Section 3.1 "Before you start" advice notes
Some "before you start advice notes".
Read all the Sub-Sections in Page 1. This will help you get an understanding of things.
Skim through all the pages to get a feeling for the project.
Any names or terms you are not sure of? Refer to the glossary of terms.
Use the contents menu at the bottom of every page to jump back and forth as need be,
or go to the Main Index page for a more in-depth listing.
Don't forget all measurements are given first in standard, followed by metric measurements in brackets. There is more about that in
Cutting the bearers and the floor joists
Cut the two 4" x 4" (100mm x 100m) bearers at 81 1/2" (2040mm) long.
Cut eight 1 1/2" x 3 1/2" (90mm x 45mm) floor joists 80" (2000mm) long.
Rip (cut down lengthwise) a 1" (25mm) strip off two of the joists. In other words, turn a couple of the 3 1/2" (90mm) wide joists into 2 1/2" (75mm) wide joists.
The six common joists need to be rebated at both ends
(see the pictures).
Do this by first clamping them together on a couple of saw-horses.
Make sure the ends are all in line and square, and that the tops are flush.
Make a pencil-line across the top of the six joists, 4" (100mm) in from each end. That defines the lengths of the rebates (the rebate area).
Set the blade on a circular saw to a depth of 1" (25mm) and make multiple cuts across the rebate areas,
and then clean them out with a chisel.
Ensure that the distance between the rebates at each end is
Making the sub-floor
Lay the two 4" x 4" (100mm x 100mm) floor bearers on a flat surface so that they are apart 60" (1500mm) overall.
Nail the two narrower (ripped) joists to a couple of the common joists.
Use 3 1/2" (90mm) galvanized flathead nails, 12 to each pair, staggering the nails along the length of the joist.
That makes a pair of double joists, one pair to go each end of the bearers.
Mark 10" (250mm) in from the ends of both sets of double joists.
That is the amount that the joists will overhang the bearers.
Place the double joists on the bearers, one pair at each end.
Ensure the bearers are 60" (1500mm) apart overall and the joists overhang each side of the bearers by 10" (250mm).
Once in position, nail the double joists to the bearers.
Ensuring the floor frame is square
Check to see that the work thus far is square.
This can be done by measuring diagonally from corner to corner in one direction, and then by measuring diagonally in the opposite direction.
If both measurements are the same, then the frame is square.
If they are not the same,
then the frame will need skewing a little until the diagonals are the same.
The page contents are listed at the bottom of every page but the main index page also lists every sub-section.
Completing the sub-floor
Run a taut string-line from the end of one joist, to the end of the other.
Pack the string-line out from the joist ends with a nail.
Lay the intermediate joists along the bearers at 16" (400mm) O.C. (ctrs) and align the ends a nail's breadth away from the string-line and then fix them to the bearers.
Cut and fix three rows of blocks between the joists. A row along each side (beginning where the rebate ends) and also a row with its centers being 48" (1200mm) from one side
(where the rebate ends). It doesn't matter which side, as
that row is solely to support the plywood floor join.
Check the diagonals again, as the sub-floor may have skewed slightly with all the hammering.
Laying the floor
Cut two pieces of 3/4" (19mm) plywood. One at 78" (1950mm) long x 48" (1200mm) wide, and another at 78" (1950mm) long x 24" (600mm) wide.
Lay the floor in place.
Screw the floor sheets temporarily. The floor sheets may have to come off when the floor is moved to its final site, to allow the bearers to be packed, dug in, or whatever will be required to level and secure the sub-floor.
However, if you are building the subfloor on site, and have it level and anchored, then by all means nail or screw the floor permanently.
Meantime, the floor makes a good working surface to make up the wall frames.