Flat Roof Perimeter Nailer Anchorage Design 1

[Forensic74]

Has anyone here been involved in the design of wood nailer plates around the perimeter of a flat roof system? I'm curious if there's any design guides out there that might establish standard design assumptions.

 

[jayrod12]

I'm not sure exactly what you mean. A lot of us have designed lots of wood nailers. But there's way too many different possible configurations to discuss in generalities.

What type of roof truss/joist? What type of wall below the nailer? Top chord bearing truss/joist or bottom chord, or top flange hanger? Seismic? What level of lateral or uplift are we talking about?

 

[Forensic74]

The edge of a membrane roof is anchored (usually nailed) into a continuous wood nailer. This nailer must be anchored to the structure, and this is a high stress connection for wind force.....so if the nailer anchorage to the structure fails, usually the roof membrane peels back and fails. This can cause very high dollar damage.

Here's a typical detail

https://www.carlislesyntec.com/view.aspx?mode=media&contentID=1521

The design of the nailer anchorage to the structure is not by the membrane manufacturer.

On a new structure, the design of the nailer anchorage to the structure falls under the responsibility of the structural engineer or architect.

On a re-roof, good roofing contractors would work with a roofing consultant (usually an engineer involved) who would design this connection.

The IBC is very specific in that the nailer anchorage must be designed for full ASCE-7 CC wind loads. I'm just looking for the standard design approaches for this nailer as it can get funky if you start considering torsion.

 

[msquared48]

The wood nailer does not appear to be a structural member.

The structure.- diaphragm and chord member - is below the insulation and nailer.

Mike McCann, PE, SE (WA)

 

[Ron]

I don't consider torsion in the structural sense. I do consider rotation of the nailer and handle that by staggered nailing or anchoring. My approach is to design the fastening for the perimeter and corner uplift condition and use a tension safety factor of 5 (remember who will be installing...roofer or framer). Each termination of a nailer, whether at a splice or end, gets two fasteners, then spread them out staggered, depending on the loading. I practice in a high wind area and have seen many uplift failures.

 

[Forensic74]

Thanks Ron. Yea, I dont think there'll be that much torsion in every case, but I have observed failures where gutters are anchored into the nailer and the wind force acting upward on the gutter (in combination with membrane uplift) will literally break the nailer down the grain.

Factory Mutual does have a document that includes some basic recommendations for perimeter nailer anchorage FM 1-49, but its not code and isn't necessarily meeting a "design" to ASCE-7 loadings depending on the configuration.

I think msquared48's reply really shows how grey of a design area it is. The PE points the finger at the roofer, the roofer points the finger at the architect, and the architect points the finger at the PE.

The design of one stupid board seems trivial until it's a $2 million lawsuit of damaged building contents.

 

[gte447f]

On a new structure, the design of the nailer anchorage to the structure falls under the responsibility of the structural engineer or architect.

Very good answer by Ron above. He may be the only SEOR who has ever considered such a connection, or maybe he considered it as a specialty consultant to a roofing contractor or in a forensics role. What I am trying to say is that in my experience the SEOR would rarely consider this connection in the design of a new structure unless specifically asked to do so. I would bet it usually falls through the cracks.

 

[Forensic74]

Yes, I've been the SEOR and understand where you're coming from. That said, there are engineers out there who detail this nailer, and I think that's the appropriate way to go as it's the design team's responsibility and the SEOR has the CC edge and corner zone loadings on hand.

 

[gte447f]

Foresnsic74, for new structures, the construction drawings should have the C&C wind loads on the drawings, so contractors, subcontractors, material suppliers, etc. should know the C&C wind loads (I think this is an IBC requirement) . As an SEOR, I don't think I would design this nail strip unless specifically asked to do so. Our contract usually specifies that we design the "primary structural system" and defines what that is. This nail strip would not fit within that definition. I do agree with you that this is an important connection/anchorage detail and needs to be designed properly by whoever is responsible for it.

 

[gte447f]

Regarding the "torsion", I guess it is really a cross grain bending problem for the wood nailer, right? As Ron suggests, it would probably be important to install the fastener near the outside edge of the nailer, or perhaps use plates or washers to distribute the anchor/fastener force and reduce cross grain bending, kind of like the plate washers required for wood shear wall sill plate anchorage.

 

[Forensic74]

I'm curious then, who exactly do you think is performing a load/strength analysis of the edge nailer, the contractor?

 

[Forensic74]

Forgot to note, the NRCA standard specifically says that the the configuration of the perimeter nailer and its anchorage is the responsibility of the designer. "A designer should specify the means of attachment, as well as the fastening schedule for all wood nailers and blocking."