What’s the best way to frame a T&G fir floor on an L-shaped porch to get sufficient slope for drainage in both directions?
A.Scott McBride, owner of Mustard Seed Master Builders in Sperryville, Va., responds: There are two good ways to deal with this situation — by running floorboards into a separate meeting strip (see top photo, below), or by weaving their ends together in a herringbone pattern (bottom photo, below).
Credit: Scott McBride
The framing is the same in either case: Run a triple 2-by girder on a diagonal from the corner of the house to the outside corner of the porch, and tie the porch joists to the girder. The joists should lie parallel to the house walls, so that water will run off along the length of the floorboards, rather than across them. Since a porch floor is typically pitched 1/8 to 1/4 inch per foot, the girder will need to be pitched as well, much like a very shallow hip rafter.
To fabricate the meeting strip, select a straight piece of flooring — preferably with vertical grain to minimize shrinkage — and glue a spline into the grooved edge, which will give you a floorboard with tongues on both sides. Fasten the meeting strip on top of the hip girder with construction adhesive and blind nails, then start laying the full-length boards from the end of the floor farthest from the corner, tongues toward the meeting strip. When you reach the transition, miter the inboard ends of the boards and groove the mitered ends with a router to engage the tongues on the meeting strip. Cut the boards long by an inch or two and let the outboard ends run wild, trimming to a chalk line after all the boards are laid.
In the herringbone method, the floorboards are chopped square, grooved across the ends, and laid alternately so that each end-groove engages the tongue of the preceding board. Practice with a few scraps to get the hang of it. Because opposing floorboards lie slightly out-of-plane with each other — owing to the change in direction of pitch — a little belt-sanding will be required to smooth the finished surface.
A limitation of the herringbone method is that the zig-zag falls along a 45-degree intersection, so it can only be used where the adjoining porches are of equal widths. It’s also more time-consuming than using a meeting strip, but the finished look is terrific. The herringbone pattern is accentuated because the alternating grain causes the light to reflect differently from the two surfaces.