The mechanism
A flat glass escalation clause indexes the portion of the contract price tied to flat glass to a published series instead of fixing it at signing. The benchmark is the BLS producer price index for Flat Glass, series WPU1311.
Flat glass is a real input cost in curtain wall and glazing, windows and storefronts, and interior partitions work. Its price can move more over a contract term than a fixed-price contingency is built to absorb, so indexing the portion of the contract price tied to flat glass moves that risk onto the published index instead of onto one party.
What's specific to flat glass
Flat glass manufacturing is dominated by furnace natural-gas cost and short-radius freight, so this PPI tends to track regional gas prices. In a gas spike, a flat-glass clause and a natural-gas clause on the same project can both trigger for the same underlying cause.
The calculation, step by step
- Base index value. The index value at the base period, usually the month of contract signing or bid submission.
- Current index value. The index value at the adjustment date, usually the month of delivery or invoicing.
- Ratio. Current value divided by base value.
- Adjustment. Apply the ratio to the portion of the contract price tied to flat glass, not necessarily the whole contract.
Take a clause with a $240,000 scope tied to flat glass, a base index of 175.0, and a current index of 190.0. The ratio is 1.09, a 9% increase, so $21,600 is added to that scope.
Where clauses go wrong
The most common mistake is not naming a series code. "The flat glass index" is not a series. BLS publishes more than one flat glass-related series, so name the exact code, WPU1311, in the contract text.
Clauses also skip the averaging window. A single month's value swings more than a 3-month rolling average, so the clause should say which one applies. Publication lag gets left out the same way: BLS releases data on a delay and revises early figures, so the clause should say whether the preliminary or final value governs.
The last common gap is no cap or floor. With no cap, an index spike passes through in full, which can make a large adjustment commercially unworkable.
Related
- Escalation clause analyzer: paste a clause and see the formula, undefined terms, and risk score.
- How escalation clause base periods work
- Price adjustment caps and floors