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It's your duty of care as a charging tradesman to follow propper and due practice, which isn't debated here. If you are being asked to install materials that your customer supplied which have been sold as fit for purpose and in your opinion aren't then you've two options, one walk away and loose that channel of potential future work or two, explain in detail your founded concerns and let them decide. It's not as black and white as some one asking you to lay red biscuit crap on a driveway, you've been asked to fit tiles that have been sold as floor tiles by a reputable manufacturer in a bathroom floor. Your not likely to be found at fault for that. 🙂The true is the fact you are the tradesman,the client not have any authority to tell you how you do your job.Is just up to you if is suitable or not,if you with your experience have any concern about the materials or surface not need to ask to your customer,just refuse to do something wrong.