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vcleo 05-17-2009 12:36 AM

Green house on flat roof?
I have a 20x24 garage with a flat roof. The roof appears to have 2x12 on 16 inch centers. I would love to put a sun deck or green house up there. How do I find out if it can support either of the two?

DaveyDIY 05-17-2009 08:36 AM

Is there any support in the middle?
20' is around the Max for a 2x12 span
Without any support in the middle of the span I would say no
Interesting a flat roof in Alaska
I would think you would want a slope for snow to fall off

vcleo 05-17-2009 12:54 PM

There is a wall dividing the one car stall from the shop, with a man door between the two. The wall runs the same direction as the roof supports.

DaveyDIY 05-17-2009 06:59 PM

The wall would need to be perpindicular to the roof rafters in order to be supporting them
But you could put a beam under the rafters- supported at that wall
Sizing would be up to an engineer based on the load

CraigFL 05-18-2009 06:14 AM

The other thing you need to consider is that the roof is probably not made for traffic. This means there would be additional weight to support some kind of decking/floor.

DaveyDIY 05-18-2009 08:55 AM

Actually depending upon the wood the 2x12's are pretty close to being rated as a floor for a 20' span. It would be a bouncy floor. Once a beam is installed to cut the span in 1/2 the 2x12's will be fine as a floor - plenty of support.
The only concern (maybe?) is you do not want to ruin the roof - IE cause leaks ?
A greenhouse would be fairly heavy - depending upon how much dirt/plants you want to put up there, Unless you are just using it to grow out seedlings

Possibly you could use one area as a greenhouse near a corner, then the rest as a deck. My only though is - why do this on the roof?
My greenhouse is on the ground, facing south - up against the house. It can get up over 100 in the Spring & fall & can help heat the house

I can see a deck on the garage if there are decent views
If not then you are better off building it on the ground

vcleo 05-19-2009 11:39 PM

Right now the garage gets wonderful sunlight from mid morning to late evening and without cutting down some mature birch and spruce that area is an ideal and unused space. I think you have me convinced to keep things on the ground just because of all the downfalls to trying to make it work, although the view is pretty nice up there.

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