working with back injury....

Hi,
Im 26, 10st, and new to body building.
I have hardly any fat on my body and im a builder by trade breaking concrete all day and laying blocks etc, so i get a fairly good workout/calorie burn during the day

The past couple of months ive been getting interested in working out more so i started weights.
Ive sorted out a diet and i eat between 6-8 times a day, 3 big meals with small snack/meals between.
My diet is even, lots of carbs and protein with fat included (was informed that fat helps produce test.)
id say my calorie intake is around 4500 a day including 2 weight gain shakes and a protein shake i have after a workout.

Currently im working my arms etc monday, legs on weds then various other excersises combining both on the friday with rest sat/sunday and days in between. (have been informed rest is important)

im taking creatine before and after a workout as well as a few other supplements which ive read about in variuous magazines and books (arginine, multi vitamins etc)

past 3 months ive been using weight gain and increased my calories with shakes and food throughout the day.
combined with the above routine i have gone from 9 1/2st to 10st.
i havent gained a great deal but i feel a hell of a lot stronger.
so much so that im doing more work breaking concrete etc and dont feel tired afterwards... so im glad for those gains.
but as with everyone im after size.

i will be doing a good search of this site for more information... the more info i get the better, but one thing that im not sure about being covered is working the back.
especially with an injury.

i crashed a car about 4 years ago and tore the tendons in my spine... was  a bad crash which left me unable to do much heavy work... hence dropping from 11st to 9st.
my back has healed but it still hurts when i do heavy work... like a niggling pain that you gets used to.
my concern though is lifting weights... and how far i can go without injuring myself....

has anyone here suffered back problems and been able to work thier back?
ive been doing arobic exercises such as lifting my hands and feet off the ground whilst laying on my stomach.
ive been able to these ok... but earlier i lefted a heavy weight above my head and my back suddenly gave way... and that got me concrened about how i can train.

i train at home with a bench and set of dumbells etc as i dont have time to go to the gym.
i get in from work at 7:30pm and up at 6am along with doing office work in the eveneings (downside of running a company)
but im managing to do an hour or 2 every other day mon/weds/fri working out.

just realised how much ive written... its like an essay, sorry about that.
so this is more of a hi, im new... and how do i work with a back injury post :)

p.s. its not the kind of injury that rest will cure, so advising me to rest up wont help my back.