Weak points in a structure usually occur at the connections (joints
and splices) between pieces of lumber. However, these connections can
be structurally sound if done correctly. Such weak points are usually
a sign of poor workmanship.
Joints are connections between two pieces of timber that come together
at an angle. The types of joints most commonly used in carpentry are butt
joints and lap joints.
A butt joint is formed by placing the end of one board against another
board so that the boards are at an angle (usually a right angle), forming
a corner. The types of butt joints are shown in Figure 6-4 and are described
Straight Butt Joint. This joint is formed by placing
the square-cut end of one board against the square face of another. The
butt end of one board should be square and the face of the other board
smooth so that they fit perpendicular to each other. Select the right
type of nails or screws to hold such a joint securely. For framing, butt
joints are secured by 8d or 10d nails that are toenailed to strengthen
the joint. The end grain is the weakest part of a piece of wood when used
in joints. A butt joint is made at either one or two endgrain
parts. It will be no stronger than the quality of those
parts. A butt joint is, therefore, the weakest type of joint.
This is especially true if the joint is made of two pieces of
Oblique Butt Joint. This joint is formed by butting
mitered end of one board against the face of another board.
Bracing is typically made with this joint. It should not be
used where great strength is required. The strength of the
oblique butt joint depends upon the nailing. The nail size
depends upon the timber size.
Nails should be toenailed to strengthen the joint; not too many
nails should be used.
Miter Butt Joint. This joint is formed by bringing
mitered ends of two boards together to form the desired
angle. This joint is normally used at corners where a
straight butt joint would not be satisfactory. To form a
right-angle miter joint (the most commonly used), cut each
piece at a 45° angle so that when the pieces are joined they
will form a 90° angle. The miter joint is used mostly in
framing. However, it is a very weak joint and should not be
used where strength is important.
The lap joint is the strongest joint. Lap joints (Figure 6-5)
are formed in one of two ways: a plain lap joint or a half-lap
Plain Lap Joint. This joint is formed by laying one
over another and fastening the two with screws or nails.
This is the simplest and most often used method of joining.
This joint is as strong as the fasteners and material used.
Half-Lap Splice Joint. This joint is formed by cutting
away equal-length portions (usually half) from the thickness
of two boards. The two are then joined so that they overlap
and form a corner. Overlapping surfaces must fit snugly and
smoothly. Overlaps should be sawed on the waste side of the
gauge line, to avoid cutting laps oversize by the thickness of
the cut. This joint is relatively strong and easy to make.
NOTE: Some useful variations of the half-lap joint are
the cross-lap, the middle-lap, and the mitered half-lap
Splices connect two or more pieces of material that extend in
the same line. The joint will be as strong as the unjoined
portions. The type of splice used depends on the type of
stress and strain that the spliced timber must withstand.
· Vertical supports (longitudinal stress) require splices
that resist compression.
· Trusses, braces, and joists (transverse and angular
stress) require splices that resist tension.
· Horizontal supports, such as girders or beams, require
splices that resist bending tension and compression.
For example, splices for resisting
compression are usually worthless
for resisting tension or bending.
Figure 6-6 shows splice types;
splices support weight or exert
pressure and will resist
compression stress only. The most
common types of compressionresistant
splices are the butt splice
and the halved splice.
Figure 6-7 shows splice stresses.
Butt Splice. This splice is constructed by butting
the squared ends of two pieces of timber together and securing them in
this position with two wood or metal pieces fastened on opposite sides
of the timber. The two short supporting pieces keep the splice straight
and prevent buckling. Metal plates used as supports in a butt splice are
called fishplates. Wood plates are called scabs and are fastened in place
with bolts or screws. Bolts, nails, or corrugated fasteners may be used
to secure scabs. If nails are used with scabs, they are staggered and
driven at an angle away from the splice. Too many nails, or nails that
are too large, will weaken a splice. Halved Splice. This splice is made
by cutting away half the thickness of equal lengths from the ends of two
pieces of timber, then fitting the tongues (laps) together. The laps should
be long enough to provide adequate bearing surfaces. Nails or bolts may
be used to fasten the halved splice. Note: To give the halved splice resistance
to tension as well as
compression, fishplates or scabs may be used.
In members such as trusses, braces, and joists, the joint undergoes
stress in more than one direction; this creates tension, buckling the
member in a predictable direction. Tensionresistant splices provide the
greatest practical number of bearing surfaces and shoulders within the
Square Splice. This splice is a modification of the
compression halved splice. Notches are cut in the tongues or laps to provide
an additional locking shoulder. The square splice may be fastened with
nails or bolts. Note: It may be greatly strengthened by using fishplates
Long, Plain Splice. This splice is a hasty substitute
for the square splice. A long overlap of two pieces is desirable to provide
adequate bearing surface and enough room for fasteners to make up for
the lack of shoulder lock.
Horizontal timbers supporting weight undergo stress at a splice that
results in compression of the upper part; this has a tendency to crush
the fibers. Tension of the lower part also tends to Figure 6-7. Splice
stresses pull the fibers apart. Bend-resistant splices resist both compression
Make a bendresistant splice as follows:
Step 1. Cut oblique, complementary laps in the end of two pieces of
Step 2. Square the upper lap (bearing surface) to butt it against the
square of the other lap. This offers maximum resistance to crushing.
Step 3. Bevel the lower tongue.
Step 4. Fasten a scab or fishplate along the bottom of the splice to
prevent separation of the pieces.
NOTE: When this splice cannot be done, a butt joint, halved
splice, or square splice secured by fishplates or scabs may be used.