Roof Top Units 3

[dik]

Is there an elevation above the roof of a building that you can place mechanical roof top units so there is enough space beneath that you don't have to consider the effects of snow accumulation around the unit? I generally consider that if there is 5' clear beneath the unit that there is sufficient space that snow accumulation is not required. Does anyone know if this is codified anywhere?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[1503-44]

Not sure if that strategy is gonna work here. They had to take the picture in the summer for obvious reasons.

A black swan to a turkey is a white swan to the butcher.

 

[bones206]

A clause was introduced in ASCE 7-16 for this. I believe it is a minimum of 2 ft clear above the depth of the design roof snow load to avoid drift accumulation.

 

[dik]

Thanks bones... I'll look into that. My 5' does that...

1503... I love the sign with the snow... our snow accumulation, during a good winter (aka lots of snow)... is about 24"...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

FYI

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[skeletron]

You could also equate the height of the ground snow level in your area using the density of snow equation in NBCC. That can give numerical backup to the ASCE prescriptive requirements.

 

[Brad805]

By the time you elevate the unit is there any cost savings? Seems like added work for the mech trade and a bit more steel. I would have never thought of this, but our basic snow load would lead to it being too high. If it is an existing structure I understand.

 

[dik]

Yup... first thing I checked was using 0.8*GSL / gamma to determine the snow height and added 2' to this and ended up with a little over 4'... my WAG of 5' was good.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

The savings is not in the platform support, but in not having to reinforce the existing roof for the 'huge' snow accumulation load. I've done that often and there is a real savings. I've used 5' clear because I thought that would provide ample clearance for wind to clear away any drifts.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Brad805]

It is an interesting idea. We have reinforced roofs before for new RTU's, but our snow load is great enough that the drifting is not usually much of a problem unless they are tall MUA. You must have small plenums thru the roof.

 

[dik]

The load from the snow accumulation may exceed the weight of the equipment.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Brad805]

My question about plenums was more about their blocking the air flow.

 

[bones206]

It's a fair question. I have a self-supporting RTU dunnage tower under construction right now. I utilized the 2 ft clearance exception to avoid reinforcing the existing PEMB structure, which would have been prohibitively expensive for the client. It satisfies the building code, but I do have some nagging doubts and will be watching closely in coming years to see how snow behaves around the plenums and also the platform access stair.

It's a relatively new code provision and I'm not sure how much real world testing they did to account for these types of obstructions.

 

[dik]

I suspect plenums do interfere with the air flow at the roof, but considerably less than the RTU, itself. The immediate area under the RTU is free of snow which helps. I've just arbitrarily been using 5' for decades (not true engineering, but it sorta worked); I now have a source that I can reference. Over the years, I've never encountered snow accumulation with my approach. It came from using the depth of snow and adding 3' to it. I thought that would allow sufficient airflow to prevent windblown snow from 'precipitating out'. I was comfortable with it and am even more comfortable with it now that I have a source.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[structSU10]

I had this question prior to ASCE 7-16. I sent an email to the author (Michael O'Rourke)of the snow guides and a big contributor to that section and he sent me a preview of that clause. I think he referenced testing but I can't track down the old email.

 

[dik]

Thanks for the added info.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[1503-44]

Dik, my father grew up in that area and he would always talk about the snow drifts they'd get. The peninsula in the middle of Lake Superior is some 1250 ft higher than the lake and the humid air takes the opportunity of the elevation change to unload every snowflake that it possibly can. Of course all the roofs up there are very pointy; even the pigeons have trouble finding a roost. Lots of houses have 2nd floor, winter entrance doorways, but no stairway down to ground level.

Glad you got such a good tip and the problem is solved. At least for most regions.

A black swan to a turkey is a white swan to the butcher.

 

[dik]

Only real snow we had in Winnipeg, that I recall, was in March, 1966. We had to exit the house from the second storey onto the garage roof which was the snow level... fortunately I was in scouts and had three sets of snowshoes, else I don't know where that 'first step' would have taken me.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik