Wind uplift on roof deck 1

[ronster]

I design many oceanfront homes in hurricane areas. Architects often specify wood roof decks placed on sleepers over the flat roof areas (1/4"/ft pitch to drain). In the past the wood decking and sleepers would be installed like pallets so that they could be removed to allow for cleaning and roof repairs. The decking "pallets" are not attached to the roof surface which brings up the concern of uplift on the roof deck or "pallet". Screwing the pallets to the underlying roof stucture will lead to water problems or using an adhesive to the bottom of the sleeper will not allow for repair of the roofing surface. Anyone come up with a solution to this problem or is uplift not a concern? How would you determine the uplift force if any? It is my understanding that the roof uplift force determined from ASCE 7 for the building would act against the flat roof sheathing below and not on the decking above.

 

[Ron]

Uplift is a concern. If you put any accessory, appurtenance or device onto a roof, particularly in a high wind area, it must be fastened to comply with code.

For the decking you mention, it must either have sufficient dead load to prevent its removal by wind or it must be fastened. It does not have enough dead load most likely, so a fastening scheme needs to be devised. I would consider a series of anchor points built into the roof membrane system and properly flashed, with bolts or threaded receivers sticking above the roof surface. Thaler Roofing Accessories makes a variety of such devices that can be integrally flashed to the roof. Next, span the devices with appropriate wood sleepers fastened to the embeds. Then fasten your decking to the sleepers in a manner that all can be removed (screw fastening would be appropriate).

With this you'll get drainage onto the roof membrane and protection for same.

Another option is to use insulated pavers, rated for wind loads and tie them together with the manufacturer's accessory clips.

 

[ronster]

But what is the uplift force? Any internal uplift force will be resisted by the roof joists below but how do you calculate the uplift suction force on the deck from the wind blowing accross the roof? You need this info to be able to design the fastners which are not that simple to flash and keep watertight.

Pavers would have the same problem. They weigh a little more than the wood deck and that may be enough but unless you can calculate the uplift force how do you know? How does clipping to an adjacent paver that is not tied down and is also resisting the same uplift force help out?

 

[Bagman2524]

I've used ASCE-7-05 section 6.5.11.4.1 to calculate uplift on a roof overhang. In my case the uplift pressure was about 60% of the horiztonal pressure.

 

[Bagman2524]

You might also want to check the IBC section on roofing. I believe it requires certain types and numbers of fasteners.

 

[ronster]

Bagman
I am not talking about a deck over open space but rather a placing decking on sleepers over a flat roof to an enclosed space. I do not believe that roof overhang uplifts forces would not apply to this situation.

 

[ronster]

Sorry for the double negative in the last post. Kindly omit one "not" as you read it.

 

[Ron]

I'm not being flippant, condescending, nor judgmental, but your original posting says that you design many oceanfront homes in hurricane areas, and you categorize yourself as "structural", so I'm surprised that you do not know how to compute the wind loads on a structure.

The uplift pressure will depend on the configuration of the structure, its height and several other factors. I'm not sure where you are geographically; but if you are in the US, you can use ASCE 7 to compute the loadings, being careful to use the proper edition according to your code requirements.

It is not practical for anyone in the forum to tell you the amount of uplift you will encounter as we do not know the locale, the configuration of the building or the roof, and other factors which must be considered in evaluating roof uplift magnitudes.

If you are uncomfortable with doing such computations, I would suggest that you engage a local structural engineer to assist you.

 

[JAE]

ronster,
Ron is correct that ASCE 7 is the document you would want to use here. It's wind section provides guidance on wind pressures for components and cladding, which you would use for smaller area pallets.

If you can't screw or otherwise fasten these to the roof itself, you'd need to have heavy enough pallets to counter the wind uplift force sufficiently. ASCE 7 provides a load combination to check this as follows:

0.6D + W

So your pallets would have to be 1/0.6 = 1.66 times the wind pressure to meet code.

There may also be local code provisions that would no allow "loose", but heavy objects on the roof (as Ron mentioned - it may be a code mandate that positive fastening be used.

 

[ronster]

I have no problem determining the wind loads on the roof. I was looking for guidance in how other engineers detailed the condition and/or guidance how the wind load is applied to roof.

ASCE7-05 is the applicable design standard, The wind force is P=Qh(GCp-GCpi). I am in a 110 psf, exposure C wind zone.

GCp is the external wind pressure force on the roof = 21psf
GCpi is the internal wind pressure force on the roof and is resisted by the flat roof framing below.

To clarify my question regarding the magnitude of the force, does the entire external wind force GCp (using the uplift condition) act by pulling out the roofing material (as a suction force) or does some of this force push up from the inside of the building on the roof structure?

If I consider all of this force acting from the exterior as a suction force (which is how I would apply it) then the pavers system mentioned as an option would not have enough dead load to be used in any high wind area and I will need to tie down the roof sleepers which the architect and roofer do not want to do out of concern for roof leaks.

 

[structuresguy]

Well, i would take the component of the force which only accounts for external pressure, ignoring the internal pressure component. So in your example, you have 21 psf as the external pressure. That is what I would use for design uplift pressure on the decking.

That said, I don't know if the FBC will allow the decking to just sit there without secure fastening. I would check with the local building official on this.

 

[burtonSTR]

uplift design using combination
0.9D+1.6W (LRFD)
0.6D+W (ASD)

 

[JAE]

ronster,
The internal pressure (GCpi) would not be used on the pallets on top. This pressure is a differential between the interior and exterior areas which are divided by the main roof structure. Thus the main roof structure would take both the GCp and GCpi pressures. The pallets would only be subjected to the GCp pressures.

 

[Ron]

Consider this a monoslope roof with no internal pressure as JAE noted. Ignore the placement of the concentrated load as noted in ASCE 7 and distribute the load over the area.

You will be in the 20 psf range for uplift.