Work on asphalt roll roof, 3 tab and architectural shingles, metal.
How to cover a building so that it is waterproof, strong against wind and long lasting has yielded several thoughts on the matter.
To try to get the new material as tight as possible to the existing underlayment emerged from a series of roofs in Connecticut that were weakly laid down over existing roofs. This led to a re-examination of the edges, where the newer material was cut flush up, re-nailed and edged with tar, a time consuming process but one which kept water from getting underneath.
Metal roofing: Never did one historically but willing to try.. several crucial factoring thoughts emerge:
1. The methods of attachment are all exposed. (the screws) rely on being driven straight, usually into wood purlins (1 x 4's) with no room for missing. Over time we've observed screws backing out requiring re-visiting a slick surface that is hard to maneuver on at risk of loosing footing and of damaging the roof. Screws we used in bridgewater also backed out.
2. Purlins suck. Cheap 3rd rate lumber that is full of knots which can leave you with a weak point of attachment or force the screw in on an angle. Plywood or composite board tends to be more uniform in character.
3. Nails vs. Screws? A nailgun delivers a precise blow that sinks the fastener despite whatever is underneath. Screwing is much more demanding. Requiring precision and eyeing the amount of force used to sink the screw in far enough to engage the seal without warping the metal.
4. Old roofs? An old roof is not necessarily flat or square. Odd shaped roofs prefer shingles that can be easily trimmed and flatten themselves against. After some deliberation it was decided to align with the top to bottom edge and deal with it piece by piece. (here it might have helped to snap some lines but other than a better picture and taking up some time would not have helped much).
5. Why the swimming pool effect? The top to bottom edge pieces designed to transition the rood to the existing A structure. Expecting the non flexing edge of that to marry flat isn't going to happen. And that it rises off the roofline at the ends creates a place where water has to deflect. Water that otherwise flies off the edge of the building will have to be deflected and directed down. Rely on a caulk bead.
6. Metal roof is great to form a fast cover on large expanses such as pole barns. New construction where it is flat and square. Lose the edging. Perhaps if one must lay it on old roof, skip the purlins. (they won't make it any flatter) and lay directly on the existing roof and go for a tarred down edge - other than figuring out how to seal the cap at the ends, this would be stronger and skips a time consuming step.