screed

a layer of material (usually cement based) of defined minimum thickness, which sets in situ and which may be interposed between the structural substrate and bedded finish

<concrete> strip of wood or metal used as a guide for striking off or finishing a surface to a required shape
<plaster> narrow strip or band of plaster laid on a background to serve as a thickness and alignment guide for bringing the whole of the undercoat to a true and even surface