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LOS ANGELES -- It's America's biggest home improvement store and, now, the country's biggest home remodeling contractor. But after hearing horror stories from consumers about The Home Depot and its remodeling services, Investigator Joel Grover decided to go undercover.

NBC4 has done a lot of stories about small, fly-by-night contractors. But this is a story about a company millions of Americans trust and many of them hire The Home Depot to remodel their kitchens, put on new roofs, and do other home repairs. But NBC4's investigation found that behind the big name are some big questions about how this company does business.

NBC4 has heard from dozens of customers who all say they have had run-ins with the biggest name in home improvement. "It's a nightmare," one customer says. "I broke down and cried," says another customer. "I wouldn't buy a box of nails from them, they don't deserve it," says another customer. These days, Home Depot sells more than just nails.

Inside a Los Angeles area Home Depot store, NBC4's Joel Grover asked questions about their roofing services. A Home Depot employee offers to send a roofing expert to evaluate the roof. "He'll go up and he'll look," the employee tells Grover. "He'll go up on the roof?" Grover asks. "Uh huh," the employee replies. But when a Home Depot salesman shows up, he never gets up on a ladder and he tells an undercover NBC4 customer she needs a new $10,000 roof. "The house needs a new roof now?" the NBC4 customer asks the salesman. "Yes," he replies. "The roofing is crumbling?" the NBC4 employee asks. "Yes," the salesman replies.

But former LA City Housing Inspector Randall Akers disagrees. "I think he's wrong," Akers tells NBC4. Akers inspected the roof on the house before that salesman came and said, "It's in great shape." Akers also tells NBC4 that the salesman "should have gone up on a ladder, walked around the roof." Home Depot's own guidelines say their salesman should inspect the roof from a ladder, and look in the attic for "interior damage". But the salesman that evaluated the house never did that. A former Home Depot roofing salesman tells NBC4 there's big pressure to sell new roofs. "Completing the deal is the number one objective," he tells NBC4. "Even if you have to lie?" Grover asked. "Yeah," the former salesman replied.

"I don't trust them," says Maureen Sandoval. She says Home Depot lied to her when they promised her a new roof on her house within eight weeks. "How long was it?" Grover asked Sandoval. "Six months," she replied. She tells NBC4 that it took 17 calls to Home Depot before roofers finally showed up, and while waiting for them her roof started leaking. "It got worse and worse every time it rained," Sandoval tells NBC4.

NBC4 heard similar claims about Home Depot kitchen remodels that took months longer than promised. "They broke this cabinet three times," one customer tells NBC4. "We had no sink, no stove, we had to eat out three times a day," says a Home Depot customer.

NBC4 also heard problems about floor installations. "The wrong color of flooring was put down," says a Home Depot customer. One of the roofing salesmen that NBC4 caught on tape tells an undercover customer, "You only deal with Home Depot. You will not deal with any contractor." But the former salesman that Grover spoke with says that's not true. He tells Grover, "They'll [customers will] have to deal with the subcontractors."

Most of Home Depot's work is done by subcontractors. But when customers complained to the company about problems, they say letters and calls often went unanswered. " "I went through four different Home Depot employees," says one customer. "We got no response. They just don't care," says another. "I think that customers should definitely be very wary of Home Depot's promises and not believe everything that they're being told," the former salesman tells NBC4.

The Home Depot salesmen that NBC4 caught on tape insist everything they said was true. NBC4's Joel Grover caught up with one of them outside the house he said needed a new roof. Grover asked, "Will you answer our questions?" "I will," the salesman replied. He immediately called his boss and told Grover, "I was advised not to say anything." Then he went to leave, telling Grover, ""I don't have anything to hide." Grover asked him, "Do you always follow Home Depot's guidelines?" "Yes, I do," the salesman replied. But Grover pointed out that he didn't and the salesman said, "If I was in error in my process, I'm willing to leave Home Depot."

As far as NBC4 knows, that salesman is still working for Home Depot. As for the company, they declined NBC4's offer to do an on-camera interview for this report. But in a statement, Home Depot says it does 11,000 home installations each day and the vast majority meet or exceed their customers expectations. They say all of their home services are guaranteed.