Ridge Board: What It Does (and What It Doesn’t)
A ridge board is the non-structural board at the peak where opposing rafters meet. Its main jobs are:
- Alignment: keeps rafters in a straight line at the ridge so the roof planes stay true.
- Nailing surface: provides a solid, consistent surface to fasten each rafter end.
- Layout reference: helps maintain consistent rafter spacing and a clean ridge line during framing.
What a ridge board typically does not do:
- It is not usually a beam. In common rafter roofs, a ridge board is not intended to carry vertical roof loads down to posts or walls. The rafters support each other at the ridge, and the ridge board simply helps connect and align them.
- It does not prevent roof spread by itself. If the roof framing relies on rafter ties (ceiling joists), the ridge board is not the part resisting outward thrust at the walls.
Practical check: If you see a ridge member supported by posts/columns down to bearing points, that’s a different component (a structural ridge). A ridge board in a typical rafter roof is supported by the rafters meeting at it, not by posts.
Ridge Board Sizing Concept (Field-Oriented)
For a ridge board to work as a nailing surface, it must be deep enough so each rafter can be fully nailed without splitting or missing the member. A common rule of thumb is that the ridge board depth should be at least the cut depth of the rafter at the ridge (so both rafters have full bearing/nailing). Always follow the plan/specs and local code requirements for minimum ridge board size.
Why Roofs Try to Spread: Opposing Rafters and Outward Thrust
When rafters are loaded (dead load from materials and live loads like snow), each rafter wants to slide down its slope. At the ridge, opposing rafters push against each other. At the lower end, that “slide” translates into a force that can push the exterior walls outward. This is called outward thrust.
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Think of it like two people leaning ladders against each other at the top: if nothing ties the ladder feet together, the feet tend to kick outward.
What You’ll See When Thrust Isn’t Properly Restrained
- Walls bowing outward near the top plates
- Ridge sag or a “dip” along the ridge line
- Ceiling cracks, drywall joint separation, popped fasteners
- Doors/windows sticking from subtle building movement
Ceiling Joists as Rafter Ties: The Primary Anti-Spread Member
In many conventional rafter roofs, ceiling joists do double duty: they support the ceiling load and also act as rafter ties. As rafter ties, they connect the lower ends of opposing rafters (or the top plates they bear on) to resist outward thrust and keep the walls from spreading.
Key Placement Concept: Low and Across the Building
To be effective against spreading, rafter ties should be placed in the lower portion of the roof assembly—commonly at the ceiling level—so they form a strong “triangle” with the rafters. The higher you raise a tie, the less leverage it has to resist the outward push at the walls (and the greater the force it must carry).
How Often?
In a typical layout, rafter ties are installed at the same spacing as the rafters (often every rafter pair). Some designs allow ties at wider spacing with additional framing details, but that must be engineered or specifically detailed on the plans. When in doubt in the field: match rafter spacing unless the drawings clearly show otherwise.
Connection Requirements: Make the Tie Actually “Tie”
A rafter tie must be connected so it can carry tension without slipping. Good practice focuses on:
- Positive fastening at each end to the rafter (or to the top plate with an approved detail).
- Continuous load path thinking: the tie must connect into the rafters in a way that transfers the thrust into the tie, not just “touch” the framing.
- Splices matter: if a ceiling joist/rafter tie is spliced, the splice must be detailed to develop tension (often by lapping over a bearing line or using specified connectors). A random midspan butt splice does not create a reliable tie.
Practical step-by-step: installing ceiling joists as rafter ties
- Confirm the intent on the plans: are ceiling joists intended to act as rafter ties? Look for notes/details about rafter ties, raised ties, or vaulted areas.
- Set joists at the correct elevation: typically on the top plates at ceiling height, spanning from exterior wall to exterior wall (or to other designated bearing points per plan).
- Align with rafter layout: place joists so each rafter pair has a corresponding tie where required.
- Fasten each end correctly: use the specified nailing schedule and/or metal connectors. Ensure nails are not overdriven and connectors are fully nailed with the correct fasteners.
- Handle splices intentionally: if joists must be spliced, do it where the detail allows load transfer (commonly over a bearing partition) or use specified hardware.
- Maintain continuity at transitions: where ceiling joists stop (vaults, stair openings), install the specified alternative ties/straps/headers so the load path isn’t broken.
Collar Ties: Uplift Resistance Near the Upper Third
Collar ties are members installed in the upper third of the roof height, connecting opposing rafters near the ridge. Their primary purpose is to resist uplift (wind trying to pull the rafters apart at the ridge) and to help keep the ridge connection tight.
