t and g is five inches wide and was allways used for those jobs, but today osb is used it comes in wide boards is easy to work with and fits togather like t and g, it is very strong, you will do the job a lot quicker, it is used for floors were their used to be wood, on the roof it will be coverd with felt ,the floors treat as t and g, but if you are tempted to do walls with it then it must have a felt covering,they are treated but will not take being continualy being wet,

So it's not a poor substitute for T&G?

depends what you are doing with it,if exposed to the weather a coat of paint would be no good,it was ment for floring in side and for that it is a grate inprovement on t&g but with ashfelt it will not worpe.,or rot outside.

horses for coarses . the reasen shiplap does so well compared to other wood is the water doesnt lay on it as theres a drip line every 5-6" .