As others have mentioned it all depends on the actual pitch of the roof. What exactly is shallow?
My own extension is 22degree pitch but it is cross battened and double felted with a 125mm headlap on the slates. The slates I used are also a wide duchess so things may vary depending on your own slate sizes. In additiona allowances also have to be made on the headlap when the roof is on the weathered side of the property (i.e. extra lap).
But in conclusion, your BC will advise you of what they require.
But, if you go below the minimum recommended pitch for any particular tile, then you must incorporate additional measures to deal with any rain which may blow in under the tile. This would invole sealing the sarking felt in some way.
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