Ballasted Roof Dead Load

[RFreund]

What do you guys/gals typically use for a dead load for a ballasted roof.
Maybe I am misunderstanding the different built up roof types...
Usually I see 12 psf used for a ballasted roof. However ASCE7 gives 6psf for 5-ply felt and gravel and 5.5 for bituminous and gravel.
Is the difference in how the roof systems are constructed (the ballasted roof holds the membrane down)?

EIT

http://www.HowToEngineer.com

 

[wadavis]

Actually getting on the roof and finding out what was used aside (or having accurate structural drawings), I would calculate the wind uplift and back calculate the required ballast.

It's not perfect, but it is a sanity check to see if you are in the right ballpark.

Wadavis
E.I.T.

 

[JAE]

I've had the same questions - I think that the 6 psf range given in many tables is for a multi-ply roofing with gravel that is impregnated into the top layer.

A loose ballasted roof (where you have 1 to 2 inches of gravel) is more on the order of 12 psf.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[jike]

JAE is correct. Multi-story buildings and perimeters will often require more than 10 psf ballast for larger uplift requirements.

 

[Ron]

Yes, JAE is correct. A typical 3 or 4 ply built-up roof with embedded gravel surfacing has about 2 psf for the membrane and 4 psf for the gravel ( 2 psf of the gravel is typically embedded in an asphalt flood coat and 2 psf is "loose" on top). This type of roof system is not often used anymore (though it is certainly one of the better performing roof systems available). For roofing terminology, this is not referred to as a "ballasted" roof since the built-up roof is bonded to the substrate.

A "ballasted roof" generally refers to a single ply roof membrane that is held in place against wind uplift by loading the surface with either stone ballast or pavers. The membrane for such systems is usually not attached to the substrate, although some systems do have supplementary perimeter attachment. Since the ballast is the primary resistance to wind uplift, there has to be a lot of ballast installed on the roof. Most systems have 10 to 12 psf for ballast. If you do the wind uplift calcs, that's woefully deficient for higher wind load areas so if that's where you are, recommend against its use.

A fully adhered or mechanically fastened single ply roof system is very light. Most of them that are commonly installed these days (PVC, TPO or similar membranes) only exert about 2 psf in additional dead load. They have no gravel surfacing and no supplementary ballast.

If the roof membrane is a modified bitumen system with ceramic granule surfacing, use about 4 psf for the loading, since those systems use heavier membranes that are bonded to the substrate, but do not have aggregate ballast.

 

[RFreund]

I see, makes sense. Thanks guys.

EIT

http://www.HowToEngineer.com

 

[BamaStrucPESE]

I don't remember where I found it, but I did some research a couple years ago when working on a project with existing roof ballast and came across a note that said 10 psf was the minimum DL that could be used for roof ballast.

 

[BUGGAR]

Anybody allow for future re-roofs?