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Roof Top Garden on 6 inch Concrete Slab

02/28/2016 3:14 PM

Hi all, Am new to this site. I found it whilst trying to find some information about a roof top allotment I am currently building. So I have 5 planters 2 @ 3.5 ft x 14ft. 2 @ 3.5 ft x 10.5 ft. 1 @ 2ft m x 10.5 ft. These in total will contain approx 2 tons of soil. They are sitting in scaffold fames, each planter has 8 legs that sit on 1ft x 1ft base plates. The roof is 6 inch concrete and is second floor. It is an old industrial building in London so hard to tell the extent of reinforcment or quality of concrete. However it all looks good with no cracks or current issues. If there any way to tell if this is ok? The roof has had machines/ other things stored on it before, and have had people tell me it should be more than strong enough... however, I would really appreciate a second opinion. Many thanks

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#1

Re: Roof top garden on 6 inch concrete slab

02/28/2016 3:51 PM

Well it's not something I would attempt without a structural engineers' OK.... he might need to take core samples...

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#2

Re: Roof top garden on 6 inch concrete slab

02/28/2016 4:43 PM

I don't know what soil mix you plan to use, but you might consider mixing a lot of peat moss and vermiculite into the dirt to keep the weight of the soil as low as possible.

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#3

Re: Roof top garden on 6 inch concrete slab

02/28/2016 4:50 PM

Well figure that every square foot of concrete that makes up the whole roof weighs ~75 pounds what percentage of the total present roof weight would your added load make up?

Personally unless you were covering the whole roof a foot or more deep in soil I would not have the least bit of concerns over a ~2 ton weight that was spread out over several hundred square feet of area with 40 legs under it.

To be honest I have driven fully loaded 80,000 pound semi rigs over old concrete bridges with 20+ foot spans between support points that were 6" thick.

To me any building that was constructed as a commercial building that could have heavy machinery like forklifts and what not driven on the roof I would not be the least bit concerned about a spread out stationary load like you planters.

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#14
In reply to #3

Re: Roof top garden on 6 inch concrete slab

02/29/2016 2:50 AM

An approximation for snow design load on a flat roof in London can be taken as 60kg/m2 (BS 6399 1-3 which is superseded by the eurocodes but still about the same). Note that this corresponds to a snow accumulation close to the parapet.

Even though it is an old commercial building, it is unlikely that forklifts were going around the roof. The load on the roof these days would be either 500 kg/m2 or 750 kg/m2 for equipment. Not knowing the age of the building but given that it is now residential and therefore quite old (in the 40s / 50s ?), the roof would not have been designed for modern heavy machinery at the time. Given the thickness of the slab 150mm, that supports that idea.

So your roof slab is quite probably designed for the snow load alone.

If you are putting the planters towards the edge, that would be safer.

The fact that you have no cracking so far is not a good indication of its viability. Wait till it has snowed about 50cm on your roof and then see if it is cracking. Or perhaps not . . .

If you need, I know a couple of good engineers in London who could also orient you towards concrete testing. You probably won't need to core through as the cover meters will probably be able to give you sufficient information on the reinforcement.

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#22
In reply to #14

Re: Roof top garden on 6 inch concrete slab

03/01/2016 9:02 AM

"Wait till it has snowed about 50cm....."
Might have to wait a long time for 50cm of snow in London.

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#4

Re: Roof top garden on 6 inch concrete slab

02/28/2016 5:06 PM

Thanks for the advice- appreciated. That's a good idea re mixing the soil, shall look into this. RE getting a structural engineer in, this is most likely my next step. As I said, I have been told that it should be fine so have been putting it off- however for the cost of an SE in relation to possible damage makes sense to me! Thanks again all!

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#5

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 6:13 PM

I agree with tcmtech, but I can't supply any scientific data to back that up, since the condition, strength, reinforcement, etc. of the concrete is unknown.

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#6

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 6:21 PM

That's 400 lbs per square foot it's concentrated on.. Not recommended

on top of that, what is the load (snow load) usually 50-75 pounds saw/ft...

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#9
In reply to #6

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 7:57 PM

Well lets hope no one ever stands on their tip toes being if I do that I can focus all 260 pounds of myself onto a less than 4 square inch area which would then equate to a equivalent force of ~7500 pounds per square foot.

