When you get a roofing estimate from a Vancouver roofing company and you've got a shiplap deck then the roofer should have a peak inside your attic. Commonplace rules amongst laminate fiberglass shingles is that if the shiplap is between six and eight inches in width and your roof is well ventilated then you have to be safe. If you have shiplap that is wider or displays gaps between the boards then plywood is very advisable.
What some roofing installers fail to realise that with unhealthy shiplap, temperature mixed with moisture is the enemy. With the extremes of winter and summer time the shiplap moves around and backwards and forwards. The roofing shingles are nailed down and transfer round to. This motion could be neutralized with good air flow, which brings me to the second drawback. On high of all this motion which is due to beneath air flow and dangerous shiplap, the shingles prepare dinner and freeze sitting on the outer layer of a excessive temperature range attic. Generally this example occurring on a house won't see the end of the warranty. Which isn't lined except the shingle was faulty to begin with, which not often happens
Plywood is laid down and a reliable roofing company will ventilate to as we speak's building code. This may guarantee a roof for at the least thirty years and in the future not have to cope with the shiplap issues. There are some asphalt shingle roofing corporations out there who is not going to deal with shiplap unless plywood is being laid down.
Ask for two estimates if potential to get an idea of the cost of your roof if plywood is a notion you might proceed with. Each home is different, I know in the Greater Vancouver area that some houses only want twenty years and are happy to put over shiplap. On the finish it's your dwelling and there is no legislation saying you need to get plywood. But you've gotten been warned, sometimes getting plywood is an effective choice to make