Collar Ties vs. Rafter Ties: Same Idea, Different Problem
| Member | Typical Location | Main Purpose | What It Does NOT Replace |
|---|---|---|---|
| Rafter tie (often ceiling joist) | Lower portion of roof (near top plates/ceiling level) | Resists outward thrust; prevents wall spread | Does not primarily address ridge uplift |
| Collar tie | Upper third of roof (near ridge) | Resists wind uplift; keeps rafters from separating at ridge | Does not prevent wall spread when used alone |
A common misconception is that collar ties can substitute for rafter ties. They cannot. Because collar ties are high, they don’t effectively restrain the outward push at the walls.
Placement and Spacing Concepts
- Location: install in the upper third of the rafter height (measured from the top plate to the ridge).
- Frequency: commonly every other rafter pair or as specified; in higher wind areas or per plan, it may be every rafter pair.
- Connections: collar ties must be securely fastened to each rafter they connect. If the design calls for straps or specific connectors for uplift, use them exactly as detailed.
Practical step-by-step: installing collar ties
- Mark the upper-third zone on a sample rafter bay so the crew has a consistent target area.
- Cut and fit ties tight to the rafters (avoid gaps that reduce stiffness and invite nail slip).
- Install at the specified spacing (don’t “fill in later” after sheathing—access and fastening quality drop).
- Fasten per schedule and add any required straps/hurricane ties that complete the uplift load path.
Correct Placement Concepts: Keeping a Continuous Load Path
Even without re-teaching load theory, you can use a simple field rule: every force needs a connected chain of wood and connectors to get to something that can resist it. For this chapter’s components, that means:
- Outward thrust chain: rafter → rafter tie/ceiling joist → opposing rafter/top plate connection (so walls don’t spread).
- Uplift chain: rafter → collar tie/straps (near ridge) and rafter-to-wall connectors (at the plate) so wind can’t peel the roof apart.
Connection Details That Commonly Break the Chain
- Ceiling joists not actually tied to rafters: joists merely resting on plates with weak or missing end fastening.
- Raised ties without engineering: moving ties upward for a “cathedral feel” without compensating details.
- Interrupted ties at openings: stairwells, chimneys, and attic access openings framed without alternate tension ties or straps.
- Improper splices: butt splices midspan that cannot carry tension.
Common Misconceptions (and Corrections)
- “The ridge board holds the roof up.” Correction: a ridge board mainly aligns rafters and provides a nailing surface; it’s not typically a load-carrying beam.
- “Collar ties stop the walls from spreading.” Correction: collar ties primarily resist uplift near the ridge; rafter ties (often ceiling joists) resist wall spread.
- “A few ties are enough.” Correction: ties must be installed at the required spacing and connected properly; missing ties create weak bays that can still spread.
- “I can raise the ceiling joists a bit and it won’t matter.” Correction: raising ties increases the tension demand and reduces effectiveness against wall spread unless the design is specifically detailed for it.
Mini-Scenario: What Happens If Ceiling Joists Are Omitted or Raised Too High?
Scenario: A crew frames a conventional rafter roof with a ridge board. The homeowner wants an open ceiling look, so the crew omits ceiling joists in several bays, or they raise them well above the top plates without any engineered ridge or alternate tie system.
Expected Framing Consequences
- Outward wall movement: the exterior walls begin to bow outward at the top because the rafter feet are pushing out with no effective tie.
- Ridge sag/deformation: as the walls spread, the ridge line can settle or appear to sag; the roof planes may look “dished” or uneven.
- Cracked finishes: ceiling-to-wall joints crack, drywall seams open, nail pops appear, and trim joints separate as the structure moves.
- Secondary issues: doors and windows may go out of square; roof sheathing and shingles can show waves as the geometry changes.
Why Raising Ties Changes the Forces
When ties are installed low (at the ceiling), they efficiently restrain the rafter feet. When ties are raised, they form a smaller triangle and must resist greater tension to do the same job. Without a design that accounts for that increased force (member sizing, connectors, and sometimes a structural ridge), the roof can still spread even though “some ties” exist.
Field-Fix Mindset (Conceptual)
If ceiling joists can’t be used as rafter ties due to a vaulted ceiling, the roof typically needs an alternate, clearly detailed system (for example, a structural ridge or engineered ties/straps). The key is not the look—it’s ensuring there is still a reliable tension path preventing the walls from being pushed outward.