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#10
In reply to #9

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 9:25 PM

Thank god I got big feet.... And you know what they say about big feet.... :p

At work I was told to lay some 18" cooling lines and to run them on top of a mezzanine that had a spancrete floor with supports every 10 feet off the floor.... After I rained on their parade, I ran the lines outside.

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#12
In reply to #9

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 10:07 PM

Pay no attention to him.

At the farm in Arkansas, 50 years ago, my father built a garage on the side of "our" hill, by the house. The terrain where it was built was steep enough that he had a two story garage. It was 40 feet wide and 25 feet deep, the 25 feet span was unsupported for 12 feet, except by the outer wall on one end and the back side. It was open on the other 12' end. One side of the floor had a 3'x4' opening for servicing vehicles from below.

I parked my car below when we weren't working on something parked above from down there. It was great for oil changes etc. No bending over or lying on the ground.

The floor was 4 inches thick and not heavily reinforced. The opening may have had extra reinforcement, I can't remember.

I lived back there 20 years ago, for a year and a half, and parked two full size Chevy 4WD vehicles in there. It's still there today and still supports two full sized vehicles and assorted tools.

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#17
In reply to #12

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 8:38 AM

Lyn gives his professional stamp of approval and with the information given, blindly says "go, dam the torpedoes, full speed ahead, reefs? what reefs, I don't see any reefs. Don't mind him, you listen to me. Full speed ahead."

Instead what the original poster should do is the right thing instead and find out what the building specs are for the slab. Don't do the ignorant thing, do the smart thing, and find out.

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#7

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 6:39 PM

do you have any idea what is directly below?the closer you are to a bearing wall the better. columns too

I'd go shallow on the soil with plenty of drainage-weep holes in the design

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#8

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 6:55 PM

What about do away with the soil all together and go with an aeroponic system, where the roots are sprayed....You could have the water tank on the first floor and pump it up to the roof...

http://www.homehydrosystems.com/hydroponic-systems/aeroponics_systems.html

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#11

Re: Roof Top Garden on 6 inch Concrete Slab

02/28/2016 9:50 PM

Beware wet soil weights more!

Beware snow loading on top of wet soil. Its a flat roof right?

Do a structure analyses. Pay for it to cover you know what!

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#19
In reply to #11

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 9:15 AM

Agree. When the soil is saturated you could expect to double the weight of the dry soil.

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#20
In reply to #11

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 10:53 PM

Make sure the SE includes WET soil and snow load in his calculations.

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#13

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 2:01 AM

I don't think you will have any problem with the slab except possibly with the concentrated loads at the legs (those 1ft x 1ft base plates should negate that).

I would be concerned with the ability of the walls to take the added roof load. Old industrial buildings are not always up to taking the additional load, hence the recommendation by another poster for a structural engineer, particularly if you are using the building for residential purposes and have modified the wall or interior support.

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#15

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 3:49 AM

The details and arrangement of what's supporting the slab from underneath are as important as the slab thickness.

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#16

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 7:14 AM

I will not recommend this. Roof is not designed to carry load of plants. Moreover the plants will grow so the weight will also grow. Also the constant presence of moisture on ceiling will weaken the RCC.

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#18

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 8:57 AM

Your local building department may have drawings or records that could lead you to drawings. A metal detector might be able to sense the presence and position of reinforcement in the concrete. See the walls/columns on the floor below to determine what the span is between supports.

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#21
In reply to #18

Re: Roof Top Garden on 6 inch Concrete Slab

02/29/2016 10:59 PM

Depending on how old the building is, the building department may no longer have the plans, but should have plans if recent modifications were done, particularly if a permit was required.

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#23

Re: Roof Top Garden on 6 inch Concrete Slab

03/07/2016 1:44 PM
% total Wt Wt each supports lb/sq ft
3.5
14
2 98 0.509091 2036.363636 1018.182 8 127.2727
3.5
10.5
2 73.5 0.381818 1527.272727 763.6364 8 95.45455
2
10.5
1 21 0.109091 436.3636364 436.3636 8 54.54545
192.5 4000
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