Snow load calculation

[masona]

When the structural engineer do the low pitch roof stress load calculation for the snow load, what are snow height they take into account or what is the building regulations maximum snow height for their calculation?

I was told 5" of snow but I'm sure it's a lot more that.

Not sure if I'm clear on this?

 

[tony1851]

In the UK, snow load is usually taken as either 0.75 kN/sq m., or 1.0 kN/sq m., depending on location.

Don't know what this translates to in terms of depth of snow, but what is more critical is the way the snow behaves - if an initial coat has slightly melted and then turns to ice, and then is overlain by a second coat, then 'depth of snow' is irrelevant.

In some circumstances, drifting of snow has to be allowed for, such as where a lower roof meets a wall. But this is usually only critical for larger industrial and commercial buildings; for small domestic works, the figures above would normally be acceptable.

 

[masona]

Thanks Tony, I'm trying to get as much information as possible for our club and I have noticed since the solar panels fitted, the roof is bowing and the high point I'm guessing are the steel trusses. I'm not sure if it's the heavy slabs from the solar panels are causing this

The structural engineer who did the calculations is visiting me on Tuesday

solar panels

• masona
• 2 Aug 2014
• 2

 

[theoldun]

Would appear that roof was never designed for solar panels hence the deflection.
If you want the weight of snow, google (weight of snow) and you will have all the information you require.
Regards oldun,

 

[masona]

Would appear that roof was never designed for solar panels hence the deflection.

I only spotted it when investigating a leaking roof!

What happened previously about 18 yrs ago, the roofing company replaced the white cement corrugated roof sheets with plywood, looks like OBS or shuttering plywood and on top of that is EPDM Sika roofing membrane and even the plywood underneath is not fitted onto flange or angle iron! There is not even any support or noggins on the plywood edge, When I walk on it, it's like a trampoline

Then came the solar panels in 2011, they have put down 648 heavy slabs weighing 54 lbs each onto the EPDM and I have no idea how much the solar panels weigh on top.

According to the Silka EPDM BBA specification, you can only use light foot traffic and concentrated load on top of their membrane

Does the structural engineer take into the account of the internal roof void structure? Or is it they are only interest that their calculations can take the weight of the slabs and solar panels?

 

[bigmikerics]

Tony's points are spot on regarding the snow.
The problem with solar panels is that they are usually installed under a competent person scheme whereby the electrician self certifies the whole installation including the electrics, the structure etc under a CPS. These schemes are ok in theory for small works where the electrician self certifies, for example, that he has not notched joists excessively, as this comes straight out of a book, but generally the pv installer has no idea of how to assess the loadings and wind uplift on the roof. There is a requirement under the feed in tariff FIT scheme that the installer assesses the structure of the roof, but, in our experience this assessment is virtually never carried out by a structural engineer, if atall. If FIT is not being claimed then the CPS does not even mention the roof structure specifically.
Your engineer will assess all issues and hopefully come up with a reason for your problems.
There are several CPSs for electrical works and my comments above relate to all of the ones we have dealt with, but there may be some who are more competent. Also, there are more than likely some very good installers who do carry out a proper structural assessment.

 

[masona]

So I'm assuming the roof problem is the responsibility of the solar panels installers rather than the structural engineer?

The structural engineer did send his calculations via email and I have noticed on each page the design has not been checked by another engineer and approved by a further engineer. From what I understand the designer should normally have their calculations checked twice by two other engineers?

What is CPS?

 

[Jackrae]

I would have thought your roof problem was due to the cr4p rework undertaken by those who replaced the roof 18 years ago and any subsequent works are merely adding to the problem. If your structural engineer had advised that "the whole roof needs replacing mate before you instal PV panels" what would have been your reaction.

 

[masona]

I would have thought your roof problem was due to the cr4p rework undertaken by those who replaced the roof 18 years ago and any subsequent works are merely adding to the problem. If your structural engineer had advised that "the whole roof needs replacing mate before you instal PV panels" what would have been your reaction.

Yes I'm blaming all three, the roofer company for no supports underneath, the EPDM company for failing to check the roof is solid and the solar panels for not checking the internal roof void.



I can see they will all blame each other and we're in the middle of it all.

Not sure about the structural engineer because they are saying the beams and metal trusses can take the weight!

