So now, let's take a look at the quarter. For U.S. Gypsum, overall wallboard volume, price and cost significantly improved compared to the second quarter of 2011. Compared to the first quarter, our volume was down slightly, but our margin increased with both price and cost improvement. Our new pricing strategy, which we instituted earlier this year, which gives our customers certainty on price levels continues to be very effective. In our Substrates business, we had strong results with our next-generation DUROCK, with margins up over 16% year-on-year, and DUROCK volume was the strongest since 2008.
Now turning to our Ceilings business. Profitability in the quarter remained strong due to the growth of our high-end ceiling tile and solid grid margins. Sales were down slightly versus the first quarter, but we do expect a pickup as we are currently in the school construction season and the seasonality of our big-box business which will happen in the fall.
The next area is our Distribution business, L&W Supply. L&W's wallboard volume and price increased in the second quarter, and we had strong performance in steel and ceilings compared to the first quarter of 2012. Our comp store sales increased 11% year-on-year as we continue to focus on the commercial contractor, where L&W provides value. In fact, sales and margins at L&W improved in all 7 core product categories year-on-year.
Our international units continued to weather various storms in their market. For example, Canadian Gypsum Company's second quarter year-on-year sales are ahead of last year, while operating profit was down. USG Mexico sales also improved year-on-year despite a very challenging market. In order to strengthen our product portfolio in Mexico, we've successfully launched both UltraLight and Mold Tough during the second quarter.Our results from the first half of this year give us reason to be cautiously optimistic with both top and bottom line improvement. Despite a continued weak market in the United States, we made slow recovery. We continue to focus on our plan to win. Read the rest of this transcript for free on seekingalpha.com