D. Barbour
Analyst · R. W. Baird
Thanks, Mike. Good morning, everyone. Thank you for joining us on today's call. We delivered another quarter of record financial performance in the fourth quarter of fiscal 2021. Sales grew 20% year-over-year, driven by 21% residential sales growth and 11% nonresidential sales growth as we continue to execute at both ADS and Infiltrator in a favorable demand environment. The residential market remained strong. Both ADS and Infiltrator Residential market sales grew over 20% in the fourth quarter, driven by favorable dynamics in new home construction, repair/remodel and on-site septic, accelerated by our material conversion strategies at both businesses.
Residential market sales have increased to 39% of our domestic sales as compared to 23% prior to the Infiltrator acquisition. The market indicators show that homebuilders continue to acquire land for future development and that there's an overall shortage in available homes, which drives the front-end new community development sales at ADS and the on-site septic system sales at Infiltrator are driven during the home completion stage.
In addition, the repair/remodel business remains robust. ADS participates in the repair/remodel segment of the residential market through retail, which is about 40% of the legacy businesses residential sales. Infiltrator's repair/remodel business and the residential on-site septic market accounts for roughly 1/3 of their business.
Growth in our nonresidential end market was broad-based throughout the United States. We continue to benefit from growth in horizontal construction, such as warehouses, distribution centers, data centers as well as the developments that follow the residential build-out. About 2/3 of our domestic allied product sales are in the nonresidential market, where sales increased 13%, further giving us confidence in the underlying market strength. Sales in the agricultural market increased 50% this quarter, driven by strong demand as the spring selling season got off to a good start. The agriculture economy remains favorable, and we continue to benefit from the programs we put in place around organizational changes, new product introductions and improving execution. We experienced strong demand in the Midwest region, particularly in Minnesota, Ohio, Iowa and Michigan. Further, we are expanding our presence in key strategic areas like Missouri and parts of the southeast to drive increased market share.
International sales also increased 49%, primarily driven by sales growth in our Canadian business which nearly doubled compared to last year. Canada is doing well across both the construction and agriculture end markets with similar trends to the United States. Additionally, this quarter, we continued to leverage our pipe manufacturing facilities in Mexico to help service the strong demand we experienced in the United States.
Finally, Infiltrator continues to exceed expectations with 23% sales growth in the fourth quarter against a very tough comparison to the prior year, and broad-based growth across the infiltrator product portfolio. This includes double-digit growth in tanks and leach field products, with strong growth in Florida, Tennessee, Alabama and Indiana, among other states. This was led by our material conversion strategy of displacing concrete septic tanks with plastic tanks and the economic advantages of septic chambers in leach field systems.
The Infiltrator business is benefiting from strong distribution presence in the Southeast and Midwest as rapidly growing micropolitan areas, which typically lack the sewer infrastructure needed to support rapid housing development.
Moving to profitability. We achieved record fourth quarter adjusted EBITDA during the period. Adjusted EBITDA margin increased to 190 basis points. The increase in profitability in both businesses was driven by leverage from the strong sales growth, favorable pricing as well as contributions from our operational, productivity initiatives, which helped to offset inflationary costs. I'm very proud of our employees and management team at both ADS and Infiltrator for bringing fiscal 2021 to a close with strong financial performance this quarter.
I would like to highlight our fiscal 2021 financial performance compared to the 2018 Investor Day plan now that we have finished out the year. We communicated a 3-year plan in November 2018, about a year after I got to ADS, and I'm very pleased to have exceeded the targets we laid out. The ADS legacy business grew sales at a 7.7% CAGR, driven by the sales programs we laid out in November 2018. We continue to execute our proven market share model, converting traditional materials to our plastic pipe products in the strong -- in the stormwater market to drive this outperformance. Our sales team is going after the significant growth opportunity for large diameter HP Pipe, which has grown at a double-digit CAGR over the 3-year period. We are focused on key -- on growth in key states namely Florida, Texas and California as well as additional priority states where we have -- where we find attractive market opportunities.
We continue to penetrate the allied product market through our existing portfolio as well as through innovation and acquisitions. Allied product sales have also grown at a double-digit CAGR over the time period. Finally, our agriculture and Canada businesses performed above our expectations, both returning to strong growth.
In the plan laid out in November 2018, we stated our intention to grow adjusted EBITDA margin to between 18% and 19%. The legacy ADS business finished fiscal 2021 with a margin of 24.3%, significantly outperforming our plan. The outperformance was driven by execution, top line growth, favorable material cost, fixed cost leverage as well as improved efficiency in our supply chain, operations and distribution. The improvement in profitability as well as execution of our working capital initiatives and the acquisition of Infiltrator drove the improvement in free cash flow conversion to 66% of adjusted EBITDA, significantly better than the 45% in fiscal 2018 and above our target of at least 50%.
Our performance over the last 3 years, coupled with the acquisition of Infiltrator, changed the growth and financial profile of the company. The acquisition of Infiltrator was a great addition to our business. Through Infiltrator, we increased our exposure to the residential market, diversifying our end market exposure and gained a very high-quality management team, set of engineers and operators who continue to execute the Infiltrator proven business model.
We believe executing on the strategies and plans laid out in 2018 increases the value of our business as evidenced by the significant increase in our stock price since issuing this plan in 2018. We will continue to focus on driving top line growth, improving our profitability and converting profitability to cash at a high rate, in turn, creating additional value for our shareholders. We will continue to pursue these proven strategies and we'll issue our next 3-year plan this fall at our next Investor Day.
In summary, we did a great job executing this quarter and fiscal year, and are pleased to continue our track record of generating above-market growth across our key end markets. In the past, we've shown our growth relative to the market. However, market statistics are a bit distorted right now due to the pandemic, making it more difficult to measure. That said, regardless of how you measure the market growth or decline we handily outperformed the market, giving us confidence that our material conversion story is intact or even accelerating. Our success in growing above market is a function of our unique advantages.
We continue to have success in gaining market and wallet share through our material conversion and water management strategies. We are more vectored to key states where construction activity remains high, and we're making focused bets in others where we see opportunities for growth. We are benefiting from broader market trends, including rapid growth in micropolitan areas and higher exposure to suburban development. And since the acquisition of Infiltrator, we are more exposed to the residential construction market, which now represents nearly 40% of sales.
And within the nonresidential market, we are also benefiting from our outsized exposure to horizontal construction, which was far more healthy than vertical construction this past year. In other words, we're an evolving and a stronger ADS today than any point in our history, and we look forward to the future. As we look to fiscal 2022, we will build on our strong market position, execution, and new levels of profitability. I want to thank our employees who are the bedrock of our success over this past year. We will stay focused on employee health and safety and on delivering the needs of our customers.
We are well positioned to capitalize on market demand while continuing to generate above-market growth through the execution of our material conversion and water management solution strategies. We remain focused as always on disciplined execution.
With that, I'll turn it over to Scott Cottrill to further discuss the financial results.