Expert: Dave Johnson - 12/14/2007

Question
QUESTION: Bought our house 3.5 years ago and it had an addition (really a sunroom) that didn't seem right but to make a long story short, I want to re-build the addition.  Original house built in 1968 in So. Calif. with slab foundation.  Addition has slab and flat roof that leaks a little.  Can I bond the addition slab to the main slab if it isn't already?  Is it better to demo the slab and pour a new one?  I want a sloped roof over the addition, so how do I make the right specifications, such as rafter size and spacing?

ANSWER: Hi Mike,
First, why do you need to attach the 2 slabs? And, if you do need to, it would probably cost as much as a new one.
In your area, I would think a lower pitched roof would be adequate, such as 3 or 4 inch pitch. (example: 3" pitch means 3" of rise in 12" of run). Trusses are a great way to go, for time and money savings. If you aren't bothered by excessive winds, 24" spacing with 5/8" sheathing will be fine.
Alternatively, you could use a "shed" design and put all your run-off in one direction...
Best of luck,
Dave
p.s. mail me if you need more

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

QUESTION: Thanks for the reply.  I was told, but not by a building inspector, I would need to bond the two slabs.  I am guessing to keep the two slabs from moving at different speeds.  My idea was to cut some channels across the slab joint and then epoxy a metal rod to connect the slabs.

On the roof, I don't think I can use a truss because I am just changing the pitch one side of the roof.  The 24" spacing is helpful but what kind of wood and should it be 2 by 6 or something larger?  Shed design is what I have now.  The high point of the shed roof meets the bottom of the pitched roof.

Answer
mike, i was thinking that in CA that there wouldn't be much movement between the slabs...but i since realize you do have the occassional tremor...so i think you are doing the right thing by joining them.
you can build your own trusses by making a pattern (jig) on the floor and making any pitch you like, as well as an uneven pitch.
if you go with trusses, 2"x4" may be used, as long as you make the proper webbing pattern.
if using common rafters, it depends on the rafter length
and if you'll be using joists and collar ties.
please let me know the desired pitch and length of rafters...and if you're having flat ceiling, vaulted ceiling etc.