Snow Drift on Existing Roof
Snow Drift on Existing Roof
(OP)
I am working on a project where it is proposed that an addition will be approximately 12 feet taller than the existing building. Due to the length of the existing roof, there is potential for a 4.5' drift to form at the junction.
Unfortunately, the existing structure is joists and joist girders, which were not designed for this type of loading. Since reinforcement of the girders is almost impossible I was considering another avenue. I have heard of similar cases where a sloped roof, mono-slope type structure was erected on the existing roof that represents the anticipated shape of the potential drift.
Have any of you heard of this practice?
Unfortunately, the existing structure is joists and joist girders, which were not designed for this type of loading. Since reinforcement of the girders is almost impossible I was considering another avenue. I have heard of similar cases where a sloped roof, mono-slope type structure was erected on the existing roof that represents the anticipated shape of the potential drift.
Have any of you heard of this practice?






RE: Snow Drift on Existing Roof
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RE: Snow Drift on Existing Roof
This is an industrial facility and is currently under production. It is not viable to add columns, etc.
RE: Snow Drift on Existing Roof
I cant think of any other option. Adding a section of sloped roof will just transfer your drift to another part of the roof.
csd
RE: Snow Drift on Existing Roof
RE: Snow Drift on Existing Roof
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RE: Snow Drift on Existing Roof
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Sea Water Intake and Jetty Construction
RE: Snow Drift on Existing Roof
The joist girder is most likely a steel bar joist (just heavier), and it is probably at the same elevation.
I don't think you can add channels to this condition, but possibly a king post on the bottom chord will help? This is done for vibrations quite frequently, maybe it will help for your strength concerns.
RE: Snow Drift on Existing Roof
csd
RE: Snow Drift on Existing Roof
RE: Snow Drift on Existing Roof
I have seen what you are describing done. I'm not sure I agree with the theary.
One solution to a joist and joist girder roof that needs to be upgraded to handle new snow drifting loads is to build a secondary roof system over the area in question. therefore by passing the existing joists and joist girders which are very difficult to strengthen.
Come out from 1 bay deep over your existing wall, and set new small post directly on top of the existing columns (poking up through the existing roof), and span a beam between them, and then frame bewteen this beam and your new structure with joists, beams, light gage studs, etc.
This has the effect of putting most of the new drift load into the new structure, where you can control the size of memebrs. The existing interior columns only see a small bit of the increase load, and if they need streghtening, it is usually easier to streghten a column that a joist or joist girder.
usually have to talk yourself into that fact the footings will be OK, since the drift load is only a short duration loading.
Just some thoughts.
RE: Snow Drift on Existing Roof
Your original post makes sense. If you put a sloped structure to match the anticipated drift snow load then theoratically you wont have the leeward and winward drift snow. Then you have to make sure the josts and the girder can take the extra DL of the new structure. Also, the addition, Does it have a sloped roof going toward the existing lower roof? You need to look at the sliding snow. Good luck! I must say I always hate remodels.
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Sea Water Intake and Jetty Construction
RE: Snow Drift on Existing Roof
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RE: Snow Drift on Existing Roof
I worded my first reply in a way that did not reflect my opinion on this subject very well.
I agree that this works if you can make the 'high' end of the new roof 'over framing' at the same (or nearly the same) elevation as the height of the new wall/roof that is creating the drift. The low end of the new 'over framing' is at the height of the balanced snow load elevation on the existing roof. Then, yes, I agree no drift is formed.
However, I have seen other engineers trying to justify using a sloped roof that was substainatially below the level of the new wall and roof that casued the drift problem. ( like 10 feet below), and trying to justify that becasue the new 'over framing' roof was sloped, no drift could form. This is the part I disagree with.
RE: Snow Drift on Existing Roof
Is the idea then that the wind would blow the snow up the new "ramp" and onto the proposed roof and no drift would form?
RE: Snow Drift on Existing Roof
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RE: Snow Drift on Existing Roof
Thus, the first new bay will take all the drift and you can design for that directly.
If this is not an option, then you are back to doing what all the great comments above suggest.
But it is usually less expensive to come in and strengthen the existing framing as opposed to building a whole "cover" roof over the existing framing.
RE: Snow Drift on Existing Roof
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Sea Water Intake and Jetty Construction
RE: Snow Drift on Existing Roof
I appreciate all your input and suggestions. Something will work out.
RE: Snow Drift on Existing Roof
I have also upgraded lower roofs with new drift load by adding more joists (you can get each joist in two pieces that are bolted together in the field--you can't install a one piece joist into an existing roof easily). Or, you can get the original joist supplier to check the existing joists for the new load, and design reinforcement as required--I have done this, and the original joist supplier provided drawings that showed where to add in angles, etc.
All of this assumes that the joist girders, columns, and footings are OK--and my experience is that they usually are.
Don't forget that the deck itself may not be OK, and so joists would need to be added just to make the deck work.
DaveAtkins
RE: Snow Drift on Existing Roof
RE: Snow Drift on Existing Roof
RE: Snow Drift on Existing Roof