What causes the wood under the roof to bend?
Since I'm not sure what you mean, I am assuming you mean the supporting frame under your roof "deck", ie the joists, purlins, ridge, etc.
The reason a roof generally sags is that it is not built correctly in the first place or because the wrong size joists with the wrong spacing have been used.
For example:
If a roof is designed to support custom orb (corrugated iron) roofs or shingles, the spacing for its joists and slats will be different from one designed to carry concrete or terra cotta shingles.
A roof designed to carry concrete or terracotta tiles can weigh up to 40 tons compared to a tile or iron roof of 4-6 tons (average).
If the roof is not "braced" correctly with the correct number of straps, then it will sag and sag.
Generally, the climate does not have much to do with the deformation of these woods. As they age, they shrink and twist from any failure in the wood.
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Just 50 years ago, wood was not "classified" as it is today.
Now each piece of wood has an "F" (tension) rating and is rated with certain requirements, for a house frame.
The degrees of stress is known by:
• Grades 'F' F4 to F34: p. Ex. F14 indicates that the basic working stress (in bending) for this wood is around 14 MPa. Most structural woods from native forests, such as white cypress and eucalyptus hardwoods, are classified using the F grade system; or
• MGP machine graded pine, MGP10 to MGP15: p. Ex. MGP10 indicates a minimum threshold for stiffness properties of 10,000 MPa. Most softwoods from exotic plantations (Pinus species) are classified using this system.
Why are houses in Mexico often made of concrete, but not in the United States?
Well, if you include concrete and cement blocks in the concrete category, I can give you a perspective from a little further south than Mexico: the Caribbean coast of Colombia.
On the coast of Colombia, at least during the 80s, when I lived there, concrete/cement block houses (and buildings of all kinds) were ubiquitous. This was for the simple reason that they are extremely easy to build, even for people with little skill or experience.
Cinder blocks also greatly facilitate the installation of ventilation - just turn the block on its side. On the Caribbean coast, there's not much in the way of winter to worry about, but it does get a bit hot during the dry season, around 90F, and at least back then, air conditioners were rare. There's a lot of crime too, especially theft, so being able to bypass real windows by just having a few areas (often just a clerestory) with the blocks turned on their side is sometimes considered better than windows.
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In fact, this was also a trend in the US in the mid-20th century: breeze blocks. They have recently returned a bit:
This adobe house (in Mexico) has had its windows filled mostly with cement blocks, almost certainly for safety reasons:
But again, it is also very easy to build in real windows. In that case, they are usually filled with metal bars or windows with glass and metal shutters.
Because cinder block houses are so easy and quick to build, they are common in areas that are often affected by natural disasters, such as Guatemala, which is prone to earthquakes:
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Roofs are usually made of tiles or corrugated metal.
Everything is cheap, cheap, cheap and also fast. Some do not have plumbing or electricity, and the poorest do not even have a floor; just stepped on land. However, the comparatively pretty houses are also built of blocks; they are usually plastered and painted in bright colors, with nice tile floors and ceilings.
Buildings such as shops, schools, and churches are also sometimes built with concrete or cement blocks. Our family was in Colombia on a church building mission, and I mean it literally: my parents (well, mainly my father) helped physically build a church out of concrete blocks.
Note: I recently took a Google Street View tour of my old hometown, and the concrete/cinder blockhouses were as ubiquitous as ever.
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Edit: As noted in several comments, another great benefit of building with concrete/cement blocks is that you don't need to finish it all at once. These types of homes are popular in low-income areas; If you can't afford to buy all of your supplies at once, you can still start and add as you can buy more.
Since supplies are much less susceptible to decay in their unfinished form (they won't rot and take a long time to erode in rain), you can leave the unfinished part exposed until you can build more.
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