Shade the original roof with a second roof : is that a good idea?

[gejohesch]

I may not express myself clearly (I'm not a pro and English is not my mother tongue). I have a house with ca. 130 sq metres of roof. Heat is a real problem in the house. The roofing sheets are old and I want to replace them. Also, I plan to install a few solar panels on top, which I expect will reduce the heat going to the house (since heat will be converted to electricity).

 

So far, this probably is a rather standard operation. However, I think of venturing a bit further: when installing the solar panels, why not using the old roofing sheets to build a "second roof", 1 ft or so above the original "real" roof, and install the solar panels on top of that "second roof"?

To summarise, I would end up with a 3-fold heat reduction :

1 - new roofing sheets with bonded insulation (I see a few discussions on this in the forum).

2 - solar panels taking some heat off and convert to electricity.

3 - "second roof" providing shade to the "real" roof.

 

Why I got to that concept is because the old roofing sheets will not cost me anything, and I expect relatively extra steel and work will be required for the installation, i.e. relatively little extra cost. Obviously, I would have to close off the space between the 2 roofs to prevent birds nesting. I guess wire screens will do at low cost.

 

I tried to find about this "2 roofs" concept on the net but could only come across one discussion, here is the link : Floating a second roof for shade above first roof.... in Alternative methods and solutions (forestryforum.com)

 

See the pic attached. In blue : the "real roof". In green and yellow : supporting bits of steel. In grey : the "second roof". Plus a few solar cells on top. All that conceptual, of course.

 

Is it realistic or far-fetched?

[Kwasaki]

6 minutes ago, gejohesch said:

Is it realistic or far-fetched?

It can be done.

[bamboozled]

I know that heat problem too well. Ugh. I like the idea. Might be an issue with wind storms, however. They can really pack a punch and I'm sure would love having a go at something like you propose.

[4MyEgo]

2 hours ago, gejohesch said:

To summarise, I would end up with a 3-fold heat reduction :

1 - new roofing sheets with bonded insulation (I see a few discussions on this in the forum).

2 - solar panels taking some heat off and convert to electricity.

3 - "second roof" providing shade to the "real" roof.

 

I would look at putting some type of shade cloth structure over the flat roof and put the solar panels on the roof that has the pitch.

 

The shade cloths are cheap enough and you can find them in 3 x 2 metres at HomePro for around 400-500 baht and they have 90% UV reduction, we have them and I swear buy them.

 

Also at the beginning of your pitched roof, and at the rear if you haven't already got those saloon type slated vents, you should put one on either end as it helps with the air flow along with whirly birds that I am recommending below. 

 

Also on the pitched roof I would put in a couple of whirly birds on either side and also put in some insulation batts in that ceiling. 

 

From the sketch you have provided, I don't know how successful it will all be, because the flat roof is definitely a problem and it all boils down to affordability, perhaps another roof but pitched so it allows air to build up in the ceiling as opposed to coming straight through.

 

I have a gable roof, wide eaves with vented eave boards, thermal reflective sisolation and insulation batts, whirly birds and those saloon style wooden vents and the flow of the air in the ceiling is like a wind tunnel, those whirly birds rarely stop spinning, naturally it all costs, but it also depends on how much you want to keep the heat out.  

[canthai55]

Done in the Gulf all the time.

Buses even made with two layer roofs.

Make sure the roof frame can support the weight of the extra panels and supporting structure.

Seal the gap REALLY well as every bug, animal and bird known to man will love to get in there

[HashBrownHarry]

i like your idea, however i would not enclose the additional roof, i'd leave it open for airflow instead.

 

i've seen this done with porta cabin accomadations in hot climates and it'll make a huge difference.

 

pls report back how it works out.

[NCC1701A]

3 hours ago, gejohesch said:

(since heat will be converted to electricity).

no. light is. 

[gargamon]

Depending on the construction style of your property, the best solution may be putting a radiant heat barrier in the attic. I did this in a townhouse in the Philippines and it made a marked difference of heat in the upper floor. The ceiling was no longer hot to the touch after the installation was done. It's a cheap update, and installation was quite easy, 3-4 hours for inexperienced applicators. I, of course, supervised closely.

