Direct Answer
Order structural steel material as soon as you have a preliminary BOM from the detailer — before shop drawings are approved. Long-lead sections (heavy W-shapes, thick plates, HSS in non-standard sizes) can have 8–16 week mill lead times. Waiting for IFC approval before ordering is the single most common cause of fabrication schedule delays.
Full Explanation
Steel mills produce structural sections in specific run schedules. W14×730 columns, for instance, may only be rolled quarterly. Heavy plate (over 4" thick) may require a mill order with 12–16 week lead time. The strategy: order the structural section material from a preliminary BOM as soon as the design is 70%+ stable. Add a contingency allowance (5–10%) for design growth. When the IFC BOM is finalized, reconcile against what was ordered and pick up additional items. Connection material (plates, clips, gussets) can typically be ordered later from standard inventory.
What This Means for Your Shop
Every week you delay a material order on a project with a tight delivery date is a week of schedule risk you cannot recover. Material lead time is almost always on the critical path.
Common Mistakes
Waiting for IFC approval to order long-lead steel. By the time IFC is received (8–12 weeks after design documents are issued), you are already 2–4 months behind on material.
Recommended Process
At project contract signing, request a preliminary BOM from the detailer within 2 weeks of modeling start. Place a mill order for long-lead sections immediately. Set a reconciliation milestone when IFC BOM is received to catch any design growth.