Could be 50. It's mostly a working minimum for the clay deteriorating I think. Even when you see a 20s estate it's uncommon to see replacements, and half of tho

Could be 50. It's mostly a working minimum for the clay deteriorating I think. Even when you see a 20s estate it's uncommon to see replacements, and half of those are rot in the timber or some such. Blocking up ventilation is a great way to cause that rot as some find the hard way. 60s and 70s houses may need earlier - quality wasn't great with some of those buildings. Great point about conservation areas, never occured to me. The slate looking one is convincing enough for the national parks. The unique pattern printing should appeal to the most fussy NIMBY. Shame they won't last 150 yrs like slate.