 

This is now the standard construction technique in the hot southern states in the US. 

[gejohesch]

2 hours ago, NCC1701A said:

no. light is. 

Sure. Thanks for the correction. And obviously, since otherwise the panels would also provide electricity at night .... which they don't.

 

That was a bit of a figure of speech. But light = photons (or electro-magnetic waves) and photons will transfer energy in the form of heat when they hit something, no? So, if the panels intercept photons from the light coming to hit the roof, those photons will not end up heating up the roof, right?

[gejohesch]

3 hours ago, bamboozled said:

I know that heat problem too well. Ugh. I like the idea. Might be an issue with wind storms, however. They can really pack a punch and I'm sure would love having a go at something like you propose.

Good point. It must be a robust construction. I would think of managing space at regular intervals between the sheets of the "floating roof" to allow airflow and reduce the lift caused by strong winds.

[gejohesch]

3 hours ago, 4MyEgo said:

 

I would look at putting some type of shade cloth structure over the flat roof and put the solar panels on the roof that has the pitch.

 

The shade cloths are cheap enough and you can find them in 3 x 2 metres at HomePro for around 400-500 baht and they have 90% UV reduction, we have them and I swear buy them.

 

Also at the beginning of your pitched roof, and at the rear if you haven't already got those saloon type slated vents, you should put one on either end as it helps with the air flow along with whirly birds that I am recommending below. 

 

Also on the pitched roof I would put in a couple of whirly birds on either side and also put in some insulation batts in that ceiling. 

 

From the sketch you have provided, I don't know how successful it will all be, because the flat roof is definitely a problem and it all boils down to affordability, perhaps another roof but pitched so it allows air to build up in the ceiling as opposed to coming straight through.

 

I have a gable roof, wide eaves with vented eave boards, thermal reflective sisolation and insulation batts, whirly birds and those saloon style wooden vents and the flow of the air in the ceiling is like a wind tunnel, those whirly birds rarely stop spinning, naturally it all costs, but it also depends on how much you want to keep the heat out.  

I already have those "saloon type slated vents" at both ends of the pitched roof, but I'm not sure they are that effective in venting the hot air out. Certainly adding whirlybirds will much improve the situation. I had not thought of it, so thanks for the idea.

 

When changing the roofing sheets, I will have access to the top of the gypsum ceiling underneath, so I will certainly think of padding it with some insulation material as well.

 

As for the shade sails you mention, I would rather not use them for the roof, as eventually they would deteriorate or even take flight away ... what with the heavy rains and strong storm winds we have so often around here (I'm in Isan, near Khon Kaen). I will buy a few shade sails however to screen off parts of the large connecting terrace I got built between our 2 houses. It's a very nice place to use .... when avoiding direct sun.

[gejohesch]

3 hours ago, canthai55 said:

Make sure the roof frame can support the weight of the extra panels and supporting structure.

Seal the gap REALLY well as every bug, animal and bird known to man will love to get in there

I don't think the additional weight will be an issue. It's all on steel and concrete.

For sure, the space between the 2 roofs will have to be screened off properly. Not completely sealed off, however, I want air to circulate around.

I think a solid metal mesh should do. 

[gejohesch]

2 hours ago, gargamon said:

Depending on the construction style of your property, the best solution may be putting a radiant heat barrier in the attic. I did this in a townhouse in the Philippines and it made a marked difference of heat in the upper floor. The ceiling was no longer hot to the touch after the installation was done. It's a cheap update, and installation was quite easy, 3-4 hours for inexperienced applicators. I, of course, supervised closely.

 

Indeed, that will be part of the solution. I'm all for attacking the heat problem from different angles! (Heat is a real issue in Isan, me coming from a temperate climate!)

[Dan O]

Creating a air space between the 2 roofs will create an insulation barrier and will help. You would be well advised to add a exhaust fan to vent the heat from that air space for maximum benefit.  You will need to make sure your construction of the original roof will support the 2nd roof and solar panels over the original one 

[bwpage3]

Your plan will work with no issues and work well. Adding screen will keep out the critters and let the air flow.

 

 

[Crossy]

The concept of a flying-roof is a good one.

