To use a plain tile below 35 Degrees you would use an underlay system as supplied by Onduline which acts as the waterproofing layer.
BUT!! (Assuming you mean new 10.5" x 6.5" not 10 x 6) According to Marley Eternit you can use their single camber plain tiles down to 30 Degrees and still have the product guarantee, I hasten to add that this goes against BS5534 and we certainly wouldn't be too happy about it it here.
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