Roof top deck.

[greentrees]

I'm getting ready to start re-designing a roof top deck. (first set of plan/structural eng. were'nt accepted by city -- set back problem).

So I'm moving the deck to the other end of the house. This will be a 400 sq. ft. deck on top of the second floor roof, (over the livingrom, over the garage)

The living room has 2x14@16 floor joists running N-S. and the garage door is on the west wall. the fireplace/chimney is on the north wall. The front (W) 7 ft. of the house is 19 ft across (N-S), then the chimney sets in about 2 ft. for about 4ft, then the house becomes 24 ft wide, although the 19ft wall continues (flanks a stair well). The N & S walls are load bearing walls, The West wall is currently designed for the existing roof ridgebeam which supports 4x6 joists to the N-S walls. (pitch is 3/12).

I hope you can visuallize that...

I'm planning to move the N.W. 7ft wall in (S) 1 ft. and put in a french door. (it would then line up with the wall underneath it, it cantelevers out 1 ft now). this will give me room to build a new tiny 2nd floor deck off the livingroom, and place a spiral stair to a new roof top deck.

I'm trying to come up with a plan to take to the structural engineer. He's old school, and his last design included a 400lb beam. searching about (and on this BBS), It looks like an engineered wood product would be a good choice.

I'm going to try and run some numbers for two different Ideas.
1) either long wood I-beams from the south to north walls. My concern here is the chimney, which blocks the joist from reaching the load bearing walls. could these i-beams be doubled and headered to transfer the load around the chimney?. or could lvl/glulam joists be used inplace of the doubled i-beams? Is is bad to mix type of joists in a situation like this?
or can I put a post down to the existing roof joist (4x6 timbers)?

2) add a large lvl/glulam beam N-S across the roof, resting on wood girders that distributes the weight across the load bearing walls, two smaller beams that run E-W createing a cross, joists would again be placed N-S but only half the distance, I figure the N&S edges would transfer their load through joists to the N-S walls, the middle of the deck would trasfer the joist load to the little beams which would transfer the load to the big beam which is supported by the N&S walls. The east & west ends of the little beams would support a small portion of the deck, and this would be transfered to the E&W walls. Since the roof is now covered by a deck, does it still need to have a live-load calc (in addition to the deck live-load)? if not, the the live load saved from the roof could be used to support the end of the little beam.

Any thoughts, ideas, suggestions?
thanks
GT

[Rich]

I'll need a little bit to digest all that information :)

[greentrees]

take your time, I've battling this deck project for almost a year, and am just trying to re-group before I start over.

[greentrees]

Corection pitch is 2/12,

Another thought I had:
3) try and use the existing 4x6@24 rafter/beams as part of a bridge-like truss system. Could remove the T&G to loft the ceiling even further... Unfortunately, I think the middle point would be thinest, so it may tend to push the walls out, if/when it defelcts.

One assumption I've made is that I would tile the deck, this way it would be waterproof, and I would just need to maintain it rather than plan for removing it for maintainence on the roof.


gt

[Rich]

I think I'm going to have to draw something for this one.. haha.

[greentrees]

o.k.
I've been playing with GP's s/w, and have decided that option 1 is probably workable.

Some follow on basic questions:

if I plan to header out a section, then the joists which the header ties to will hold an extra load, and the joist which tie to the header will be shorter, (and so realitively stiffer). can I expect the subfloor to hold this togeather, or will the differences cause problems? How is this usually handled?

Along the same line if my North wall is "jagged", would I need to do anything special when the joists change from 20' to 16'?

if I'm concerned about the joists surrounding the chimney header, would using a "solid" engineered wood (e.g. lvl) cause problems.

I'm thinking that the differnent flex characteristics of the long/short & i-beam/LVL might be more than the subflooring can distribute.

I'm also wondering about a good connection between the exisiting angled rafters and the vertical cripple wall under the new beam, any good hints, or can I just use Simpson ties?

GT