Why don't they make roofs out of plastic?
December 18, 2009 2:54 AM Subscribe
Why aren't roofs made out of plastic?
It's waterproof, keeps the heat in, can be ruggedised and protected against UV, and it's easier to transport than slate or clay. So why don't they make modern roofs out of plastic?
It's waterproof, keeps the heat in, can be ruggedised and protected against UV, and it's easier to transport than slate or clay. So why don't they make modern roofs out of plastic?
"Keeps the head in" is a con in an awful lot of places, especially where cheap roofing is going to be appreciated. I can only imagine how revolting a plastic roof would be in an Australian summer, let alone an African or South Asian one.
Oh, and it seems like a 'cheap' option.
posted by Jilder at 3:40 AM on December 18, 2009
Oh, and it seems like a 'cheap' option.
posted by Jilder at 3:40 AM on December 18, 2009
In many places clay tile is used for roofing. In others, slate. These are relatively cheap to produce, have good solar reflectivity, and don't turn into a toxic dripping ball of flame in a houise fire.
posted by le morte de bea arthur at 3:52 AM on December 18, 2009 [5 favorites]
posted by le morte de bea arthur at 3:52 AM on December 18, 2009 [5 favorites]
Best answer: UV stabilization of plastic is only relative not 100%, a stabilized plastic still degrades under full sun. Look at how bad plastic playground equipment looks after a decade or so. A fade/colour change like that on homes isn't considered acceptable. There are polycarbonate panels available that are often compatible with fibreglass or metal sheet systems. It's pretty expensive compared to standard roofing and is only really used in special applications.
Even asphalt shingles or built up tar and gravel aren't really UV/Heat stable, it's the sand/gravel/rock over the petroleum that prevents the solar degradation of the waterproof sub layer. This is why shingles have to be replaced regularly. A good asphalt shingle will have a max life of 30 or so years; most are much less. Standing seam metal runs 50+ years (and can be painted) and slate is good for a couple centuries. Besides the UV degradation your water shedding layer has to handle the expansion and contraction from changes in temperature. Around here for example it wouldn't be unusual for the top layer of a roof to see temperatures as low as -40 and highs in the 60-70s. Vinyl siding for example expands about 1% over a 100F temperature change. My roof isn't atypical at ~50 feet wide, a solid sheet of plastic with the same thermal expansion coefficient as vinyl siding would be 6-10 inches wider in the peak of summer verses the low of winter. In the winter especially you'd see a lot of expansion between 6AM and 1PM and back again every sunny day.
Also generally speaking you don't want your waterproof layer to keep the heat in. The roof deck in most construction is ventilated to lower deck temperatures in order to extend the life of the roofing material.
Finally few people, in the US/Canada anyways, will pay for real durability in building products. If it cost product A lasts 25 years and product B lasts 50 few consumers will pay even modest amounts extra for A.
Having said all that there are a couple synthetic rubber membrane roof technologies. Self adhesive or torch down EPDM is the most common. There are also assorted roll on roofing coatings. These are all rubber products so have a lot of stretch.
posted by Mitheral at 4:00 AM on December 18, 2009 [8 favorites]
Even asphalt shingles or built up tar and gravel aren't really UV/Heat stable, it's the sand/gravel/rock over the petroleum that prevents the solar degradation of the waterproof sub layer. This is why shingles have to be replaced regularly. A good asphalt shingle will have a max life of 30 or so years; most are much less. Standing seam metal runs 50+ years (and can be painted) and slate is good for a couple centuries. Besides the UV degradation your water shedding layer has to handle the expansion and contraction from changes in temperature. Around here for example it wouldn't be unusual for the top layer of a roof to see temperatures as low as -40 and highs in the 60-70s. Vinyl siding for example expands about 1% over a 100F temperature change. My roof isn't atypical at ~50 feet wide, a solid sheet of plastic with the same thermal expansion coefficient as vinyl siding would be 6-10 inches wider in the peak of summer verses the low of winter. In the winter especially you'd see a lot of expansion between 6AM and 1PM and back again every sunny day.
Also generally speaking you don't want your waterproof layer to keep the heat in. The roof deck in most construction is ventilated to lower deck temperatures in order to extend the life of the roofing material.
Finally few people, in the US/Canada anyways, will pay for real durability in building products. If it cost product A lasts 25 years and product B lasts 50 few consumers will pay even modest amounts extra for A.
