Roof Dead Load Calculation

[msquared48]

When figuring total vertical loads for gravity or seismic loads, does anyone adjust the roof dead loads for the slope of the roof?

I just looked at a 12:12 pitched roof, and if you compensate for the pitch, the flat projected dead load is increased by about 25%. I've never done this before, but I will now as it seems reasonable to me. Obviously, for lower pitched roofs, the results will not be as dramatic, and at some point, neglected. However, I feel that the results here are worth considering. The effect is to increase the normal 12 to 15 psf dead load to more in the range of 15 to 19 psf.

This seems so basic, but I have never heard of it before. I know. I'm dangerous.



Mike McCann
MMC Engineering

 

[DaveAtkins]

Yes, I do consider this on a steep slope as you mention.

But like you I am lazy and ignore it on "low" slope roofs such as 4:12 and less.

But I typically round every dead load up, which compensates for this somewhat. For example, I assume a GWB ceiling, which really weighs 2.5 psf, to weigh 3 psf.

DaveAtkins

 

[Ron]

Mike,
I hadn't thought about it much either. In special cases like your high slope, yeah it makes a little difference.

In most of my analyses, dead load is a non-conservative consideration (high uplift) so less is more for me.

Ron

 

[msquared48]

I'm not really worried about it, as for most major beams in the roof, I have always use one size greater than what is required by calculations. And, I too, round up the dead load for vertical and seismic, but deflate it for uplift.

Thanks for verifying that I am not crazy.

Mike McCann
MMC Engineering

 

[ChadV]

I do this as well - my first job doing structural design, I asked a more senior engineer about it and he replied that he'd never thought about it.

 

[bootlegend]

I assume you are implying that you will account for the "true" area of the roof rather than the projected "plan view" area. I have always accounted for it and didn't know that people neglected it. Of course, it would be small for low slopes.
Interesting to read the different processes.

 

[demayeng]

I included it in my beam design spreadsheet however my boss doesn't account for it... but he uses 0.5kPa for a sheeted timber roof with ceiling - 25% more than the 0.4kPa I use.