Sistered joist fastening questions

ntruroDecember 10, 2009

When sistering a new section of floor joist to a water damaged end of a floor joist, how should the new piece be attached to the existing one? The joists are 2x10 hemlock, 16" on center, span is 10.5', and in a crawlspace. The damage is only to a few inches of the end of each joist.

From searching the web, in the UK, the preferred method is with through bolts or (threaded rod), large square washers, and large nuts. Another UK option is to use something called a Bower Beam - essentially a steel trough made of two L-shaped (cross-section) steel plates that is attached with carriage bolts to the end of the damaged and new joist. Does anyone know a US supplier of Bower Beams?

Many US websites recommend using lots of 16D nails. Others recommend using lots of 3" long deck screws - some say the holes should be pre-drilled. I've also read that all fasteners (bolts, screws, nails, etc.) should be placed a minimum of 2" inches above the bottom edge of the joists. And that if using screws or nails, they should be driven in from both sides - new and old joist sides.

Questions:

Are through bolts better than nails or screws?

How many is "lots" of screws?

Assuming 3" decking screws, what pattern should be used, i.e. how should they be spaced? Should they be driven in from both sides? Should holes be pre-drilled and, if yes, then what size should the holes be?

Thanks in advance.

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sierraeast

Here's a link that might help. You can also get the nailing schedule requirements from your local building dept. Out here on a 2x10, it's two 16d's, top & bottom of the joist, every 16" on one side of the joist, with two 16d's on the opposing side, other joist, between them, esssentially giving you two 16d's every eight inches.

Your building dept will have nut ,bolt, washer sizings as well, similar to requirements on deck connects.

You might also look into flitch plating if it's a major structural concern.

Here is a link that might be useful: sistering

    Bookmark   December 10, 2009 at 11:08AM
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brickeyee

"... in the UK, the preferred method is with through bolts or (threaded rod), large square washers, and large nuts."

The problem that occurs with bolts is you need a decent number of smaller bolts to not exceed the allowed loading of the wood (compressive loading) at the fasteners, and to make sure the loads are transferred as evenly as possible.

I would guess that they are not using 2x lumber, but something thicker (and possibly even hardwood).

16d nails are worth around 300-600 pounds of shear load per nail. The actual value allowed depends on wood species and grade.

Lots of 16d nails has the advantage of spreading the loads out among the fasteners to avoid exceeding the compressive strength of the wood.

even in flitch beams, fastener size and hole clearance through the metal is critical.

There have been tests using powder actuated drivers to run the nails through the wood and metal in flitch beams. It appears to work well since the nail punches a tight hole through the metal flitch.

    Bookmark   December 10, 2009 at 1:19PM
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macv

Sistering is adding a full length joist and only minor attachment is needed to the old joist. Reinforcing a portion of the joist is called scabbing. At this point on the joist the stress is almost entirely shear so a metal plate secured with nails should work well. Metwood makes some preformed reinforcing plates for notches and they might work for your condition.

Here is a link that might be useful: repair plates

    Bookmark   December 10, 2009 at 1:36PM
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macv

If you want to use a British repair plate try this company.

Here is a link that might be useful: British plates

    Bookmark   December 10, 2009 at 1:38PM
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