Having said all that there are a couple synthetic rubber membrane roof technologies. Self adhesive or torch down EPDM is the most common. There are also assorted roll on roofing coatings. These are all rubber products so have a lot of stretch.
posted by Mitheral at 4:00 AM on December 18, 2009 [8 favorites]
Synthetic roofing material is common for flat roofs. I just had a sheet membrane roof put on my garage.
posted by octothorpe at 4:29 AM on December 18, 2009
posted by octothorpe at 4:29 AM on December 18, 2009
Probably because other things are cheaper.
Shingles are generally made out of asphalt/bitumen. This is a petrochemical product, it's true, but it's a product of the cracking process that can't easily be used for other purposes besides paving streets. It's highly viscous, releases a ton of smoke and not much heat when burned, and can't easily be used to make other petrochemical products. It's benefits are that it's pretty durable, largely impervious to water (the ancients used it to seal wooden ship hulls), and plentiful. It is what it is. As such, it tends to be pretty cheap, whereas plastics, which are made out of different components of crude petroleum, tend to cost more because of their greater usefulness. You can do a ton of stuff with plastics that you can't do with asphalt, but not the other way around.
So basically, you can use a perfectly adequate cheap product or a more expensive product which doesn't necessarily have any benefits. This seems like an easy decision.
Materials other than petrochemical products, including metal but especially slate, are also used. While these can perform even better than asphalt shingles, they're also much heavier and orders of magnitude more expensive. A slate roof can add 100% to the cost of a building due to the need for much stronger supports and the cost of the stone itself. Metal roofs are frequently used on commercial and industrial buildings. While an expensive asphalt shingle roof can be expected to last 30 years without much trouble, metal roofs can last 50 and slate can easily last over a century. You get what you pay for.
posted by valkyryn at 5:52 AM on December 18, 2009
Shingles are generally made out of asphalt/bitumen. This is a petrochemical product, it's true, but it's a product of the cracking process that can't easily be used for other purposes besides paving streets. It's highly viscous, releases a ton of smoke and not much heat when burned, and can't easily be used to make other petrochemical products. It's benefits are that it's pretty durable, largely impervious to water (the ancients used it to seal wooden ship hulls), and plentiful. It is what it is. As such, it tends to be pretty cheap, whereas plastics, which are made out of different components of crude petroleum, tend to cost more because of their greater usefulness. You can do a ton of stuff with plastics that you can't do with asphalt, but not the other way around.
So basically, you can use a perfectly adequate cheap product or a more expensive product which doesn't necessarily have any benefits. This seems like an easy decision.
Materials other than petrochemical products, including metal but especially slate, are also used. While these can perform even better than asphalt shingles, they're also much heavier and orders of magnitude more expensive. A slate roof can add 100% to the cost of a building due to the need for much stronger supports and the cost of the stone itself. Metal roofs are frequently used on commercial and industrial buildings. While an expensive asphalt shingle roof can be expected to last 30 years without much trouble, metal roofs can last 50 and slate can easily last over a century. You get what you pay for.
posted by valkyryn at 5:52 AM on December 18, 2009
Here in Tucson one sees a ton of white plastic roofs. We'll have weeks on end where it doesn't dip below 100F. White roofs make quite a difference in the summer. This link tells a little about a few different types of white plasticky roofing material.
posted by The Potate at 6:42 AM on December 18, 2009
posted by The Potate at 6:42 AM on December 18, 2009
odd to see what folks call "a plastic roof" maybe a set of general areas/zones/regions within a roof might help us remain on the "same page?"
I would think of the roof structure - basically the stuff above/ resting on the beams, walls, posts, structure of a building (plywood, lathe of some sort - forget what they call the little boards that hang slate from, rafters or purlins, etc)... would never be plastic, but then again there is "plastic lumber"
The substrate that lay upon the structure... be it the tar paper, tyvek, plastic sheeting... depends upon how "built up" or engineered the roof type is (esp when talking about flat roofs - think "green roofs," and pebble covered roofs)...
Then there's the somewhat final layer of membrane - again depending upon type (might be talking about grass here if u r talking about a green roof)... here *maybe* plastic could appear??? but UV stable stuff like rock (slate), sand, dirt (clay), thatch (relatively stable... an odd/unique roof type), etc... etc... etc...
But the truly final layer... the "paint" when used... esp when talking about a highly reflective white vinyl/plastic compound... sure... there's an area where one might see more plastic (though it's not "plastic" in the sense of a plastic toy or or cup or such).
