Reinforcing existing flat roof for rooftop garden
Mark Anderson
and so far haven't found much. Next Spring I want to expand my garden
onto the main roof of the building. I currently have a rooftop garden on
a flat garage portion and it has been excellent for the last 3 years now.
The main flat roof is 20' x 75'. The roof is supported by 2x10s on 16"
odd centers spanned the entire 20' from outside wall to outside wall. As
part of a renovation several years ago I did a complete tear off taking
off 100 years of roofing material; there were 9 roofs piled on top of
each other. The roof seems pretty solid. There seems to be no flexing
and several people can walk around without any bounce or anything.
I'm getting confused after reading about live loads and dead loads which
are stated in pounds per square foot. Some tables I've read only specify
live loads and don't seem to take into consideration dead loads. Along
the outside walls I want to place 2' x 8' x 12" deep planter boxes which,
when filled with soil and water can weigh up to 60 psf, which I
understand is considered dead weight. There's also a snow load to
consider as well. I plan to build a structure that secures to the
outside wall so I assume that the roof will have to endure 30 psf and the
wall gets the other 30 (the wall is plenty strong).
Here's where I get confused. Logically, the roof should be much stronger
near the wall and weakest at the middle of the span but I can't find
anywhere that can give me an indication as to what the strength of the
roof is at various points and what kind of dead weight I can place at
various places. I'm also concerned about the wear and tear on the beams
having to hold this weight for many years.
So I thought about reinforcement. As part of building an access port to
the roof, I probably can get 10' long 2x10s to the underside of the roof.
I'm thinking of sistering 2x10s but since I can't get a 20' piece of
lumber up there, I can only sister in 2 sections. Is this feasible? I
read up on furrings (sp?), those blocks you place between the beams to
make the structure stronger. I'd like to add them as well and it would
be nice to know what kind of load the roof can bear.
Living in Chicago we get a lot of porch collapses where people don't
build them strong enough and someone has a big party where people are
shoulder to shoulder and all of a sudden a joist gives way. I'd like to
understand a little more about building to these kind of worst case
scenarios. I will eventually have a structural engineer sign off for
the permit etc. but right now I'm in the pre-planning stages and I'd like
to understand a little about how the forces and stresses propagate on a
system like this. Many many years ago I took Statics and Dynamics in
college and the book is still sitting there on my bookshelf staring at
me. :-)
Ed Kliman
Given all the variables, whatever you need to pay a consulting
engineer would seem to me to be money well spent since you're looking
at dead load, snow load, etc. The weight of the water alone for any
kind of extensive gardening would be of concern for me. If you're in
the city, won't you have to have some kind of permitting to cover any
extensive additions to the building? If so, you might as well bite the
fiscal bullet and get an engineer to sign off on your construction
plans.
Good luck on building the Hanging Gardens of Schaumberg...
Tio Ed
Austin, TX
Eunoia Eigensinn
You might want to have a look at the classic "Simplified Engineering for
Architects and Builders" by Harry Parker.
It will have the beam diagrams & equations to help you understand shear and
bending moments for the loading situation which causes you confusion (as per
you note below).
That being said, if your planters are over or near the walls, the additional
load of the planters shouldn't really be a concern for the joists (ie flexure
isn't an issue).
Sistering 10 ft long members onto the side of 20 ft long 2x10 joists (ie a butt
joint at midspan) would be bad news.
It would be better to use 16 ft long 2x4's along the bottom edge, one on each
side, each one starting from the opposite end of the existing joist, and using
a sheet metal gusset plate at the butt joints of the 16 ft and 4 ft pieces.
Or use 20' long steel flitch plates.
On Sat, 19 Jun 2004 00:54:30 -0500, Mark Anderson <m...@nospambrandylion.com>
wrote:
Mark Anderson
> Given all the variables, whatever you need to pay a consulting
> engineer would seem to me to be money well spent since you're looking
> at dead load, snow load, etc. The weight of the water alone for any
> kind of extensive gardening would be of concern for me. If you're in
> the city, won't you have to have some kind of permitting to cover any
> extensive additions to the building? If so, you might as well bite the
> fiscal bullet and get an engineer to sign off on your construction
> plans.
I do live in the city of Chicago and a rooftop garden project does
require a permit. Mayor Daley and ComEd placed an experimental garden on
top of City Hall which you can see at:
http://www.ci.chi.il.us/Environment/rooftopgarden/
Other than the access port and reinforcements, this can't be considered
an addition. I think the city is encouraging people to do this but they
just want to make sure that your roof doesn't cave in.
For my purposes, I don't want to do what the city did with their rooftop
although it is beautiful and it does work for that building. They make
the roof like one giant flower pot with a prescribed set of layers to
make sure it drains properly. I don't trust that method and prefer a
more modular approach where I make the containers that have the proper
drainage and just place them on the roof structure wherever I want.
Maintenance paths would be just the roof platform (protected of course).
This makes it easier to maintain and if anything goes wrong, the plants
can be removed or the layout reconfigured. The bottom line, either way,
the roof has to hold the same weight (i.e. the weight of the dirt).
I fear after having to deal with the Department of Buildings in the past
that they will force me into their prescribed way of thinking when all I
want is to reinforce the structure and understand its limitations.
Personally, I think my approach is safer for a homeowner building with
wood rafters whereas City Hall's approach may work for commercial
buildings with already established strong steel beam supported roofs.
Mark Anderson
> You might want to have a look at the classic "Simplified Engineering for
> Architects and Builders" by Harry Parker.
>
> It will have the beam diagrams & equations to help you understand shear and
> bending moments for the loading situation which causes you confusion (as per
> you note below).
Thanks for the reference. I just checked it on Amazon and it's a 5 star
book albeit an expensive 5 star book. :-)
> Sistering 10 ft long members onto the side of 20 ft long 2x10 joists (ie a butt
> joint at midspan) would be bad news.
>
> It would be better to use 16 ft long 2x4's along the bottom edge, one on each
> side, each one starting from the opposite end of the existing joist, and using
> a sheet metal gusset plate at the butt joints of the 16 ft and 4 ft pieces.
>
> Or use 20' long steel flitch plates.
I don't think I can get anything longer than 10' up there without tearing
open the top of the roof which is something I really don't want to do. I
was hoping for some simple things I could do to strengthen certain areas
and then know the limitations. This won't have to hold wall to wall
people (or even anywhere close to that) like in those porch collapse
situations but I'd like to know the limitations. It sounds like that
book has what I'm looking for. Thanks again!