Roof Framing: Rafters and Trusses

Why roofs push

A rafter leaning against a ridge pushes down and out at its foot. Two rafters meeting at a ridge act like a giant pair of legs: loaded with snow, they try to splay apart and shove the tops of the walls outward. The UDC requires that this horizontal thrust be taken up by wall ties, ceiling joists, a structural ridge beam, or an engineered system ¹.

In ordinary stick framing, the ceiling joists do it. They’re nailed to the rafter heels and to the wall plates, and where they lap over an interior wall they overlap at least 3 inches and are fastened with three 16d nails, so the whole ceiling acts as a tension tie across the house ¹. That’s also why rafters go up only after the ceiling joists are fastened ².

Rafters and ridge

• Slope: written rise in 12, like 6 in 12. The UDC’s rafter and joist span tables are valid at 3 in 12 or steeper; flatter roofs need engineering or a structural ridge beam ¹. On roofs steeper than 30 degrees (7 in 12), the design snow load may be reduced ¹.
• Rafters are notched to sit on the wall plate (the birdsmouth) and fastened to it, with at least 1 1/2 inches of bearing on wood ¹.
• Where rafters meet at a ridge, they attach to a ridge board at least as deep as the rafter’s cut end: 1-inch nominal if the rafters line up opposite each other, 2-inch if they’re offset more than a rafter’s thickness ¹. A ridge beam, which carries the rafters instead of just aligning them, uses engineered hangers ¹.
• Collar ties go in the upper third of every third pair of rafters or every 48 inches, whichever is closer; they steady the ridge but don’t replace the low ties ¹.
• Valley rafters without support below are doubled and 2 inches deeper than the common rafters ¹.
• Gable overhangs over 12 inches need ladders (lookouts) reaching back into the roof at least as far as the overhang, hung on a rafter or truss ¹.

Holding it down

Wind lifts roofs. The UDC requires roofs and overhangs to resist 20 psf of uplift ¹, and:

• Rafters or trusses spanning more than 6 feet from the roof edge are fastened to the plate with engineered clips, straps, or hangers (hurricane ties).
• Those spanning 6 feet or less may be toenailed.

That’s virtually every house rafter, so expect a metal clip at every rafter or truss heel.

Trusses

Most new Wisconsin houses use prefabricated trusses: chords and webs joined by metal connector plates into triangles, designed by the manufacturer for the span, pitch, and snow zone. They go up fast, span the full width without interior bearing walls, and come with drawings showing bracing and bearing.

The rule to remember: never cut, notch, or bore a truss. Every member is part of a calculated triangle, and the code forbids altering them ¹. Damaged or cut trusses get an engineered repair from the truss designer.

What to take from this

Rafters push walls apart; ceiling joists fastened to the rafter heels and lapped over interior walls tie them together. Use the span tables at 3 in 12 or steeper, seat rafters with a birdsmouth on the plate, run them to a ridge board as deep as their cut, add collar ties every third pair or 48 inches, ladder gable overhangs over a foot, and clip or strap anything spanning over 6 feet against 20 psf of uplift. Trusses are engineered triangles: never cut one.