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Ground Floor

Exterior wall framing

Correct interior wall framing-. Same height of cross grain wood as in exterior wall

Incorrect interior wall framing-. Less cross grain wood than in exterior wall

1. Equalizing Cross Grain Wood in house Framing

1. Masonry components of a wooden building, such as chimneys and exterior facings, do not shrink appreciably when compared with the wooden rrame. Details of structural attachments and flashings that connect the wood and masonry must be designed to accommodate the differential movement. A sliding masonry tie will allow the frame to shrink without stressing the brick facing.

: The flashings and counterflashings around a masonry chimney slip freely to allow the roof to drop a fraction of an inch in relation to the brickwork.

2" clearance

3. Chimney Flashing at

Roof

As its moisture content changes, wood shrinks and swells a great deal across its grain, but very little along its grain. For purposes of detailing ordinary wood buildings, we assume that the longitudinal shrinkage and expansion is essentially zero. Movement in the cross grain direction is large in magnitude and is difficult to quantify, because tangential movement (movement along the growth rings) is substantially larger than radial movement (movement perpendicular to the growth rings). In the plainsawed lumber that we use for framing, we can't be sure whether cross grain shrinkage in a joist or a plate will take place at the radial rate, the tangential rate, or something between the two. The best we can do is to assume that the rate will be something between the two.

1. In platform framing, which is the most common way of structuring a house, each level of floor framing interrupts the vertical studs that form the walls and that support the upper floor and roof. The studs will shrink verv lit-

j tie in the vertical direction of the building, but the floor framing, even if it consists of seasoned lumber, will shrink bv a substantial amount, often Va" to per floor (6 to 9 mm). When detailing wood platform framing, it is important to provide equal amounts of cross grain wood at each level so that shrinkage will not cause tipping of floors or distress in interior finish materials. It is good practice to leave a space of about Vi (12 mm) in the sheathing panels at each floor platform, because the platform will shrink considerably and the panels will not.

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