How to Build a Strong Frame
by Damien Andrews
There are an awful lot of projects that the do-it-yourself carpenter tackles which begin with a wooden frame. Some of the frames are small and simple, some are large and complex. Some frames seem to disguise themselves as something other than frames, but they're frames nevertheless.
Let's begin by dispelling a common myth: lots of lumber equals a strong frame. Take a length of standard wall, for example. Let's say you have a wall frame for a wall that's intended to be 30' in length and 8' in height. We'll say there are no doors or windows to be installed in our fictitious wall, so we just have studs – no cripple studs or headers. The studs for our wall will be nailed on 16" centers. So we now have a 30' x 8' x 3½" frame that contains 9-10' x 2" x 4" as plates, and about twenty-five 2" x 4" studs, including the corner studs. That's a lot of lumber by any measure. And the frame is able to withstand a tremendous amount of STRAIGHT downward pressure. But it cannot take much horizontal pressure, especially not if the pressure is applied horizontally to either the top or bottom. Add some cross bracing and the wall frame is now up to withstanding great amounts of horizontal pressure as well.
Way back when, we used to cross brace stud wall frames using lumber. What size lumber we used depended on the size of the studs used (2" x 4" or 2" x 6") and the amounts of stress the wall would have to withstand. The normal cross brace was a 1" x 4", but on occasion 2" x 6" lumber was used. It was quite time consuming to install these wooden cross braces, too. The studs had to be notched out so the selected cross brace would fit into the notch, and leave the wall's outside flush, to accept fascia materials properly. Today, thanks to the genius of some industrious fellow, one can buy "T" metal cross braces that only require a slit to be cut into the accepting studs. They're cheaper, and so much faster and easier I can't begin to explain…
Besides the "T" metal cross braces that are designed for use in stud wall frames, there are countless other metal braces that are made for practically every imaginable application. No doubt, irrespective of your project, you could find some metal braces to employ that would do an admirable job of strengthening your frame. I've seen brass corner braces that are so small that I needed tweezers to pickup the included screws. And on the other end of the spectrum, 1/8" thick steel braces are relatively common.
Sometimes steel braces really are the way to go – whether it's because of the job's parameters, or simply the time involved in installation. Professional carpenters have a constraint that most do-it-yourselfers don't have to worry about – time. Professional carpenters bid jobs based on parts and time. With top professional carpenters earning hourly wages of $75 and more, it's very often cheaper to buy and [quickly] install metal bracing, versus making wood bracing. And professional carpenters will have trouble getting jobs if their bids get out of line.
Bracing a frame with wood is not usually a very demanding task, as long as you understand one or two things. The cross braces must run at angles to the square of the frame and braces should not weaken installation points due to attaching hardware. 45° cross braces are hard to beat, but when they're not possible, other angles may be engaged. Don't put too many screws and/or nails into the corner you're strengthening or the wood will actually become weak and allow for an easier violation of the corner.
Referring to Diagram 1, frame A is very weak, frame B is stronger, frame C is stronger yet, and frame D is at least as strong as frame C. NB: the use of screws in frames always makes for a stronger corner or cross brace. This is not to suggest that screws should always be used, quite the contrary – they are rarely called for. But they do add to the strength.
In Diagram 1, C is A with four 45° cross braces installed. Some jobs may only call for the installation of two 45° braces. If that's the case, put them both on the top or the bottom. Use appropriately sized attaching hardware.
In Diagram 1, B uses 45° angle corners. This is quite common in small, light duty frames such as those used for pictures. It's really quite strong, but inadequate for large projects. This type of corner is usually reserved for use on smaller lumber, so pick your attaching hardware carefully. This is the perfect place to use a finishing stapler, should you happen to have one. If you use brads, consider pre-drilling to absolutely prevent any splitting.
In Diagram 1, D uses plywood gussets in all four corners. As in the case of C, two may be adequate for some applications. The thickness of the plywood depends on the job – but in this application, even ¼" plywood makes a formidable brace. You can also use a router, or saw and chisels, to notch out the frame so that the face of the work is all flush. The addition of wood glue between the gussets and the frame wood makes an amazingly strong brace. Avoid putting your attaching hardware right at the corner – so it doesn't weaken the frame wood where the corners are initially attached to each other. Once again, this is the perfect place to use a finishing stapler.
In closing – if you're a pretty new do-it-yourselfer, this might be a good time to consider creating a dry place to save ALL of your leftover lumber. You never know when you'll need a small piece of ¼" plywood, or just a few inches of 2" x 2" or 1" x 3". It's not just the money you'll save by not having to purchase wood, it's the time and money you'll save by not having to travel to the hardware store or lumber yard to get it!