Over the years, I’ve noticed myself getting more and more away from miter joints. In fact, just before writing this point, I was contemplating using a miter joint for a detail on our new house, but quickly rejected the thought. In this post, I’ll go in depth on why I think a simple butt joint is superior to a miter joint – most of the time – despite seeming more primitive.
Both miter and butt joints serve the same function: they connect two building materials together where the ends meet, such as the corner of two walls, a table, or a picture frame.
When to Use a Miter Joint
In woodworking and carpentry, there are very few situations in which a miter joint is absolutely necessary. Miter joints are most commonly used to connect right angles at corners, but this is merely an aesthetic choice. I’ll admit, a miter joint is a more stylish and impressive looking joint, but again, this is just a superficial detail. Most of the time, both a miter joint and a butt joint can get the same job done.
Some scenarios that may require a miter joint, however, include: the point at which two rafters meet each other in an A-frame or gambrel style roof, and the webs of a roof truss. Miter joints may also be preferable when working with materials other than wood, such as steel tubing or pipes.
In woodworking, miter joints are infamously weak. To produce a strong miter joint, a spline – a thin piece of wood inserted in a bored out slot – or a dowel must be used. On the other hand, it is much easier to produce a strong butt joint.
Suppose we are connecting two boards at a right angle – one vertical board and one horizontal board. We can overlap the edge of the horizontal board with the edge of the vertical board, and fasten it down to the horizontal board, using gravity for added strength.
If two horizontal boards are meeting, and we can’t take advantage of gravity, nor are fasteners alone – screws or nails – enough to produce the strength necessary, we can reinforce the joint with dowels, a biscuit, or a spline. In my experience as a carpenter, though, I have never needed anything more than fasteners and/or wood glue to produce a strong enough butt joint.
Joint Failure Due to Expansion and Contraction
A wooden miter joint will gradually push itself apart due to the natural expansion and contraction of wood. This happens because wood expands and contracts across the width of the board but not the length – not to any significant degree.
As the width of the boards expands, the outer seam of the miter will get pushed apart. Conversely, as the width of the boards contract, the inner seam will separate. After repeated instances of this phenomenon, the joint will eventually separate entirely. This is usually not an issue for indoor furniture, cabinets, trim, etc… but miter joints should never be used outside, where the expansion and contraction of wood is much more frequent and dramatic, unless absolutely necessary.
A butt joint, on the other hand, is far less likely to push itself apart because only one of the boards is expanding into and contracting away from the seam of the joint.
Though a miter joint is not exactly a hard joint to make, it is more difficult than a butt joint. The easiest way to make a miter joint is with a miter saw, hence the name. However, if you don’t have a miter saw, a circular saw can also get the job done, it just takes a bit more carefulness and precision. A butt joint can easily be made with any saw since it is just a straight, perpendicular cut that does not require any angles to be measured.
A miter joint always produces more waste than a butt joint. This is the main reason I no longer like to use them if they can be avoided. For example, think of it this way…
Let’s say I want to put trim around a window that is 4′ tall and 2′ wide, using standard 1×4 boards. I’ll need to cut my vertical board to a length of 4’7″ so that I have a 3-1/2″ overlap on the top and bottom. I’d then make a 45º cut at each end of the boards, removing a small triangle. For the horizontal boards, I’ll need to cut them to 2’7″ and do the same thing. I now have a miter joint at each of the four corners.
So, how does this produce more waste? After all, even if I use a butt joint instead, I still need to cut the boards. Well, small lengths of rectangular boards come in handy. I find myself using scraps like that all the time. However, I’ve never been in a situation where I found myself saying, “You know what I really need right now? A small wooden triangle of no specific dimensions.”
Most people probably won’t care about this nominal amount of waste, but I am a scrap hoarder. I can never have too many pieces of wood cuttings that could potentially be used in a future project. That said, I know those tiny triangles are just gonna get binned.
By no means am I saying miter joints are stupid or that they don’t have their place – they certainly do – but more often than not, a basic butt joint is all you really need.