Oh what a mess

 

[tony1851]

[

Not sure about the structural engineer because they are saying the beams and metal trusses can take the weight!

The beams and trusses may well be able to 'take the weight' according to the engineer, but what exactly does that mean?

By 'take the weight', they could mean that the structure won't actually collapse (which is the least you would expect). But it can still 'fail' by excessive deflection and even experienced engineers sometimes get caught out by this.

Excessive deflection under dead load and live load such as snow, can affect the structure, sometimes to the state where it becomes unserviceable. Joints in the covering can open up (allowing water in where it shouldn't be); ceiling finishes can crack; occupants can feel un-nerved when they see parts of the structure bending excessively etc.

(As for whose fault this is, I wouldn't know!)

 

[masona]

Will let you know the outcome after the meeting tomorrow

 

[masona]

Untitled

• masona
• 6 Aug 2014
• 2

Untitled

• masona
• 6 Aug 2014
• 1

Untitled

• masona
• 6 Aug 2014
• 1

The meeting and investigation with the structural engineer, solar panels installers and their rep was all blah blah blah. As far as the structural engineer is concerned it's safe, he doesn't considered the extra weight of the slabs and panels on the roof is heavy! The total weight for 648 slabs is 16,200kg and the panels 4,400kg therefore total weight added to the roof is 20,600kg or 45,415 lbs. They put a string line at the top approximately 1m down from the ridge and quoted it's only bowing 5-10mm. Surely they should've done the string line test indown the middle of the roof because nearer to the ridge will be too rigid for any bend?

If you look at the photo's you can see the plywood bending and dropping with no supports and hardly and any screw fixings to the purlins. Surely there should be another extra beam or purlin in between the existing purlins plus noggins on the edge of the plywood?

The metal roofing system we have used to have cement corrugated roof sheeting and from what I understand is, these type of purlins are made of lighter steel than girders since they're not used to hold up heavy loads. Am'I right on this? Also these flanges or purlins previously has roofing hook bolts onto them, converted from corrugated roof sheets to flat plywood roof and now they have screws into them from the top, would that weaken the purlins

We have the Sika-Trocals Roof Waterproofing System on top of the plywood and according to the BBA (British Board of Agrement) you can only put light concentrated loads onto the membrane, again they considered it's not heavy. They are reporting their investigation by this Friday

Resistance to foot traffic
Tests indicate that the membranes can withstand, without damage, the limited foot traffic and light concentrated
loads associated with the installation and maintenance operations. Where traffic in excess of this is envisaged
(eg maintenance of roof-mounted plant or regular access to plant rooms), walkways must be provided (eg using the
WBP materials), as recommended by the Certificate holder. Reasonable care should be taken, however, to avoid
puncture by sharp objects or concentrated loads (see section 14, Table 2 Physical properties — general).

 

[jeds]

The problem really stems from the roof alterations. Carp detailing carried out poorly. The length of those boards without noggins at joints was bound to eventually cause an issue. The extra load sounds a lot (200kN) but it isn't that much really. It wasn't wise for the solar people to add that extra load given the state of the roof. Had it been the original roof it would probably have been fine.

 

[masona]

I have checked with the Law Society website regarding building regulations and the Building Regulations approval is required if the solar panels will add more than a third to the weight of the existing roof covering. So are they saying if the solar panels are more than 33% of the total weight of the plywood and EPDM membrane?

http://www.lawsociety.org.uk/advice/articles/the-pitfalls-of-solar-panels/

 

[tony1851]

I have checked with the Law Society website regarding building regulations and the Building Regulations approval is required if the solar panels will add more than a third to the weight of the existing roof covering. So are they saying if the solar panels are more than 33% of the total weight of the plywood and EPDM membrane?

http://www.lawsociety.org.uk/advice/articles/the-pitfalls-of-solar-panels/[/QUOTE]

Don't know where they got that 33% figure from but Approved Document A (Structure) regards anything over a 15% increase in the weight of the roof covering as needing to be checked.

When the engineers declared the 5-10mm deflection (of the trusses or the purlins?- we don't know) that would not have allowed for snow load, so ultimate defection will be greater.

In all probability, the roof won't collapse (hopefully) but it's not a good state of affairs.