ICF wall installers must give special consideration to the construction and installation of wood door and window bucks.
Methods of Wood Buck Construction
There are three main methods of constructing wood door and window bucks:
1. External- Run the wood buck all the way across the width of the insulating concrete form
2. Internal- Inset or recess the buck into the cavity of the form
3. Combination- Use plywood in combination with 2x cleats
Pre-building the bucks to the rough opening dimensions off-site will increase on-site productivity. Regardless of the wood buck method you use, the buck must be anchored to the concrete wall using nails, screws or anchors.
The first method is probably the most commonly used by contractors. The 2x buck is placed so that the ends are flush with both faces of the ICF wall. Depending upon the form width, the 2x will vary in size. Some form sizes may require taking a large width 2x and ripping it to match the width of the form.
Bucks are fastened to the ICF wall with a frame of 1x4s attached to both sides of the buck. Some manufacturers allow the builder to secure the buck by driving 3-inch drywall screws through the 2x into the ICF blocks.
The second method is often used for projects that will have a stucco exterior finish or when thermal breaking is a concern. It also uses less materials than the first method, and is useful when the total wall thickness exceeds 10 inches. The “internal buck” uses 2x material cut to the width of the concrete cavity, and is placed flush into the cavity of the form. Long drywall screws are driven through the EPS to secure the buck. Use a large plastic washer, such as a Win-Lock style fastener to ensure the screw doesn’t go completely through the EPS.
This method uses less lumber than either of the other methods, which may save on the cost of the buck, because of the higher costs of lumber recently. It also provides a wider flange for fastening around the opening. It may, however, take more time to assemble.
In this method, a 2x6 ripped in half is fastened to ¾'' pressure-treated plywood. A slot in the bottom or sill of the buck must be built to allow for proper placement and consolidation of the concrete below the opening.
Regular dimensional wood should never be in direct contact with the concrete. If you must use regular lumber for bucks or sill plates, a moisture resistant barrier must be placed between the buck and the concrete in accordance with the 2003 International Residential Code (IRC), Section R319.1.
Otherwise pressure-treated lumber is used for the bucks and sill plates. Since CCA treated lumber is no longer available and has been replaced by ACQ treated lumber, extra care must be taken in fastening the ACQ lumber to the concrete. You can’t really tell the difference between the CCA lumber and the ACQ lumber by sight. To be sure what kind of treated lumber you own, look at the inspection mark or tag on the end of the lumber. Section R319.3 of the IRC requires that fasteners used with pressure-preservative treated wood be hot-dipped galvanized, stainless steel, silicon bronze or copper. You can’t use the same metal fasteners with ACQ lumber as you did with CCA lumber, because ACQ lumber will corrode ordinary galvanized fasteners. Consult with the manufacturer or supplier of fasteners to be sure that you are using the properly coated fastener.
Bracing the Bucks
The buck must be braced every 2' both horizontally and vertically to resist concrete pressures from bowing the buck. The buck must also be kept square by bracing the corners or running a diagonal across the buck. Typical wall bracing is used along each side of the jamb to laterally brace the wall.
Kelvin Doerr is the Director of Engineering and Technical Services for Reward Wall Systems. Article reprinted with permission from InFORMation, Fall 2004.
Five Steps to Foolproof Wooden Bucks
Unless you or your builder chooses to use plastic bucks available from specialty manufacturers, your ICF building will need wooden window and door bucks. Nearly all brands of ICFs follow the same steps in installing these bucks.
1. Window and door bucks should be pre-built to increase jobsite productivity. Measure the diagonals of all window and door bucks to ensure they are square, and brace them securely. The bottom sills of window bucks need either a 3-inch void running the length of the sill or a series of 3-inch diameter holes for concrete placement and consolidation. Numbering the bucks will keep the jobsite organized and eliminate the possibility of error.
2. Place your door bucks first. As soon as the foundation is cured, mark the location of all doors and secure the pre-built door bucks into place. The ICF walls can then begin.
3. Once walls have reached the level of the bottom of the window, mark the height of the window bottom on the wall and cut the foam down to window height with a hand saw.
4. Lift the window buck into place. A pair of “handles” made from scrap lumber nailed across the bottom sill will make it easier to position the buck. Ensure the buck is level and plumb, adjusting if necessary, prior to fastening it into place. External bucks are fastened using a frame of 1x4s. Internal frames are fastened into place with 3-inch drywall screws and large nylon washers.
5. Once the bucks are in place, continue building until the top of the buck is reached. Fasten the sides of the buck into place at maximum six inch centers. If using an internal buck, trim off the excess foam with a hand saw after the buck has been securely fastened. The top row of ICF blocks are placed, fastened, and trimmed just like the sides. Wall construction continues as normal above the bucks.
Use only pressure treated lumber and exterior grade plywood to build your bucks. Building codes prohibit regular dimensioned lumber from being in contact with concrete.
Pre-building the bucks off-site will increase on-site productivity and keep the jobsite clean. If you can’t build them off-site, build them in an out-of-the-way location prior to assembling the walls.
Use two 2x4s on edge for the bottom of the window buck. The gap between the boards will be used to place concrete and ensure consolidation. It’s much easier than the alternative method of cutting a series of 4-inch holes in a solid sill plate.
If the window opening exceeds three feet in either dimension, brace the buck every two feet to prevent bowing.
Use hot-dipped galvanized nails to construct the buck. The new ACQ lumber will corrode other fasteners.
Hammer nails every three inches or so to the outside of the buck. The concrete will bond to the nails and increase buck’s resistance to shear pressure.
With large bucks, attach blocks to the sill to help lift and position it on the wall.