metal roof slope and water 1

[reyzers]

What's the typical slope for a 6 meter by 6 meter metal roof and 12 meter by 12 meter metal roof?

Is the slope double for area that is twice larger?

Is there a rule of thumb formula for the size of metal roof, how fast water move down it?

 

[hokie66]

The required slope for metal roofing depends on the intensity of rainfall, the length of slope, and the type profile used. The roofing manufacturer should have the guidance you need.

 

[Ron]

In addition to hokie66's comment, most metal roof manufacturer's limit the minimum slope to 2:12.

 

[swazimatt]

i agree it is normally the manufacturer that will give you slope guidelines (IBR is 5deg, brownbuilt cliplok is 1deg etc) also need to take into consideration if you are using single length sheets or needing to overlap shorter lengths

 

[coloeng]

Also, in areas that have heavy snowfall you usually have steeper roofs, sometimes 8:12 to 12:12.

 

[hokie66]

The attached document by Bluescope Steel gives recommendations for various profiles used in Australia. Some of the concealed fastener profiles are recommended down to 1 degree, but I always prefer at least 2 degrees. See the recommendations in the table. Of course, this disregards snow, as we don't have that problem except in a small area of the country.

http://www.lysaght.com/files/dmfile/RoofingWallingInstallation2013Chapter2.pdf

 

[reyzers]

If you have a concrete roofdeck and you didn't waterproof it. How long before rains erode the bars inside slab and beams if there are delays in the waterproof or cover? Would rainwater pass thru the concrete and into the bars and cause rust inside or no oxygen, no rust?

 

[Ron]

Short term exposure to weathering is usually not an issue.

 

[hokie66]

Agree with Ron that short term exposure to rainwater will not cause structural problems, but whether or not there will be leakage through the slab depends to a large degree on how the slab is reinforced.

 

[reyzers]

what is short in short term? 2 months? 5 months? 1 year? 2 years? Please define your concept of short as things are always relative.

How exactly is concrete permeable to water? If you glue a piece of concrete in the middle of a 4" diameter test tube with water on top and seal tight. How long before 10% of the water get through the concrete at uniform diffusion assuming the the concrete is at least 5000 psi and plant mixed?

 

[hokie66]

Short term? I wouldn't want to leave a concrete roof without waterproofing more than a couple of months. As to your question about permeability, you can do the research. But permeability is only a small bit of the reason why concrete slabs can leak...cracks are more important.

 

[Ron]

I would consider short term less than 60 days. Keep in mind that bridge decks are exposed for years without significant rebar deterioration...but then, who cares if a bridge leaks!

Concrete is permeable to water because it has interstitial voids and capillarity that will allow moisture/water to migrate into and through concrete. Concrete will hold water without leaking until saturation occurs, then gravity will cause leaks. Leaks can occur before this from condensation collection from the higher moisture content of the concrete.

As hokie66 noted, when there are cracks in the concrete, leaks occur directly and quickly.

 

[reyzers]

Would you know to what extend air can get into the interstitial voids and capillarity and rust the bars inside? Have you actually seen if rusts form in the bars inside bridges or old decks that didn't have waterproof and how rusty it can get?

 

[swazimatt]

the difference with bridge decks is that they are normally surfaced and as such waterproofed to an extent. They also generally have a drainage system to ensure that the water flows off the bridge. A concrete roof may have some ponding that would increase water ingress

Saying that i have seen lots of low level crossings that are fully submerged at least once a year and while they are only lightly reinforced i have not seen anythin g working through to the surface. I have seen some concrete harbour works where the rusting steel has caused the concrete to spall. Also i think the concrete cover required for extreme conditions is about 75mm and there are no requirements for waterproofing so i think it is safe to assume that there is a low risk of any major damage