There's part of your problem right there, the coax you're using sucks raw eggs on those frequencies. You really should be using RG-8 type such as Times Microwave LMR-400 or equivalent, 58 is just fine for mobile use though since rarely more than about 18' is needed.
The other half of your problem is lack of height and you can't use more than a 10' mast on a vent pipe safely. Uh oh, plastic ties are the WORST possible way of mounting it, they were never meant to stand the strain and worse, they deteriorate quickly when exposed to sunlight. Large metal compression type hose clamps are not a good idea either, while they'll stand the strain the mast won't stay upright any more than it will with tie wrap, neither are designed to withstand lateral stresses. Why didn't you use a vent pipe mount in the first place?
Oh I almost forgot! All is well if the pipe is metal and there are no joints above the roof line BUT if it's PVC forget about it altogether. Put lateral stress on that stuff and it will snap, particularly if it's old because the older types tend to get brittle with age. In any case be grateful your landlord allows this and keep him happy by not causing damage of any kind, not in the slightest.
