When a curtain-wall package lands 10–15% over budget, the instinct is to blame the market: billet prices, glass, logistics. In my experience running estimation and production for an aluminium façade factory, the overrun is usually already written into the bill of quantities — quietly, in three places.

1. The cutting yield you assumed but never checked

A BOQ that prices profiles by net length is a fiction. Real fabrication buys stock lengths, and the difference between a 92% and an 84% cutting yield across a tower's mullions is a full truck of aluminium. If your estimator can't tell you the assumed stock length and the optimization method behind the yield figure, the yield figure is a guess.

Ask for the cut list, not just the totals. The cut list is where the money is.

2. Brackets, gaskets and the 'small items' line

Accessories are routinely estimated as a percentage of profile cost. On standard window systems that shortcut survives. On a bespoke curtain wall, brackets are engineered items: cast or machined, hot-dip galvanized or stainless, each with its own supplier lead time. I've seen 'small items — 8%' turn into 19% of package cost by handover.

3. The interface details nobody owns

Where the curtain wall meets concrete, waterproofing, stone or steel, someone has to supply the closure pieces, membranes and fire-stopping. If the BOQ doesn't name the owner of each interface, the fabricator prices none of it and the project pays for all of it — at variation rates.

A one-hour review of interface ownership before tender award is the cheapest insurance a façade budget can buy.

If you're carrying a BOQ you're not sure about, that's exactly the kind of document to put in front of me in a written consultation — with the drawings, before procurement locks it in.