Doubling up joists from support to support improves carrying capacity. Two other reasons for sistering involve strengthening joists for structural support. Refasten the deck boards with approved fasteners that won’t cause rot. Cut out the rot and sister a board of equal width and thickness where the damage is, or the full length. The damage may be superficial or only where nails or screws penetrate, leaving much of the plank structurally sound. Some joists experience rot and decay more quickly than others. The added piece may be the same length as the original or extend two to four feet beyond the weakened area. Lifting and leveling the damaged section and then sistering it with timber of equal dimensions strengthens and repairs the problem. Over time a joist may sag, become spongy, cracked, or split. This often requires the use of bolts or lag screws for fastening the sandwiching boards with the joist, especially if the joint is unsupported.Ī common reason to sister joist is to effect a repair. Sandwiching the butt joint of two joists between lengths of similar material will also lengthen a joist to span acceptable distances if longer boards aren’t available or won’t fit. It utilizes an existing support structure but may require the opposite end of the new deck to be supported. Overlapping new joists with older joists where they cross a support beam or wall will tie the new deck in with the old. There are many reasons you may need to sister deck joists in one way or another.Įxtending an existing deck is an obvious reason to sister joists. Sistering is the overlapping of joists to extend the span or length, or pairing or doubling up to repair or strengthen an existing joist. Scabbing however often is done using plywood or mismatched lumber. This is also known as double sistering or scabbing. The term sistering can also apply to sandwiching a joist between two boards, or when joining two planks together by sandwiching the butt joint between two shorter lengths of similar dimensions. This is frequently done to strengthen a rafter or joist. The sistering piece may even run the full length of the existing joist except where it is supported at the ends. The two planks should be similar dimensions and be fastened securely together from both sides.Īnother way to sister is to attach a length of similar lumber along 1/3 or more of the length of an existing joist or rafter. Adding a second joist along the full length of another is also known as doubling up, or a double joist. This may be done during construction or later in the life of the structure. Pairing two boards together along their full length where they are supported at both ends is another way to sister joists or rafters. The longer the overlapping distance, the stronger the structural connection produced. It may be used to bond new construction to an existing structure, or when lumber of required lengths isn’t available. It may refer to overlapping and securing two planks of the same thickness and width where they cross a beam or bearing wall to extend the length. Sistering depends on the purpose and structural requirements, and any code parameters. Sistering is a term with a variety of structural purposes where a length of similar dimensional lumber is attached to a joist or rafter for reinforcement or extension.
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