Expert: Bruce E. Johnson - 9/7/2008

Question
QUESTION: if my decking boards are 32mm thick and im using 100x50mm joists (on concrete base) what length decking screw would you recommend?
kind regards

ANSWER: Hi dave, if your joists are on edge you can use a 76mm screw (if there is such a thing, I use 3" screws which work out to be 76mm) So whatever metric size you have that is close will work.  If your joists are laying flat then you will need to use something like a 63mm screw (2 1/2") Generally speaking your screw should be at least twice as long as the depth of the decking so if your decking is 32mm then 63-64 mm screws should be fine.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: hi thanks for that!!
just one more quick question!
the area i am decking is 14ft x 12ft with the main joists running the 14ft long length!! i have read that they need to be spaced out at .75 meters or about 2.5ft so i will need 5x 4x2's but at what distance do i put the cross sections in at to join them together? is this also at .75 meters? i know i will have to offset them slightly so as to screw them to the main joists!! they will be lying on the edge not flat!!
kind regards
dave!
ps this site is awesome!!

ANSWER: Hi again dave, The way I would frame this floor is to lay my studs (joists) on edge as you are, then I would have a rim joist on each end, similar to framing a wall with a top and bottom plate. I would nail or screw my top and bottom plates into the ends of the studs.  Then I would put one row of bridging (joist material on edge and inserted perpendicularly between the joists)  down the center of the floor.  This will keep the joists from twisting over time.  Normally bridging is only required every eight feet or approximately 2 1/2 meters.  In your case with 14' joists a row of bridging down the center or at 7' will fall within the minimum requirements.  And yes you can offset the bridging blocks back and forth from the center line to facilitate the nailing to the joists.   I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Hi Bruce!!
thanks for that again!!
got another question sorry!!
if the concrete base is not level in places what would put under the joists and would what ever it is need to be fixed to the joists or concrete??
kind regards
Dave

Answer
Hi again Dave, if you are talking about just a low spot in the concrete here or there the joists you are using should span close to a meter without the need for additional support.  Otherwise a tapered shim can be used to fill in the gap.  Where I live I can buy cedar shims or shingles that taper from a half inch down to nothing and when you use two together from opposite sides it makes for a nice solid fill.  If you are working with a sloping slab and want to raise your entire floor system to level then I would determine the lowest point and figure how much that end needs to be raised to make it level.  Say it is the thickness of a 1x4.  Then I would tapcon a 1x4 onto the slab all the way across that end.  Then in two to three foot increments I would shim the joists up the remaining distance back to zero.  If you get my drift.   I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com