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source: FL Division of Emergency Management |
Roof sheathing failures were widespread during the active hurricane seasons of 2004 and 2005, and many of these failures were due to staples pulling through oriented strand board (OSB) roof deck panels.
Experience gained since Hurricane Andrew in 1992 has revealed several points regarding structural panel roof sheathing:
- Plywood is more impact resistant and more resistant to fastener pull-through than an equivalent thickness of OSB.
- Plywood is also more resistant than OSB to deterioration from repeated wetting and drying.
- Thicker is better – nominal 5/8” (19/32”) thick roof sheathing is commonly recommended for areas in which the Basic Wind Speed is greater than 100 mph.
- Staples are an inferior fastening system for roof sheathing – full headed 8d common nails are the minimum recommended fastener.
- Missing only 1 required fastener reduces uplift resistance by as much as 25%.
Reinforcing the Attachment of the Roof Sheathing
Once again, when considering ways to reinforce the attachment of the roof sheathing to the trusses or rafters, the main question is, “Is the existing roof covering ready to be replaced?” During the re-roofing process, any damaged sheathing at eaves, gable ends, valleys or roof penetrations can be replaced. In addition, the entire roof deck can be nailed to current code requirements. In short, these code requirements are:
- 8d full-headed common nails as the minimum required fastener (8d ring shank and 10d common are recommended upgrades from the minimum requirement).
- 6” on center spacing at panel edges and ends.
- For Basic Wind Speed zones from 110 to 140 mph, 6” on center spacing at panel intermediate supports (with 4” on center spacing at all zones marked #3 in the next figure).
source: 2004 FL Building Code |
The following diagrams from FEMA Technical Fact Sheet no. 17 illustrate these critical areas where uplift forces are strongest.
source: FEMA Technical Fact Sheet no. 17
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source: FEMA Technical Fact Sheet no. 17 |
Panel edges between trusses or rafters within 6’ of gable ends should also be supported by 2x4 or 2x6 blocking with construction adhesive. Similar 2x blocking is also recommended at openings for roof vents whether ridge or off-ridge type. All other panel edges between trusses or rafters should be supported in the middle by a panel clip properly sized to match the thickness of the sheathing. These clips not only support the edges of the panels between trusses or rafters, they also ensure a 1/8" spacing gap between panels to allow for expansion.
When the existing roof covering is not ready to be replaced, several steps to reinforce the attachment of the roof sheathing can be carried out from the attic access area. For example, a continuous 1/4” bead of sub-floor type construction adhesive can be applied to both sides of the connection between the truss or rafter and the sheathing. This method is surprisingly effective at gluing the sheathing to the trusses or rafters.
source: FL Division of Emergency Management |
It is important that the bead of adhesive be at least 1/4” thick and that it firmly embed onto the surface of both the sheathing and the truss or rafter.
source: FL Division of Emergency Management |
An alternate to the continuous bead approach is to install 6” (minimum) 1x2 or 2x2 blocks that sandwich construction adhesive to both sides of each truss or rafter and the underside of the sheathing.
There should be 3 of these blocks on each side of each truss or rafter for each 48” panel – 1 in the middle of the panel and the other 2 within 6” of each edge. These blocks need to be tacked in place while the adhesive cures.
source: FL Division of Emergency Management |
At the gable end where only 1 side of the truss is accessible, install a continuous 1x2 or 2x2 wood strip sandwiching adhesive to the truss or rafter and the underneath side of the sheathing. Field experience has shown that these glued connections are extremely resistant to removal – either through storm uplift forces or during renovations that require the removal of the sheathing.
source: FL Division of Emergency Management |
Spray polyurethane foam is an alternative to the construction adhesive attachment system outlined above. This is a specially formulated foam adhesive used in the manufactured housing industry and is site-mixed before application by trained installers. Unlike the types of expanding polyurethane foam available at home improvement stores, this foam is formulated to resist deterioration at the high temperatures typical of attic access spaces.
A final alternative for strengthening the attachment of the sheathing to the supporting trusses or rafters is to spray the entire underside of the roof deck with an insulating foam such as Icynene. Strengthening the attachment of the sheathing to the supports is a by-product of the process which is actually aimed at relocating the thermal barrier of the home and reducing energy costs. The process changes the nature of the typical vented roof system to an unvented roof system and so has to be designed and installed with great attention to the overall ventilation requirements of the home. Despite building science research and field experience demonstrating the advantages of unvented roof systems, most local Building Departments have yet to accept them as alternatives to the current code-mandated requirements for roof ventilation. So, unless the roof system for the home is being completely redesigned for other reasons, this final approach to securing the sheathing to the supports is not the most feasible option.
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