Hi,

I have good friend who has a solid concrete roof on which he wants to install 4 x rows of 6 x 550w solar panels using A-Frames.

The main point here is that he does not want to drill into the roof and would prefer to use Concrete ballasts to hold the A-frames down.

Can anybody advise how much ballast (weight) he would need to use per A-Frame to hold the panels in place (not to blow away in the wind).

He stays in Cape Town where the wind is gusty and can be relatively stong, especially when the South Easter blows.

Below is a picture of the A-Frames he wants to use (note: this is not a picture of his installation....just a picture of the A-Frames)

The A-Frames will be placed 1.2m apart - can anyone advise how much weight is recommended per A-Frame?

Edited November 14, 20231 yr by Marcodp

5 minutes ago, Marcodp said:

Hi,

I have good friend who has a solid concrete roof on which he wants to install 4 x rows of 6 x 550w solar panels using A-Frames.

The main point here is that he does not want to drill into the roof and would prefer to use Concrete ballasts to hold the A-frames down.

Can anybody advise how much ballast (weight) he would need to use per A-Frame to hold the panels in place (not to blow away in the wind).

He stays in Cape Town where the wind is gusty and can be relatively stong, especially when the South Easter blows.

Below is a picture of the A-Frames he wants to use (note: this is not a picture of his installation....just a picture of the A-Frames)

The A-Frames will be placed 1.2m apart - can anyone advise how much weight is recommended per A-Frame?

Let's hope @Steve87 can share his experience. 

Hi guys, if his spacing is 1.2m apart then a weight of 70kg every second A frame row will do. I base this on the Valsa Ballast block calculations & certification testing. They assume you place a block every 2 panel spacing at least. The Valsa product has a block at the front & at the back each weighing 35kg. 

The pics below show a property in Midrand that has stood up to massive Gauteng storms including last night's destructive hail storm. The owner sent me the images this morning & was very relieved to tell me that all 18 LONGi 575W panels survived & actually had a good wash...

Here is a link to the installation...must say the LONGi's have shown to be very strong & good yields coming from them. A mix of residential & large commercial sites & they are really performing well. 

 

This is awesome feedback - thank you @Steve87 & @Scorp007.

@Steve87, this is a seriously impressive installation - so pleasing to see that some still take pride in their work in SA - well done to you and your team.

 

@Marcodpthanks so much for the kind words...I hope your buddies project turns out a success. The Cape winds are very unforgiving. Rather over do it than under because the damage involved if you get it wrong can ruin a person. 

 

 

There are formulas to calculate the force that wind will produce on the pannels. Once you know the force in kg produced by the wind, just add the necessary ballast.

As an example:

Calculation of the force for one typical 550w panel (2,23x1,13m @ 45degree tilt)

Calculation of the effective surface that pannel opposes to wind:

Surface= panel area x sen α = 2,23 x 1,13 x 0,707 = 1,78 m2

The force that the wind exerts on the obstacle surface of the solar panels is given in the form of pressure. At about 130km/h, the pressure exerted by the wind on a surface perpendicular to its direction is about 79Kg/m2.

Now, part of this force that is exerted on the solar panels is lost when it slides across the surface of the modules since they are not completely perpendicular to the wind. For this reason, the total force exerted on the panels can be decomposed into two components with different directions;

F1: its direction is parallel to the surface of the panel and does not exert force on it when the wind slides and

F2: its direction is perpendicular to the surface of the panel and is what actually exerts the load on the support elements. The value of F2 is given by the tilt angle of the structure:

F2= F x sin α = 79 kg/m2 x sin45 = 55.85 kg/m2 (for 130km/h wind)

Therefore, the force exerted by the wind on each panel is given by the formula:
F = P x S = 55.85 Kg/m2 x 1.78 m2 = 99.42 Kg

Conclusion: The force exerted by a backside wind of 130 km/h on a solar panel measuring 2.23 x 1.13 m installed at 45 degrees is 99.42 kg.

Typical concrete density is 2400kg/m3. You just need to add enough ballast according to the number of installed pannels and their tilt angle.

Worst case is the first row as the wind will be slightly deflected on 2nd 3rd and 4th.

 

Keep in mind roof maximal load too!!

Edited November 14, 20231 yr by Mauritius B

@Marcodp I also have a flat concrete roof and went with a KD Solar mounting system instead of the Valsa mounting system. KD Solar recommended 6x 12KG concrete ballasts per A-frame.

2 in the front (bottom part of the A frame where the panel is closest to the roof) and 4 at the rear (where the panel is furthest from the roof).

 

I am in Johannesburg, so do not have the crazy wind you guys experience in Cape Town. If I were in Cape Town I may add some additional ballasts for peace of mind.

On 2023/11/16 at 5:28 PM, PsyCLown said:

6x 12KG concrete ballasts per A-frame.

How many panels per A-frame?

I read the design specs for my flat roof, they specifically mention a weight limit per SQM.

Can you imagine the fuss the insurance company would make if they did their sums and found out the roof structure was not designed to take the additional ballast + panel weight.

40 minutes ago, FixAMess said:

How many panels per A-frame?

I read the design specs for my flat roof, they specifically mention a weight limit per SQM.

Can you imagine the fuss the insurance company would make if they did their sums and found out the roof structure was not designed to take the additional ballast + panel weight.

Umm, KD Solar recommend an A-frame piece every 1400mm to 1600mm if I recall correctly.

Most panels are just over 1100mm in width (portrait).

 

That would be a big issue. A friend of mine is a civil engineer so he checked it out and did the sums for my place for me.

I have just one question.

Why not drill and use chemical anchors? Those vials with the cement epoxy mixture is waterproof and will not cause structural weakening at all.

Bearing in mind we are talking Cape Town here, I would seriously not risk this in an area where we frequently have gusts exceeding 100kPh in any one season, and those aren't even the maximum.

If there's a problem with the base, (the roof being thin cast) or lacking reinforcing then the addition of all the extra weight is potentially dangerous as well?

@Marcodp I recently went through this excersize with my panels and I found this online calculater that aasisted me.

https://www.omnicalculator.com/physics/wind-load

Its pretty self explanatory. For wind speed look at historical data for the closest airport, usually posted on their website. you can add 20% (or more if you wish) to the max measured as a safety factor. Angle is the panel angle to the roof, about 30 degrees is optimal for South Africa year round. The rest is pretty self explanatory. Once you get the Newton value devide by 9.8 and you have the required weight in kg

On 2023/11/27 at 8:29 AM, Sass said:

I have just one question.

Why not drill and use chemical anchors? Those vials with the cement epoxy mixture is waterproof and will not cause structural weakening at all.

Bearing in mind we are talking Cape Town here, I would seriously not risk this in an area where we frequently have gusts exceeding 100kPh in any one season, and those aren't even the maximum.

If there's a problem with the base, (the roof being thin cast) or lacking reinforcing then the addition of all the extra weight is potentially dangerous as well?

Usually it is because of the waterproofing warranty on the roof itself, or at least that is why I didn't drill through mine. Once you drill through this, you loose your warrantee and any issues that arise from that is yours to fix and could cost more than you bargained for.

Did this install recently...i think the main reasons for non intrusive ballast weights is for firstly warranty purposes & secondly if any waterproofing will be done in the future that the entire system can be removed (disassembled) and then reassembled without any special tools or equipment. The entire system is nut & bolt modular. 

 

 

Edited December 3, 20231 yr by Steve87

In total each sub structure weight 325kg. This being in Gauteng, to move 325kg would take some doing. In Cape town yes probably would need some extra weight & head scratching.