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Repairing Floor Joists - Methods used for repairing floor joists can vary. If the damage is fairly minor and confined to a few inches to maybe a foot from the end of the joist you can probably just add about a 4 foot scab to the side of it and bolt it to the original. A BASIC rule is scab must be at least 3 times the length of the damage. If more of the joist is damaged a full length sistered joist is recommended. If a joist is 2 boards thick or is 2 boards with spacers between it is likely directly under a wall and may need to be replaced or at least require more special attention.
Repairing fireplace hearth basket The area supporting the fireplace hearth is commonly known as a mortar basket. It is generally constucted of 2 doubled floor joists with a header joist running between them that the floor joists in that area will connect to and also helps support the hearth.
Sometimes you will have damage to just the first few inches of the double joists where they sit on the sill plate. You see this often with water damage. If this is the case you may want to consider placing support posts under the double joists next to the foundation wall. If the damage is more severe the double joists will need to be replaced. Sistering may be ok but chances are to properly support the header joist they will need replacing. Before removing the damaged floor joists the mortar basket (hearth) will need to be removed or supported because you will be cutting out the stuff that holds it up. Your support system for the hearth needs to be very stable because that big chunk of mortar, bricks and concrete will fall on your head. Now you can cut out the damaged joists and header and rebuild them around the basket. If you are an ametuer please tear out the hearth first. It's much safer.
Replacing Header Joists
A header joist is a floor joist that has other floor joists butting into it. Usually at a 90deg angle. Due to it's supporting other joists, sistering or scabbing is not generally acceptable. Typical areas header joists are used would be at stairwells, Fireplace hearths, inset concrete porches and other areas that the joists can not be run full length. Before you start cutting anything firmly support everything that the header supports. Be sure hearths, inset porches are securely supported because if your cutting out the header your probably cutting out the double joists that supports it and you will have a slab of concrete pretty much floating free and yes it will hurt you badly if it falls.
Now if the header has a ledger board. (This is usually a 2x4 nailed to the header that the joists are notched to fit over.) With a sawzaw cut out the ledger and then cut thru the header from top to bottom next to each of the joists that but into it and remove those pieces. You should have little pieces left at the ends of the floor joists. Use the sawzaw to cut the nails holding on the pieces. Get all nails cleaned out of the way. A Dremel tool is handy for that. If you are also replacing the double joists that the header connects to, get them done first. Now slide your new header straight up between the double joists and fasten it in place. Install ledger if applicable and reattach the floor joists to the header. If there is no ledger joist hangers are a good idea. Slowly and carefully remove your supports.
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