Repairing a Rotten Subfloor Floor.
Rotten Floor Due to Damp.
Also, not enough ventilation.
Peeling back the laminate floor, this is the last thing you want to see, Rot. Many suspended timber floors suffer from rot due to bad ventilation and poor damp coursing in external walls and block beam supports. The rotten floor in this image suffered from both of these conditions.
Firstly, all the rotten floorboards and joisting need to be cut out. Open the floor enough to reach in with a torch and see the extent of the damage.
Removing The Rotten Floor.
Using a measuring tape beneath the floor. Measure how far back the effected area reaches. Now mark this distance above the floor, length and width. If possible, cut the floor at the next good joist. Do this by locating the nail line of the good joist and ping a chalk line along it's centre(or a straight edge of timber could be used). Now measure back the width of a joist and ping another line. This is now your cut line. The overhang will centre on the new sister joist. Using a circular saw, cut along your cut line. A hand saw or reciprocating saw is needed to finish the cut close to any walls.
If you cannot reach the next good joist for whatever reason, cut the floor paralell to a joist, and fit a new joist. Clean and remove all debris.
Installing an underfloor Vent
Placement of the underfloor vent, should be centred between two joists(check spacing of original floor joists, 400mm on centre, will usually suffice). Where the vent exits on the exterior wall, this should be at least 150mm from the ground. When working close to a corner, keep the vent roughly 900mm in from the corner. I'm using an Air brick Ventilator, there is an entire range of suitable vent types. Measure and mark the size of your vent on the wall. Now using a masonary drill, with long masonary drill bit(longer than width of wall), drill holes centred on your marked lines(12mm - 15mm diameter drill bit). Then drill some more holes through the centre of area to be chiselled out. Using a demolition hammer, chisel out the weakened block/brick(a good masonary drill might have a hammer action function suitable for this). Fit vent and fill gaps with mortar.
Floor Joist Layout.
Fixing Wall Plates and DPC
The wall plates(mudsills) were also rotten and had to be cut back and removed. Using a roll of dpc(damp proof course). Place the desired length on the block beam supports before placing the new wall plates, cut to required size. The wall plates may require a couple of express anchors to fix them in place. Now mark your layout for new floor joists at 400mm centres, from your first joist. Cut joists to length, place on layout and fix with a couple of 90mm toenails to wallplate.
Finishing the Subfloor
Bridging and Sheathing.
From this point sheathing or flooring boards can be used to close in the area. With sheathing 18mm exterior grade plywood, or OSB3 can be used. Some bridging may be required between the joists at right angles, 90 degrees, where the edges of the sheathing meet. Again the bridging/blocking should be centred under the joint.
Screws and nailing detail
See in this image, osb3 is being used and fixed to the 6x2 floor joists. Notice all the edges are supported by either floor joist, or bridging/blocking. After the sheathing has been cut to size. Now fix with 40mm screws or galvanised grip nails every 150mm on edge, and every 300mm in the field(the field represents the centre area of the sheathing). The new floor covering can now be applied, carpet, laminate flooring, hardwood flooring or tiles(IMPORTANT: If finishing with tiles, make sure all edges and joints between sheathing surfaces, are supported with bridging. The fixing detail also changes to 100mm on edge and 150mm in the field)
Floor Joist Span Table.
Homebond 4th edition
This floor joist span table may help you when working out the distance between joists. The easiest method is to continue the existing layout. More images of a full sub floor replacement.