 

We have our solar panels on the car port roof. There's a gap of about 3" between the panels and the roof, not sealed at all so plenty of airflow.

 

The reduction in temperature of the roof inside the car port is significant although, sadly, I never actually measured it before installing all the panels.

 

To be honest, I wouldn't bother with the second roof itself and just get a structure up for the solar panels although if the roofing material is zero cost then why not.

 

Do note that solar panels are HEAVY (23kg a pop) so our 22 panels weigh half a ton! Make sure your structure is up to it. I would be tempted to take the steel support columns down to ground level unless you are certain of the existing structure.

[Dazinoz]

10 hours ago, 4MyEgo said:

I would look at putting some type of shade cloth structure over the flat roof and put the solar panels on the roof that has the pitch.

Agree about the pitch. Not sure where the OP is located but the ideal siting of solar panels in Chiang Mai is facing due south at an angle of 18.7 degrees with out shading. Panels should be pitched at the same angle of the latitude where they are placed. As i said in CM need to be 18.7 degrees and mine in Brisbane, Australia were at 27 degrees facing north.

[ICELANDMAN]

 

The practical demonstration of how to do the difficult things when you can do the simple things.

 

Laying a single metal roof with insulation has not yet come to your mind?

[bbudd]

I've installed second roofs successfully and they work well at reducing heat

Just look at the multiple roof construction on most Watts.   Built to vent heat

 

Even used that second roof construction in Canada to stop ice dams

Also reduced heat gain in the summer

[millymoopoo]

13 hours ago, gejohesch said:

Heat is a real problem in the house.

We recently built a new home.

Heat deflection, passive/active insulation and ventilation are not priorities for Thai builders, cost is the priority.!

If you are building new or renovating you will need to take charge of the built and insist on including what measures you want.

A second roof is a good idea but may not be needed.!

Standard thai roofing (aluminium with 10mm foam insulation underneath) is completely useless.

Here is what we used, which may be useful for you.

1: Corrugated iron roofing with chrome like finish (like zincalume), the shinier the surface the greater heat reflectability. The other advantage to this is it's strong enough to walk on (for solar panel installers) try to walk on aluminium roofing and see the result.

2: +40cm gap between the roof and ceiling (40cm at lowest point).

3: 150mm fibreglass insulation throughout. the roof cavity, I think this is Thai rated R115.
It is one of the thickest and highest rated available, if this is not available then use 2 layers of the lesser insulation.

4: Install large air vents into the walls at or just above ceiling level.

5: Install one industrial sized or several smaller 'whirly bird' (rotating roof vent) at the roof peak.
Note 1: item 4 and 5 are required together to create a flow of air into and out of the roof cavity, thus removing built up heat.

6: Shade roofs (may be angled) or verandas on sun exposed sides, (Thai built walls do not have insulation). If they are cavity walls add insulation to them.

7: Build on a slab, Thai builders built post and beam construction, with floors raised up on little foundations, this creates an air gap between the ground and the floor, with the average ground temperature in Thailand being 23c any cooling advantage offered by contact with the ground is lost, thus building on a slab is recommended.!

Note 2: Solar panels offer little insulation as they absorb light, not heat.. They are dark with a glazed surface, most of the heat passes through, they only offer shade.

Note 3: When you have decided on your plan, stay in the shadows, get your other half (assuming they are a Thai national) to do the price negotiations, then when everything is agreed on, step out into the light and ensure everything is done to agreed specifications. Given the chance a Thai builder will take short cuts.!

Good luck with it.

 

 

[inThailand]

Adding a second roof over the original roof with a good attitic space will reduce heating up the building. But that alone may not make a huge difference, you need to allow the hot air to escape the inside of house and the original attic. Together this should make a huge difference. Suggestion would be to insure your original roof ventilation is maximized before spending money on a second roof. 

Edited June 29, 2021 by inThailand

[MrJ2U]

12 hours ago, 4MyEgo said:

 

I would look at putting some type of shade cloth structure over the flat roof and put the solar panels on the roof that has the pitch.

 

The shade cloths are cheap enough and you can find them in 3 x 2 metres at HomePro for around 400-500 baht and they have 90% UV reduction, we have them and I swear buy them.

