Softwood plywood is used extensively in the construction of homes. Plywood sheathing covers walls and roofs, as well as being used for subflooring. In all of these applications, the plywood is expected to span the gaps between supporting structures. While it does this admirably well, it is easy for the plywood to sag over time. When that happens, it tends to happen unevenly, leaving the edges of the plywood not meeting up well and creating opportunities for damage, especially to roofing.
One potential solution to help prevent this is to increase the number of structural members in a home’s construction, reducing the space between adjacent studs and roof trusses. But this option is unrealistic from a cost point of view, as it can double the cost of framing the home.
Roof sheathing is much more susceptible to this than wall sheathing or flooring. Therefore, architects may choose to specify additional support along the edges of plywood sheets used for roof sheathing. Blocking and the use of tongue in groove plywood are the traditional means of accomplishing this, in use before the H Clip was invented.
Rather than increasing the cost of framing a home, even by adding blocking, it makes much more sense to attach the edges of the plywood sheets together, between the existing frame members. Hence the invention of the inexpensive H Clip. The H Clip essentially performs the same function as tongue on the edge of plywood sheets, eliminating the need for blocking and saving the extra milling steps required to make tongue and groove sheets of plywood.
The H Clip, more properly known as a “panel edge clip” gets its name from the fact that the cross-section of the clip forms the capital letter H. This allows one piece of plywood to be inserted into what can be considered to be the top portion of the letter and the other piece inserted into what can be considered the bottom portion. While small and unobtrusive, these clips are easy to install and provide sufficient support to ensure that one piece of plywood doesn’t sag, while its neighbors do not.
When H Clips are used, any weight placed on the panel is distributed in part to the adjacent panels, reducing the chance for deflection and damage caused by that deflection. This increases overall roof stiffness, as there are no weak points created at seams between trusses. However, H Clips cannot prevent panels from buckling or warping, merely help keep them aligned with adjacent sheets, should that happen.
These H Clips are different from the H Strip used for installing siding. That H Strip is a decorative plastic strip, used to cover the gaps between pieces of siding, when they are butt jointed together. While mostly decorative in nature, they do provide a way of ensuring there is no gap in the siding where water could seep in. However, they have no structural purpose and do not strengthen the siding in any way.
H Clips are not load rated; therefore they are not required by building code. Rather they are normally considered to be at the architect’s discretion. However, a builder or do-it-yourselfer can choose to add H Clips to their project, without approval.
While H Clips are mostly used on roof sheathing, manufacturers make them in various sizes, to accommodate plywood from 7/16 through ¾” thick. Be sure when buying H Clips for your project, that they fit snugly enough onto the edge of the plywood that they won’t slide off, while not being so tight that it requires more than a light tap with a hammer to get them to seat onto the edge. Tight enough to need that light tap is ideal, as they won’t slide then.
The H Clips are installed as the plywood sheathing is being installed. It is recommended to put them every eight inches. So one should be put between the roof truss, if the trusses are 16” apart and two should be installed if the trusses are 24” apart. Attach the clips to installed sheets before the next piece is put in place. Clips are usually attached with the smaller side up. Then slip the new sheet of plywood into the clips.
Additional clips can be added in places where there may not be a structural element available for support, such as places where the roof is cut off at an angle. With modern roofs on homes being more complicated and made of more surfaces, there are many such places where H Clips can be used to provide additional support.
Keep in mind that H Clips will leave a slight gap between the sheets of plywood. This allows for expansion of the sheet, due to moisture, without causing buckling. The 1/8” gap will not cause problems for roofing materials, because the roofing felt will bridge the gap handily, without problem.