Lowest profile flat roof ballasted system

[Mattg4321]

I've got a spare MPPT on my Sunsynk inverter.

 

I've got a garage (flat) roof on the side of my house I could fit 5 (or possibly 6, I've not bothered measuring yet!)  panels on, landscape to reduce height. It's south facing, with only moderate shading early/late in the year. Half the year it will be in full sun all day.

 

I'm sure it would payback within a couple of years or so as I can fit myself.

 

I don't want any penetrations through my flat roof and I don't want it to look awful. Are the ballasted systems any good? Roof structure is early 70's and in good condition. 4x2 timber. 2.4m span. Not sure if I will require structural engineer?

 

What's the lowest profile flat roof system? Do I require planning?

 

All input appreciated!

[TedM]

Hmmm. A quick look at good old AI suggests:

According to standard UK span tables (BS 5268), a 2" x 4" (specifically 47 x 100 mm) joist typically has a maximum allowable span of only 1.7 to 2.1 meters depending on the wood grade and spacing. At a 2.4m span, the joists are "undersized" and prone to significant sagging or structural failure under standard loads.

 

So maybe no planning required but some acro props probably will be as whatever you put up there may well try to find its way downstairs fairly quickly 😬

(The 70's has a lot to answer for!)

 

"I'm sure it would payback within a couple of years or so as I can fit myself."

 

Possibly a bit longer if you take roof repairs into account 😉

 

 

Edited yesterday at 17:55 by TedM

[Mattg4321]

46 minutes ago, TedM said:

Hmmm. A quick look at good old AI suggests:

According to standard UK span tables (BS 5268), a 2" x 4" (specifically 47 x 100 mm) joist typically has a maximum allowable span of only 1.7 to 2.1 meters depending on the wood grade and spacing. At a 2.4m span, the joists are "undersized" and prone to significant sagging or structural failure under standard loads.

 

So maybe no planning required but some acro props probably will be as whatever you put up there may well try to find its way downstairs fairly quickly 😬

(The 70's has a lot to answer for!)

 

"I'm sure it would payback within a couple of years or so as I can fit myself."

 

Possibly a bit longer if you take roof repairs into account 😉

 

 

 

I think you're right. Will be giving this a miss then.

[BotusBuild]

FYI - It is not unheard of for sloping roof trusses needing to be strengthened when PV panels are fitted, so based on @TedM's findings I would be looking to add extra wood to the existing timbers.

A friendly SE's input would not go amiss in  this situation, especially as you are talking about ballast systems

[Mattg4321]

2 hours ago, BotusBuild said:

FYI - It is not unheard of for sloping roof trusses needing to be strengthened when PV panels are fitted, so based on @TedM's findings I would be looking to add extra wood to the existing timbers.

A friendly SE's input would not go amiss in  this situation, especially as you are talking about ballast systems

The problem with that is that it starts to push out the payback time. 
 

I suppose it depends what strengthening work might be required. Anyone have any idea what might be typical?

[jack]

It might be just a case of spreading the load across as many timbers as possible, but you'd need an SE to confirm the mass needed, and whether your roof has the necessary load capacity.

We have a low-angle ballasted system on a flat roof that uses concrete council-type slabs as ballast, but with a frame system specifically designed for PV. I can't easily check, but it's something like a couple of slabs per panel, I think.

 

I'd have preferred a through-membrane system bolted down to the roof deck, but unfortunately didn't plan ahead and so it wasn't a reasonable option at the time. Hasn't moved in 10+ years.  

I appreciate it's desirable to avoid roof penetrations, but if done properly it needn't be a significant leak risk. You'd still need SE calculations for load, lift, and fastener pullout, but I still think that's preferable to using ballast. Even it's within your roof's load capacity, you'll be putting considerably more pressure on the membrane and underlying structure.

[TedM]

36 minutes ago, jack said:

I appreciate it's desirable to avoid roof penetrations, but if done properly it needn't be a significant leak risk.