 

Also at the beginning of your pitched roof, and at the rear if you haven't already got those saloon type slated vents, you should put one on either end as it helps with the air flow along with whirly birds that I am recommending below. 

 

Also on the pitched roof I would put in a couple of whirly birds on either side and also put in some insulation batts in that ceiling. 

 

From the sketch you have provided, I don't know how successful it will all be, because the flat roof is definitely a problem and it all boils down to affordability, perhaps another roof but pitched so it allows air to build up in the ceiling as opposed to coming straight through.

 

I have a gable roof, wide eaves with vented eave boards, thermal reflective sisolation and insulation batts, whirly birds and those saloon style wooden vents and the flow of the air in the ceiling is like a wind tunnel, those whirly birds rarely stop spinning, naturally it all costs, but it also depends on how much you want to keep the heat out.  

The wirly birds is what I want on our roof also.  I think for both of us it would be great to suck the hot air plume out of the rafters.

 

[VocalNeal]

Land Rover used the same principle for years called a Safari Roof. Think Africa 

 

[Sydebolle]

The answer is a clear and clean yes.

Observe a strong structure as you might have (depending on where you live and in which direction the house is built) strong winds blowing between the old and new roofing. 
Use galvanized metal bars, get them properly welded together as well as properly connected to the building structure and do not forget to paint anti-rust paint wherever they welded. 
For the solar panels ensure protected wiring with easy access as well as the possibility to replace single cell panels.
You will find websites showing the exact path of the sun over your property which will indicate the degree of tilt you need to consider for your panels. There are rigid and moving panels; stay with the rigid ones and optimize the degree for between 10am and 3pm. 
You will find websites showing the expected wind directions over your property (this is more general but it helps. Like this you will know, if you need to keep what kind of distance between solar panels to allow wind coming from an "unfortunate" angle to pass through without stressing the panels, its fittings and the roofs underneath too much. 

Good luck! 

 

[rosst]

Certainly a good idea, commonly referred to as a "tropical roof". 

A few things to be aware of, the typhoons tend to be able to attack them, as the sides must all be open for the air to circulate freely. I suggest stranded stainless steel wire across the roof to hold the sheets as per cyclone construction technique. 

It is a great idea and works very well. 

[jvs]

Have you considered not making a double roof but investing in more solar panels instead?

The extra power can be used to run aircon units nearly all day.

 A lot of sun? A lot of cooling.After the sun goes down it will not heat up a lot anyway.

[sandyf]

9 hours ago, Dan O said:

Creating a air space between the 2 roofs will create an insulation barrier and will help. You would be well advised to add a exhaust fan to vent the heat from that air space for maximum benefit. 

There would be no need. The outer roof should be sloping with an opening at bottom and the apex. The temperature of the air in between will rise and by default will make the air rise creating a draught between the 2. 

The air moving across the primary roof will prevent the temperature of the tiles from rising above ambient, which is what happens when roof tiles are exposed to direct sunlight. It is the rise in roof tile temperature that would normally be conducted through to the internal roof space.

[CaptainCarrot]

When we built an extension some years ago the architect selected flat concrete slab roof and I thought no more about it. But my guests complained that the bedroom was excessively hot, so when I had the roofers in to replace the existing tiled roof I got them to add a second, zincalume/galvalume steel, roof over the flat roof. The improvement is substantial.
I would not recommend welded steel for the frame. The standard for galvalume is to use galvalume C section which can be screwed together with self tapping screws and the sheet screwed to it. 

[sandyf]

11 minutes ago, jvs said:

Have you considered not making a double roof but investing in more solar panels instead?

The extra power can be used to run aircon units nearly all day.

 A lot of sun? A lot of cooling.After the sun goes down it will not heat up a lot anyway.

In theory a logical assumption but in reality I suspect the gap between the solar panels and the roof wouldn't be nearly as effective as a second layer about 200 mm from the primary roof. I would also see the solar panel fixings as an impediment to a smooth airflow between them and the roof.

I see quite a few buildings now fitting a decorative layer away from the main structure to provide that "tunnel effect"

[whaleboneman]

Add a second roof and hope the main roof never needs repair?