or truss peak line. The plan may not fully define this—sections/notes often do.

How Hips and Valleys Appear on Plans

Hips and valleys show up as diagonal lines because they are the intersection of two roof planes. The key is recognizing whether the diagonal line represents an outside corner (hip) or an inside corner (valley).

Visual cues

• Hip: diagonal line running from an outside corner up toward the ridge. It “caps” an outside corner where roof planes meet.
• Valley: diagonal line running from an inside corner up toward a ridge intersection. It “collects” at an inside corner where roof planes meet.

Practical check: Trace the roof outline. If the diagonal originates at a re-entrant corner (an inside notch), it’s a valley. If it originates at a projecting corner, it’s a hip.

Common Symbols and Notes You’ll See

Rafter spacing (o.c.)

Spacing is typically written as inches on center: 16" o.c. or 24" o.c.. It may appear as a note like:

• 2x8 RAFTERS @ 16" O.C. (TYP.)
• TRUSSES @ 24" O.C.

Reading tip: “TYP.” means typical—assumed throughout unless a specific area is called out differently.

Truss callouts

Truss roofs often use tags that reference a truss schedule or truss package. You might see:

• T1, T2, etc. placed along the roof footprint
• GIRDER TRUSS at concentrated load points
• TRUSS DESIGN BY MANUFACTURER notes

Important: A roof plan may show truss layout intent, but final truss profiles, webs, and reactions are typically in the truss submittal drawings. Don’t assume web configuration from the roof plan.

Sheathing thickness

Sheathing is often specified in notes, for example:

• ROOF SHEATHING: 7/16" OSB
• 5/8" PLYWOOD ROOF SHEATHING

This affects fastening schedules and edge details, and can matter where roof meets fascia or where flush conditions are required.

Typical section references

Plans often include section markers that point you to a cut-through drawing. A common format is a circle or bubble with a letter/number, such as A/5 meaning “Section A on sheet 5,” or 2/A-401 meaning “Detail 2 on sheet A-401.”

• Use sections to confirm: ridge detail, eave build-up, overhang construction, and bearing conditions.
• If a plan shows a ridge line but not whether it’s a ridge board, ridge beam, or truss peak, the section/detail is where that’s clarified.

Structured Practice: Interpret a Small Sample Roof Plan

Use the simplified sample below to practice extracting the key information. This is not a full construction document; it’s a reading exercise.

Sample plan (text-based)

ROOF PLAN (SAMPLE)  -  RECTANGULAR HOUSE WITH SMALL REAR BUMP-OUT  (PLAN VIEW)  OVERALL WALL FOOTPRINT: 28'-0" (E-W) x 18'-0" (N-S)  EAVE OVERHANG: 1'-0" TYP.  MAIN ROOF: 6:12  FRAMING NOTE: 2x8 RAFTERS @ 16" O.C. (TYP.)  ROOF SHEATHING: 7/16" OSB (TYP.)  RIDGE LINE: RUNS E-W, CENTERED ON 18'-0" WIDTH  REAR BUMP-OUT: 8'-0" WIDE x 4'-0" DEEP (CENTERED)  VALLEYS: (2) DIAGONALS FROM BUMP-OUT INSIDE CORNERS UP TO MAIN ROOF  SECTION REFERENCE AT EAVE: 2/A-301

Task 1: Locate the ridge

Step-by-step:

• The note says the ridge runs E-W and is centered on the 18'-0" width.
• Centered on 18'-0" means the ridge is at mid-width: 18' / 2 = 9' from either the north or south exterior wall line (before considering overhang).
• Because it runs E-W, it spans the long direction of the house footprint.

Task 2: Determine roof pitch

• The plan states MAIN ROOF: 6:12. That is the pitch for the main roof planes unless another area is called out differently.
• Check if the bump-out has its own pitch note (none shown in the sample). That’s a missing item to verify (see Task 4).

Task 3: Identify framing type (rafters vs. trusses)

• The framing note reads: 2x8 RAFTERS @ 16" O.C. (TYP.)
• That indicates a rafter-framed roof intent (not trusses). If it were trusses, you would expect notes like TRUSSES @ 24" O.C. and truss tags (T1, T2) or a truss schedule reference.

Task 4: List what information is missing and must be verified on site (or via sections/details)

Even with the notes above, several items remain unclear or unconfirmed. Flag these before framing:

• Ridge support detail: Is it a ridge board with opposing rafters, or a structural ridge beam requiring posts/footings? The plan alone may not specify; check 2/A-301 and any ridge sections.
• Ceiling/rafter tie requirements: The roof plan may not show where ties occur or how they interact with interior layout; confirm in framing plans/sections.
• Bump-out roof configuration: Does the bump-out have a shed roof tying into the main roof, or is it under the main roof plane? The sample mentions valleys, implying an intersecting roof plane, but the bump-out pitch and ridge/edge conditions are not explicitly stated.
• Exact overhang reference: 1'-0" TYP. is given, but from what line (stud face, sheathing, finish)? Confirm with eave detail 2/A-301.
• Valley framing detail: Valley rafter size, any required valley beam, and how jack rafters are supported—often in details rather than the roof plan.
• Ventilation and edge conditions: Intake/exhaust vent locations and whether there are special soffit/fascia requirements are typically in sections/specs.
• Openings and penetrations: Skylights, chimneys, or mechanical penetrations (if any) may be on other sheets; the roof plan might not include all framing headers/crickets needed.
• Material and fastening specifics: Sheathing thickness is listed, but nailing pattern, clips, and edge blocking requirements may be in structural notes.
• Field dimensions: Any “verify in field” conditions, wall out-of-square, or as-built differences affect ridge straightness and valley alignment.

Quick Checklist When You First Open a Roof Plan