My late dad was an architect and always said to never put anything through a flat roof unless there was absolutely no other choice.  5 panels would probably mean 20 holes, meaning that it would potentially be 20 x more likely to leak at some time in the future 😬

[TedM]

12 hours ago, Mattg4321 said:

I suppose it depends what strengthening work might be required. Anyone have any idea what might be typical?

You would basically need to strengthen each existing joist - two options might be:

• bolt a steel strap to one side of each joist (we've done that before to a pair of roof purlins with  the blessing of our SE)
• add new joists of the correct diameter - but they will def be deeper which would then affect the ceiling height of the room below.

There may be other options but I would imaging they would all involve considerable intrusion into the room below ceiling and it's associated repair costs.

[JohnMo]

Ballast buckets, do a good job of spreading loads 

[saveasteading]

17 hours ago, BotusBuild said:

sloping roof trusses needing to be strengthened 

garages are generally designed without the factors of safety required for a house. So the timbers will be on the light side already.

I.e. it's unlikely anyone would die if it collapsed in a storm, and cars don't matter.

The snow load will still apply as it can sit on the solar panels, so this is extra load even beford ballast.

So the joists will need help. The easiest solution may be to sister the existing ones.

But uplift in wind will be the critical force As the oaneks act as a wing and kite.  You don't want holes in the roof, quite rightly, so you may need a gantry to spread the load, perhaps strapped to the walls.

I'm reinventing the wheel here... there must be a standard way, although don't count on contractors or vendors to know or care.

Is it a building regs thing? I'd have to check but I think it is  as it's integral with the house. I don't think many people ask, or do it right.

[BotusBuild]

3 hours ago, saveasteading said:

cars don't matter

All the buildhubbers who are classic car owners are now gnashing their teeth (incl me) 🙂 

[BotusBuild]

6 hours ago, TedM said:

intrusion into the room below ceiling

It's a garage - I don't think this is of much concern in this situation 🙂 

[saveasteading]

After you've had a look at gantry options, Id be interested to know what you come up with, and we can look at support and fixings.

ballast is still possible, but with reinforcement of the joists, which should be quite easy: fixing the same size of timber to the sides wont intrude either.

Edited 3 hours ago by saveasteading

[saveasteading]

1 hour ago, BotusBuild said:

cars don't matter

It shocks some people that agricultural buildings are basically allowed to fail, whether for hay, tractors or living animals.

I asked a farmer once if he knew of buildings collapsing, as I hadn't ever heard of it being a problem. He told me it was quite common especially in heavy snow, and was reported in farming journals.

 

1 hour ago, BotusBuild said:

classic car owners

might want to have their garage designed with a FOS. It's about 40% in approx terms.

[TedM]

1 hour ago, BotusBuild said:

It's a garage - I don't think this is of much concern in this situation

Oh! ......Didn't know.

 

[BotusBuild]

25 minutes ago, saveasteading said:

FOS

?

[JohnMo]

33 minutes ago, BotusBuild said:

?

Factor of safety 

[Mattg4321]

No worries about intruding on a room below. The flat roof is above a utility room 80's extension with 6x2 joists (same 2.4m span) and also a garage as previously mentioned with the 4x2. There's nothing below the joists in the garage so easy access.

 

No idea of the cost of a structural engineer for something simple like this, anyone like to guess? If it's as potentially simple as sistering the joists, I may look into it as without strengthening the joists and the costs involved in that, payback time is going to be around 2 years according to AI, so pretty attractive.

 

Current joists go into pockets in the brickwork of the house and then overhang the wall plate on the other side by a few hundred mm to create an overhang outside. Presumably any sistered joints would just need to sit on the wall plate (not all the way to the fascia outside), but if the joists are deeper than the current ones, the bottom would need notching out at the wall plate? Assuming these would generally then be bolted to the existing joists.

 

I'm also not sure what the approved practice would be at the junction with the house. I assume create a pocket in the brickwork for the joist to sit in.