i wonder if any of you good souls could give me some advice ,i have a upvc door on my house it is two bricks up from the surface of the drive to the bottom of the plastic step (the same height as the plastic damp proof strip that is sandwiched betwen the bricks and the door is flush with the outside wall(there is no porch).myquestions are: what are the gaps in the vertical brick jointsat or just above ground level, i read somewhere they are cavity drains??? or vents?? also why are they below the level of the d. p.c (every 10 or so bricks,this is the level of the solid house floor) and also can i seal all the way around the out side of the upvc door frame they have done the sides and the top of the frame(in view) but not under neith the threshhold (out of sight)!!p.s there are none of the slots in the vertical brick joins under the threshhold so could i silicone seal all the way around the undernieth of the step( the only reason i wonder did they not do it was it is ia little hard to access i was just thinking it isnt very high if it snows moisture could creep in between the bricks and the bottom of the door sill sorry to waffle on a bit but i know nothing about this subject so all donations will be gratefully recieved!!