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 22 hours ago 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 🙂