Second story addition using panels and pre-built roof
Does anyone here have experience or thoughts on using panelized walls and a trussed roof assembled on the ground (all craned into place) as a way to add a second story addition?
It (or something like it) seems like the obvious way to go for a second story addition, but I have no actual experience with it, and it doesn’t get talked about much in that context.
My thinking is that craning the walls and roof (with at least sheathing on it, if not more) would not only significantly reduce dry-in time, but would likely save money. The most significant “drawback” I see written about is the need for planning and making sure things align. What other big picture issues am I not thinking about?
For context, the hypothetical project would be a simple 30’x34′ 8/12 gable roof. I live in Maine where you really can’t count on long stretches of dry weather. There’s plenty of ground space for assembling the roof assembly.
I am actually a bit perplexed how people manage to do second story additions conventionally. It would seem things are guaranteed to get soaked.
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I have done pretty close to that by pre-fabbing wall section and prepping all rafters on the ground. Sheathing was pre-cut as well so it could all be nailed up quickly once everything was lifted up. The wall sections were light enough that it didn't even need a crane. It went up pretty quick but still had to rely on tarps to keep the rain out.
Beside building the structure, a lot of time and work goes into prepping the existing house for the new floor. Some of it you can do from the attic, lot of it you can't do until the roof comes off. This can be fussy work, so won't be quick.
Basically you build, at the end of the day if it looks like weather, you tarp. If you are lucky and tarp well, water won't get in even in a big rainstorm. Partially constructed sections are not easy to tarp, this is usually where rain will get in especially if there is a lot of wind.
We haven’t pre-fabricated panels for a second floor on an existing house but we have pre assembled both walls and roof for new builds.
We find this process to be very efficient, safe and comfortable with dry in being very quick. I would definitely consider this method for your application. Dry in will be a little slower as you will have to prep the floor deck over the existing house etc but overall this is still worth it.
The absolute most important thing to keep in mind is to pre-plan and verify all measurements accurately. We temporarily pre install our plating and map out everything directly on the plates. This works very well as a template and greatly reduces the chance of error and is well worth the time.
Over an existing house, I would consider using Advantech XFactor subfloor with taped seams which will serve as a very effective temporary roof over the main floor while you construct your panels.
Yeah I need to think through the flooring prep. It actually already has joists sized the same as the downstairs, so I am wondering if I can/should leave them (technically both are undersized by modern code standards, but they're not bad-- 2x8's for a 15' span).
The issue is that they are cut down over the eave wall plates where the roof comes down, so there's no way to place a new exterior wall on top of them. I am debating if I could just sister some short sections that serve to block out under the new walls, or if I will need to run new joists sistered all the way back to the center bearing.
Regarding the roof: I am trying to get this work done without a large crew, so my thinking is that I can pick away on the roof on the ground without the rush of an exposed 1st floor, then hoist it all in a day and have it covered. I'd just need to think through the pick points and whether a crane company is willing to do that sort of thing.
Your point about the crane operator’s willingness is key. We always have our crane operator walk our sites before we fully develop our plan. Our operator has been at it for decades and is a wealth of lifting knowledge and often provides very useful information regarding lift points, acceptable weight, reach and crane placement and where we should stage our assembled roof sections. I highly recommend consulting with your operator before starting.
I think the idea is great but I think the realities of making it happen means it will make the project much more expensive and the finished product will not be as good.
If it happens to rain while the roof is off and the tarps should leak it is not the end of the world. Some more drywall will need to be worked on and a few more walls will need some paint.
Seems next to impossible to get the accurate measurements needed to prefab the new walls before the old roof is removed. The one thing you can be sure of when you open up an existing building is that you will find something you were not expecting. With the prefabbed wall on site, you will be unable to make adjustments when the real world happens to be different from what you were expecting.
Walta