Ballast requirement for Flat Roof PV ?? ( Help Please!!!!! )

[sunshine8]

Hi All

Could some one please advise me on how/ where can i find information on how to calculate ballast requirements for laying panels on A frames and screwing them to the ballast. I did consider screwing it straight into the roof. But I've had some of my friends who had it that way and had water leaking cracks around it even when they had roof mats ( dont know what they are called) for protecting against water leaking.

Its a 5 X 8 m roof ( Normal Rafter purlin roof with rafter separation at 40 cm).

Can some one please advise me how to start the calculation or is there a better sensible way to do a Flat roof PV system??

 

[Earthstore]

Make sure you have a structural survey carried out on the roof, when you add a ballasted system you will be adding a huge load onto it.
You also have to take location/wind into account.
Screwing through a flat roof is not good practice, as your friends have found out the hard way.
There are pod systems available which are free standing and individually ballasted, or you can use a mounting frame and ballast that with slabs.
Hope this helps.

 

[solarsparkie]

It was a concern for us on a job too , load is easily spread to outside walls contact your local metal fabricator ( galvanising framework will take 2 weeks another week to design and surprisingly inexpensive )

 

[sunshine8]

Thank you for your reply. May be as you suggested having the mounting frames ballasted would be a better option . It will reduce the yield maybe by 10%. Any ideas how to calculate how much load will be needed or if you know somewhere to look at?

 

[yellowvanman]

Look at this document - it has some info on ballast requirements.

http://www.solaradvancedsystems.co.uk/pdf/Console 2007 manual.pdf

 

[moggy1968]

Note though that document is for the ballast bins and the ballasting is based on certain assumed factors (location, height, wind speed etc) you will need to adjust them to take account of local conditions, but it's a useful start point

BRE wind load calculation is the standard, if somewhat complicated, document.

Any framework you have fabricated has to meet the required EN standard. On an inspection you will be pinged for this if you don't have the certification.
I recently used 16panels on a 5degree tilt roof and bolted each of the 30 brackets (made up in aluminium by a local fabricator for a third of the price of k2 brackets) to a 7.5KG concrete block. The brackets were connected with K2 rails. That made for 225kg of ballast, plus about another third of a ton for the panels and abit for the rails. I reckoned I was cutting it fine on the ballast but it was as much as the roof would take, but it's a single storey in a fairly sheltered area with a 20 degree tilt on the brackets and a space at the bottom for the wind to blow under. They have survived the recent storms so I guess I was about right!
There are many factors to take into consideration. you certainly wouldn't get away with my system on a 3 storey block of flats on the coast in Scotland!!

 

[adamcharvet]

Hi Moggy,

Your recent post was very useful. I'm currently trying to design a 3.75kWp system (15 panels) which is mounted on a flat roof. The client wants the panels to be at 0 degree incidence ie flat parallel to the roof surface. I have suggested using a large number of concrete slabs to which are fixed the rails, then panels clamped to rails in normal way. It's a single storey flat roof around 30m above sea level I estimate.

I've had a look at BRE 489 but am not fully confident with where to find the right figures etc so got structural Engineer booked to look at the job. I guess we'll see what he says, but I've been scratching me head on this one for the last few days! It would be nice if there were a more simple rule of thumb for ascertaining how much ballast to use, but I guess there are so many factors, it's impossible to generalise.

 

[screwdriver]

Have a look at the Solion sunmount system, it is relatively expensive at approx £50 per panel but is one of the only systems we have been able to use on very high roofs due to its very low coefficient of lift.
2 week lead time and a very knowledgeable staff also help, we are getting a few in for a 50kw job by the coast as there was no other alternative that didn't have massive ballast requirements.

they have a little excel design spreadsheet and can advise you on ballast amounts required for your job.

Hope that is of help!

on the other hand we have used a rail system fixed down to a flat roof but we got our roofing team to encapsulate a decent bit of timber onto the roof for us to fix through into and that seems to be a long lasting solution for a flat roof too.

 

[yellowvanman]

Hi Moggy,

Your recent post was very useful. I'm currently trying to design a 3.75kWp system (15 panels) which is mounted on a flat roof. The client wants the panels to be at 0 degree incidence ie flat parallel to the roof surface.

You need to avoid flat panels if at all possible. Being flat they won't be able to self clean (rain running off), so dirt will accumulate, water will not drain off, all this will effect efficiency and potentially shorten life of panels, and probably the manufacturers would invalidate the warranty anyway!