MCS & System Design

[VMan]

Hey Guys,

so my calcs area as follows:

PV array is to be mounted <200mm above the roof surface in the central available roof area
Module Width 994 mm
Module length 1652 mm
No. Of Modules 9
PV Array Area 14.8 m2 (Aref)
The site is located in ---( Zone 3 Figure 1)
Building Ridge Height 10 m
Site is in hilly area (Use Table 2)
Site altitude is approximatly 160 m above sea level

Dynamic wind pressure 2954 Pa (qs) (Use Tabel 2)

Wind Uplift -1.3 (Cp,net) (Mounted >100mm <300mm from edge of roof)
Wind Downward pressure 1 (Cp,net)

Wind Force F= qs Cp net Ca Aref
Upward Force = 2954 x -1.3 x 1 x 14.8 = -56754 N = -56.8KN
Downward Force 2954 x 1 x 1 x 14.8 = 43657 N = 43.7 KN

So now i need kN/m2??

-56.8/14.8 = -3.84 kN/m2
43.7/14.8 = 2.95 kN/m2

Say i use 4 brackets per m2, so i take 2.95/4 = 0.73kN (each bracket must withstand 730N right??)

how do i find out if the brackets can take 730N???Click-fit just gave me there calculator, but the spreadsheet only allows upto 1038Nm2 where i have 2950Nm2!!!

Argh!!! any ideas where to go from here??

Do i need to calculate how many brackets are needed for MCS or would calculating wind & snow lods be sufficient then passing these onto the supplier to supply necessary brackets ect?

Thanks

 

[Bryan McGrory]

Hi Guys,

I am the director of BMG Roof Surveys and we specialise in providing Desk Top Roof Loading Reports.

These reports have been accepted by both Building Control and MCS.

You can find out more information at www.bmgroofsurveys-solar.com

A Residential Property Report is priced at £70 and Flat Roof (including Ballast Calc) and commercial properties are priced at £90.

We carry PI Insurance and by having BMG carry out the calcs by qualified engineer and it gives all parties the piece of mind that they are carried out correctly.

If you have any questions please do not hesitate to contact me.

Many Thanks,

Bryan McGrory
Company Director
[email protected]
07738 470 160

 

[moggy1968]

I've never been asked for wind load calcs on my assessments. I've been asked about it so had to show I have considered it, but never been asked for calcs. I'm with ELECSA

 

[Bryan McGrory]

Hi Moggy,

In order to comply with the MCS regs you have to carry them out and MCS always look for sight of the Roof Loading Report for every install. To be honest I know very little about ELECSA however on reviewing thier documentation (Part A - Structure) it most definatley refers to establishing the static and wind loadings.

I find it mad that most Installers are not carrying out these checks. I have loads of roofs fail the check and the panels have already been installed. I would imagine its only a matter of time before we all hear about a major fail. Heres hoping im wrong!!!

We are here if you require ourservices and I am going to contact ELECSA to find out if they are aware of the MCS reg for this.

Many Thanks,

Bryan

 

[moggy1968]

I know that round this way 1m or less spacing on brackets is ok for uplift. We provide additional support for 35mm joists or joists with greater than 80cm spacing.
They seemed happy with that.
I'm sure they are aware of the regs and they were satisfied that we had considered it but didn't ask for detailed calculations.
when you say you have loads of roofs fail, can you quantify that. What sort of roofs and what is the cause of failure?

 

[Bryan McGrory]

There have been varying reasons for roofs failing. By failing I dont mean the roof has collapsed or anything, only failing when checking the loadings.

The most common factor is properties which are exposed and on high gruond. I would estimate that around 10-15% of the reports we have had fail.
Another common factor is snow loading. I woudl estimate that a good 15-20% of roofs fail due to this.
Costal areas and countryside areas are the most common areas where roofs fail.

We have now carired out over 1000 reports since our company forming in November last year so that gives you an idea of number of roof fails.

 

[Bryan McGrory]

I have just stumbled accross the ELECSA stance on wind loading. Elecsa - Adding Loadings to a Roof

It basically says "Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading". I would say they should be carried out on every install.

 

[Helio]

I have just stumbled accross the ELECSA stance on wind loading. Elecsa - Adding Loadings to a Roof

It basically says "Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading". I would say they should be carried out on every install.

In full it says (my bold font):

'Approved Document A stipulates requirements for roof loadings with respect to weight of loads imposed and wind loading.

Both of these issues will need to be considered by contractors installing solar photovoltaic panels. In many cases this may involve structural checks and/or calculations being undertaken. Decisions involving these issues will form part of Microgeneration Certification Scheme (MCS) assessment so must be documented. '
​

Now, while I agree that it is no small matter, ELECSA (who we are currently using) do not say that you must undertake calculations; only that you have to justify your decision. I have asked ELECSA to clarify this in relation to MIS 3002, issue 2.1, which says in section 4.4.4 (again my bold text):

'The contractor shall ensure that the roof structure is capable of withstanding the
loads (static and wind loads) that will be imposed by the PV modules and their
mounting arrangements. If there is any doubt, a structural engineer must be
consulted. Guidance on the mechanical installation and wind loads are given in
BRE Digests 495 and 489 or BS 6399.'

So, if I read things correctly, currently, I could say that I have considered the roof's integrity and decided that my mounting system and modules are acceptable and that would be enough to satisfy the MCS inspector (at least for ELECSA) as long as it's documented. This seems vague and should be clarified. Surely, it should be that we either do need to have calcs done or not and the smart money, as Bryan would no doubt agree, would be on having them done.

We too were told to use common sense on our 2399 course and inform domestic customers if we thought a survey should be done, taking care not to offer any advice on structural matters which we are unqualified to do. The additional £250.00 for a structural survey cuts into margins but if we all have to do it, there isn't an unfair price advantage for those who do not have them done every time.

Scottish Borders Council Building Control, for example, require a survey with all PV installations.

It does feel though, that more and more burdens are being placed on installers all the time. EPCs, FiT chaos, Green Deal etc........

 

[jason121]

Were do you stand on PV solar kits, do they allow for wind load loading ?

 

[Bryan McGrory]

At least they have provided a bit of clarification on the matter.

The only issue I can see is can everyone work out wind loadings (including uplift) on every install. If not then how can you be certain your installation is safe?

Our reports confirm the static loads, the imposed wind loads and the uplift wind loads which from reading the MIS 3002 4.4.4 is what every intallation requires.

Our reports are priced at £70 for a Residential Pitched Roof Property and £90 for Both Commercial and Flat Roof (including ballast calc) properties. If £250 is being quoted walk away. Thats way too much for what you require.

 

[jason121]

I have been told you cannot calculated wind loading on a pitch roof all calcs are based on 90 degrees not 25 to 45
Any answers

 

[Bryan McGrory]

You can caluclate the wind loading on any angle, you just need to know how to do it. Whoever has provided you with this information has also been misinformed.

 

[jason121]

Not much to choose from post 70's , either fix at 600 centres or 1200mm.
We use one size rail on every job and mainly fix at 1200mm centres will noggins

 

[Bryan McGrory]

You have to refer to BS6399 as this will confirm the requirements for wind loadings.