Maximum Radius In A Curved Roof To Avoid Ponding At The Apex

[YuleMsee]

Hi, what do you use as a maximum radius in a curved roof to ensure water does not pond at the apex. Or is there a method to ensure water still falls at the area where the slope is less than manufacturer recommended minimum pitch.

 

[KootK]

1) At a minimum, I'll require that a secant line from high point to low point satisfy 0.25"/ft. There is precedence for this in warped OWSJ roofs.

2) I'll try to get #1 to really be 0.50"/ft.

3) I see what you're getting at. At some monstrous roof size, does the large flat top pond independently? I think that mostly comes down to judgement and I would do thinks such as:

a) Look at expected deflections near the top and see how that might affect the secant.

b) Look at construction tolerances and consider how that might affect the secant.

c) Look at any potential for snow buildup or ice damming part way up the roof that might cause the flat top be pond independently of the rest of the roof.

 

[JLNJ]

I'm not sure I like KootK's secant theory. For a given radius you can increase the included angle to increase the secant slope, but it doesn't change the relatively flat section at the top.

With modern programs, you can model the incremental deflections to see if you really have a problem as the rain accumulates.

I did one roof where the members were oriented down the slope rather than across it. It seems to me that this could limit pounding issues.

 

[Ron]

Is this a metal roof, a fabric roof or a membrane roof (or something else)? It makes a difference as to how critical the slope conditions are. Most manufacturers will allow up to 72 hours of ponding; however, I'm in an area that can see multiple days of significant rainfall, so I'm always cognizant of the potential for progressive deflection from ponding and subsequent rains.


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[YuleMsee]

@Ron It's galvanised steel sheets
@kootk luckily I'm in the tropics so no snow, just rainfall.

 

[KootK]

So how large of a radius are you contemplating?

 

[hokie66]

I don't think "ponding" at the apex is what you are really concerned about. Leakage, yes.

Refer to page 21 of this link for recommended maximum and minimum radii for this situation. Note the requirement for sealing the side laps.

https://cdn.dcs.lysaght.com/download/lysaght-roofing-walling-installation-manual

 

[BAretired]

If the roof is a spherical dome, it drains in all directions. If it is a cylindrical curve, it drains in only two directions 180[sup]o[/sup] apart.

Using the KootK recommendation of 1/4" per foot from springing line to apex, it makes no difference how large the radius is; the secant slope is 1:48 and the radius to the springing line slopes at approximately 2.49[sup]o[/sup] to vertical.


BA