In reply to jkarran:
You're looking 250+kg with a decent set of holds on it.
My 8x11 was lifted with a 125kg lift which picked it up from flat at a 45 degree angle of pull (at the 2/3 point. And then lifted it with holds from 45 degrees. So I would estimate that the board was hovering around 200kg.
2.6 is about .7kg per foot for a weight of 5.6kg per length of board. I used five lengths and two ends for a weight of 40kg max. Each sheet of plywood is approx 28kg hence my three sheets were 84kg and holds (moonboard three sets) were about 50kg so... 40kg + 85kg + 50kg is approximately 170kg and my pulley would need to lift half of that for about 85kg
For a 12x12 board you can scale that up by 1.8 so approximately 300kg. At a 60 degree angle (very steep), this would be a force of half that ( assuming angle of pulley intersects at about 90 degrees) so 150kg (you'd need a bigger winch). Split across two chains would have 75kg per chain.
Obviously throwing yourself around on it would apply a fair bit extra. Lets say double your weight max. At 45 degrees, you'd apply about 70% of your weight on the chains so lets say 80kg weight, doubled = 160kg 70% makes that 112kg extra. So each chain anchor would need to hold half that which will make it 85kg + 60kg = 135kg.
So your chain anchors would have to take about 135kg. add 50% for a minimum safety margin and you have 200kg per attachment point. That's not going to work attaching to single studs so you'll need to span quite a few studs.
In my opinion you'll need to get into the roof space and attach the chains and winch to the roof. If you're wall is parallel to roof joists, you'll be OK I would imagine but if it's 90 degrees, I'd want to put in a beam that spans multiple joists. A healthy ceiling joist should easily hold that, an 80kg person jumping up and down would hit that easily.
I'm not an engineer but given the specifications, I think I'd be happy with that..