Yes, Placi - mainly slate. The local slate which was quarried in Abereiddi and nearby Porthgain and Trefin was of a poor quality and had to be covered with a coat of mortar to make it waterproof.
When I read this it became clear why the roofs look so funny (to us). They look as though they're not yet finished. They put narrow slats of wood over the slate at intervals and then cover the whole lot with mortar. You can probably just make it out in two of the photos.
They then discovered that slate waste could be used to make bricks, which led to another Porthgain industry.
There was a lime kiln there too at Porthgain. Limestone and anthracite shipped in from South Pembrokeshire were burnt in this kiln to produce lime. It was carted away by farmers and spread on the fields to 'sweeten' the acidic local soil. It was also used in building - as a mortar and as a wash for walls.
You can tell that I've got a leaflet detailing all this