Sorry, maybe that's not helpful... but I just found it odd to see folks talking about "plastic roofs" as there is so much more to a roof than just the part that the sun first touches.
posted by Jiff_and_theChoosyMuthers at 7:50 AM on December 18, 2009
I would think of the roof structure - basically the stuff above/ resting on the beams, walls, posts, structure of a building (plywood, lathe of some sort - forget what they call the little boards that hang slate from, rafters or purlins, etc)... would never be plastic, but then again there is "plastic lumber"
The substrate that lay upon the structure... be it the tar paper, tyvek, plastic sheeting... depends upon how "built up" or engineered the roof type is (esp when talking about flat roofs - think "green roofs," and pebble covered roofs)...
Then there's the somewhat final layer of membrane - again depending upon type (might be talking about grass here if u r talking about a green roof)... here *maybe* plastic could appear??? but UV stable stuff like rock (slate), sand, dirt (clay), thatch (relatively stable... an odd/unique roof type), etc... etc... etc...
But the truly final layer... the "paint" when used... esp when talking about a highly reflective white vinyl/plastic compound... sure... there's an area where one might see more plastic (though it's not "plastic" in the sense of a plastic toy or or cup or such).
Sorry, maybe that's not helpful... but I just found it odd to see folks talking about "plastic roofs" as there is so much more to a roof than just the part that the sun first touches.
posted by Jiff_and_theChoosyMuthers at 7:50 AM on December 18, 2009
It's worth pointing out that it hardly ever rains--or freezes--in Arizona, so sheets of plastic are probably a better option there than for places in wetter/colder climates.
posted by valkyryn at 7:50 AM on December 18, 2009
posted by valkyryn at 7:50 AM on December 18, 2009
Most roofing materials need to have to have some weight to them or they blow away, so the plastic needs to be bonded to something or anchored in some way. I remember back in Seoul in the 80's there were lots of shanty towns covered with orange or blue sheet plastic, held down by lathe and old tires.
I have seen rubber/hemp shingles that look like wood shakes.
posted by bonobothegreat at 9:07 AM on December 18, 2009
I have seen rubber/hemp shingles that look like wood shakes.
posted by bonobothegreat at 9:07 AM on December 18, 2009
Having said all that there are a couple synthetic rubber membrane roof technologies. Self adhesive or torch down EPDM is the most common. There are also assorted roll on roofing coatings. These are all rubber products so have a lot of stretch
There's also PVC (polyvinyl chloride) and TPO (thermoplastic olefin), which are widely used on flat roofs, and they're full-on plastics. TPO is getting a lot of momentum behind it for ease of installation, as well as that it's typically white with very high solar reflectance values, which is becoming more important with the growing importance of LEED.
posted by LionIndex at 8:08 PM on December 18, 2009
There's also PVC (polyvinyl chloride) and TPO (thermoplastic olefin), which are widely used on flat roofs, and they're full-on plastics. TPO is getting a lot of momentum behind it for ease of installation, as well as that it's typically white with very high solar reflectance values, which is becoming more important with the growing importance of LEED.
posted by LionIndex at 8:08 PM on December 18, 2009
TPO sounds awesome as, well, something that is really awesome.
posted by Mitheral at 8:29 AM on December 19, 2009
posted by Mitheral at 8:29 AM on December 19, 2009
"Keeps the head in" is a con in an awful lot of places, especially where cheap roofing is going to be appreciated. I can only imagine how revolting a plastic roof would be in an Australian summer, let alone an African or South Asian one.
Jilder, what does "Keeps the head in" is a con in an awful lot of places mean?
I can't parse that English.
posted by IAmBroom at 8:18 PM on December 20, 2009
Jilder, what does "Keeps the head in" is a con in an awful lot of places mean?
I can't parse that English.
posted by IAmBroom at 8:18 PM on December 20, 2009
IAmBroom,
what does 'Keeps the head in' is a con in an awful lot of places mean?
- read 'heat' for 'head'.
- 'con' not as in 'scam' but as in the opposite of 'pro'---i.e., 'bad thing'
IOW, "there's a lot of places where you probably don't want to live in a greenhouse", which is basically what OP suggested.
posted by FlyingMonkey at 3:46 PM on July 6, 2010
what does 'Keeps the head in' is a con in an awful lot of places mean?
- read 'heat' for 'head'.
- 'con' not as in 'scam' but as in the opposite of 'pro'---i.e., 'bad thing'
IOW, "there's a lot of places where you probably don't want to live in a greenhouse", which is basically what OP suggested.
posted by FlyingMonkey at 3:46 PM on July 6, 2010
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posted by plinth at 2:59 AM on December 18, 2009 [3 favorites]