I
know what you mean about there being gaps between the sub-floor planks.
The main floor on our 1907 project house has that type of sub-floor (but
not on a 45 degree angle) and there are some gaps big enough to lose a
dog through.
Right now we are in the middle of
remodeling a first floor bedroom. After we tore up the old fir flooring
we laid down a layer of 7/16" Oriented Strand Board (OSB). In
hindsight, plywood would have been better for longevity. (I recently
read about a study that demonstrated how OSB tends to trap water vapor,
sometimes leading to fungal growth and rot.)
I like adding another layer of
sub-floor structure (even 3/8" plywood will add a lot of strength)
because many old houses have structural wood that is cracking badly. New
plywood will help tie together the old framing members, and it will
sandwich the old sub-floor in place. This is also an opportunity to pull
out any loose nails and replace them with deck screws, thus eliminating
squeaks. And the best approach to installing plywood is to use deck
screws (we like Deck Mate screws because they are very strong) instead
of nails. Pre-drilling the holes helps to ensure that the screw heads
don’t stick up. I see no reason why the plywood edges need to break
directly on top of the floor joists. But make sure to mark the location
of the floor joists (on the wall) before you cover them. Plywood needs
to have fasteners all around the perimeter and also "in the
field", or middle section of the panel. These latter fasteners
should be located so they hit the floor joists. The proper length for
these fasteners is to achieve 1 inch of penetration into the joist.
I view plywood and 1/4" thick
cement-board as better than just 1/2" thick cement-board. I have
seen many people just nail their Durock or WonderBoard down with
galvanized roofing nails. This is supposed to be adequate. But when I
read about using thin-set mortar to bond the cement-board to the wood
sub-floor, that seemed like a superior method to me. (It was recommended
in a book called "Setting Tile" from Taunton Press.) But to
work properly, the sub-floor should be coated with a good primer.
I once spoke to a field representative
about the "Rock-On" brand of screws mentioned in the
article. He explained how the screws have a coating that resists the
caustic properties of cement-board, better that simple galvanizing. And
the serrated screw heads really do a good job of digging a countersink
hole, so the heads hardly ever stick up.
If thickness of the flooring is a major
issue and you cannot add plywood, you certainly can lay down
cement-board on top of the existing sub-floor. If you do bond it with
thin-set, you are bound to drip a lot of mortar through the gaps. This
could be a problem if there is a basement below (but a simple drop cloth
could prevent any mess). Again, the wood should be cleaned and primed,
not much will adhere to old bare wood. Maybe in this scenario your best
solution is to simply lay down cement-board with screws, if the floor
seems to be intact and not to springy. Most people don’t go to the
extreme of bonding the cement-board to the floor. But I am very pleased
with the results of the floor in the article. It’s been 9 months and
there has been no sign of movement, no cracks, no bounce, no squeaks.
Solid as a rock.
When installing cement-board, the edges
don’t have to lie over the joists, just make sure to secure the
perimeter of the panels using fasteners every 6 to 8 inches. If you do
use roofing nails, 1-1/4" long should work fine most places, but
where the nails can enter the joists, 1 inch of penetration is best.
The main reason I prefer using screws
is that nails tend to back out over time. You could get new floor
squeaks from this problem, or a springy spot, though it might take years
or decades to show up.
We received a sample of special
spiral-shank roofing nails made by Maze. They are intended for nailing
cement board. But I don’t know what stores carry them.
Bruce W. Maki, Editor.
