I'm building a single storey extension with block walls. The external skin in 7n 100mm concrete blocks and the internal skin thermalite 100mm blocks. My question is, what mortar mixture is required for both skins ? I was going to use a 1:5 for both with a plasticiser but wondered whether they should be different because of the density of block ! Also are weep holes rquired at the bottom of the block work to allow any cavity moisture ?