A shed roof that must carry snow is a small structure with a serious load problem: snow is heavy, it arrives all at once, and it stays. Collar ties, the horizontal members connecting opposing rafters, keep the rafters from spreading and the walls from being pushed outward under that load. Sizing them correctly for the span and the snow load is the difference between a shed that shrugs off winter and one that slowly spreads its walls until something gives.
What collar ties do under snow
Snow loads push down on the rafters, and the rafter geometry converts that downward load into outward thrust at the wall plates. Without restraint, the walls spread, the ridge sags, and the roof flattens. Collar ties resist this by tying the rafter pairs together, putting the tie in tension as the rafters try to separate. They work together with the ceiling joists or rafter ties at the wall plate level, which handle the same thrust lower down.
Position matters: collar ties are most effective in the upper third of the rafter span, where they restrain the rafters' tendency to bow and spread. Too low and they interfere with headroom while duplicating the rafter ties' job; too high and their leverage against spreading diminishes. The upper-third placement is the code-recognized sweet spot.
Snow load: the sizing driver
Snow load is specified as ground snow load for the location, converted to roof snow load with factors for slope, exposure, and thermal conditions. Northern and mountain regions see loads that dwarf the structure's own weight; a shed roof designed for a light snow zone fails quickly under a heavy one. Use the locally adopted snow load from the building department, never a guess or a neighboring region's number.
The load per tie follows from the tributary area: tie spacing times the rafter span it serves. Wider tie spacing means each tie carries more load; closer spacing shares it. Sizing is the balance of tie cross-section, lumber grade, and spacing against that tributary load, with connections sized to transfer the tension into the rafters.
Sizing the ties themselves
Collar ties are tension members first, so their sizing is about cross-sectional area and connection capacity rather than bending stiffness. Standard dimensional lumber, typically 1x or 2x material depending on span and load, suffices for most sheds when spaced correctly. Longer spans and heavier snow push toward larger sections or tighter spacing; there is no virtue in oversized ties beyond what the load requires, but there is real hazard in undersized ones.
Lumber grade matters in tension. Higher grades with fewer defects carry tension more reliably, and the tie's weakest point is usually at the connection, not mid-span. Size the member for the load, then make sure the connections can actually deliver that load into the rafters.
Connections: where ties succeed or fail
A collar tie is only as strong as its end connections. Nailing schedules for collar ties specify the number and size of nails at each rafter, and those numbers assume the full tension load. Undersized nailing is the most common collar tie failure: the tie itself is fine, but it pulls off the rafter under load. Follow the nailing schedule exactly, and consider through-bolts or structural screws for heavy snow zones where nail withdrawal is a concern.
Connect to sound rafter wood, not near splits or knots, and keep the tie ends bearing fully on the rafter faces. A tie connected with three nails where five are required is a decoration; the snow does not care about intentions.
The whole roof system
Collar ties do not work alone. Rafter ties or ceiling joists at the wall plate level carry the primary outward thrust; collar ties supplement them up high. Ridge beams, where used, change the load path entirely by carrying the rafters' downward load as a beam. Understand which system your shed uses before sizing any single member, because a member sized for one system may be wrong for another.
Finally, respect drift and unbalanced loads. Snow does not fall evenly: wind piles it against obstructions and on leeward slopes, creating local loads far above the uniform design value. The code's drift provisions exist for this; in shed design, generous sizing and solid connections are the practical hedge against the winter that exceeds the average.
Size collar ties from the real snow load, place them in the upper third, space them for their tributary area, connect them like the tension members they are, and design the roof as a system. Winter is a load case, not a surprise; build for it and the shed stands straight every spring.
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