Direct Answer
Stock steel is material that service centers maintain in inventory and can deliver within 1–2 weeks. Mill-order steel is ordered directly from a rolling mill on a custom run — lead times are 8–20 weeks but it is available in non-standard lengths and in sections that aren't maintained as stock. Common structural W-shapes in standard lengths are typically available from stock; heavy sections, non-standard lengths, or specialty grades often require mill orders.
Full Explanation
Service center stock typically covers: W-shapes from W4 through W33 in the most common weight designations, in standard 20', 30', 40', and 60' lengths. Very heavy W14 sections (W14×455, W14×730) are rarely stocked and usually require mill orders. Long custom lengths (over 60') for long-span applications require mill orders. ASTM A992 is always available in stock for common sections; specialty grades (A913, A709, A1085) often require mill orders. The practical strategy: identify non-stock items from the BOM early and place mill orders immediately.
What This Means for Your Shop
A preliminary BOM review against your service center's available stock inventory identifies long-lead items within 2–3 weeks of project kickoff. This allows you to place mill orders for non-stock items while standard sections are ordered from stock at the normal time.
Common Mistakes
Assuming all sections in the BOM are available from stock. A W14×370 column, for example, may require a mill order and 12–16 weeks lead time. Missing this at project start is a critical schedule risk.
Recommended Process
When you receive the preliminary BOM, send it to your service center immediately with a request to flag any items not in current stock. For any non-stock item, get a mill quote and lead time within a week. Place mill orders before IFC approval if the schedule requires it.