Calculate horizontal outward roof thrust on exterior load-bearing walls and size required ceiling ties or structural ridge beams.
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The Rafter Thrust Calculator determines the outward horizontal force exerted by pitched common rafters onto the top plates of exterior bearing walls. Without horizontal ceiling joists or a structural load-bearing ridge beam, rafter thrust pushes walls outward, causing roof sag and structural failure.
H = \frac{W_{\text{total}} \cdot L_{\text{span}}}{8 \cdot h_{\text{ridge}}} = \frac{V}{\tan(\theta)}Where W_total is total uniform load on the rafter pair, L_span is building width, h_ridge is height of the ridge above wall plates, and theta is roof pitch angle.
Scenario: A 24-foot wide garage with a 4:12 pitch roof, 40 psf total design load, and rafters at 24 inches on center.
Tributary Area per rafter pair: 24 ft × 2 ft = 48 sq ft.
Total Load on rafter pair: 48 × 40 psf = 1,920 lbs.
Vertical Reaction at each wall: 1,920 / 2 = 960 lbs.
Ridge Height: (24 / 2) × (4 / 12) = 4.0 feet.
Horizontal Outward Thrust: (1,920 × 24) / (8 × 4.0) = 46,080 / 32 = 1,440 lbs.
A ridge board is a non-structural 1x board used to align opposing rafters; it relies entirely on ceiling joists to prevent wall spread. A structural ridge beam is an engineered load-bearing beam (e.g. LVL or steel) that carries the rafters in vertical bending, eliminating horizontal thrust.
No. Collar ties in the upper third of the roof resist wind uplift at the ridge; they are mechanically ineffective at preventing wall spreading. Only rafter ties in the lower third or a structural ridge beam prevent outward thrust.
Walls lean outward out of plumb, drywall cracks along the ceiling corners, the roof ridge line sways and sags in the center, and doors and windows bind in their frames.
If ceiling ties are raised above the top plate (e.g. to create a tray or coffered ceiling), horizontal thrust increases: raising ties to 1/3 of the ridge height increases thrust by 50%.
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