Don't even think of replacing that tank. You must also replace the O ring. If so much as one tab cracks off, and it will, you are done. I did an experiment on my own car that had a cracked plastic tank. It started out as a 1 inch crack where they over tightened the bolt to hold the fan on. I used plastic weld. It held for awhile but the crack grew even after drilling the " holes ". After the crack grew to about 5 inches i replaced the rad. I took the tank off. Well, you should have seen the inside of the tank where the crack was.It was completely etched away and the plastic tank was about as thick as a dollar bill where the hot gasses and anti freeze leaked out eating away at the plastic tank. There was nothing left of the plastic where the leak was as it ate away the plastic from the inside.You know, the place you never see. John you were so bloody lucky. I wasn't. A 390 came into the shop one day. Just off the highway. Supposed to have the cap and wires replaced.Opened the hood, removed the cap and BANG the bypass hose let go. I was very lucky. Only spent 2 days in the burn ward and 4 weeks off work. When i got new skin it was the softest skin i've ever had. Baby's bum. My whiskers still grew back so that didn't change.Lost the scar under my left eye and the freckels on my nose. Lost the liver marks on my neck but they came back. I actually made more money on compensation than i did in the whole 2 months before the accident. Good stuff. Well if you want to call having all your faceial skin fall off good stuff. Yes it does actually fall off. I'm actually very lucky. I only have one " floater " in my left eye. The right eye lid feels tight but that is probably old age and I can still see with both eyes. Thank god. Yah he was probably with me under that hood. Actually he was under the hood with me. I can still see. could have lost my sight. Anyone that has been burned with hot antifreeze knows what i mean. Where i live water boils a 207 degs. F. anti freeze under pressure boils can boil at 260 F. Now we are getting into science.

Gasfitters quite often run their lines through the attic. Do you know if the pipes are black iron (heavy, screwed together into fittings, old-style) or copper? There's a new gas line out now, it's a flexible stainless steel with a yellow jacket. I really like it, and it, or copper will probably be the fix for your problem, when they find the leak if they have to run some new line. Black iron lasts forever, but it can leak. Gasfitters are supposed to pressure test it when they put it in. Why something would leak seven years later is a mystery, have you done any work on the house - installed an alarm system for example, that would involve drilling up into the ceiling? As for the smell, you might have gotten used to it, or it might just be the effect of that mercaptan (the scent agent they add to natural gas to allow you to smell a leak, it's otherwise odourless.) Have you or anyone else looked up there yet? The other possibility of the tar smell is you have some sort of issue with your roofing, assuming you have asphalt shingles. Let us know what the outcome is. I know you said the house is 7 years old, you MAY still have some building warranty left, or your builder may stand behind it for good will, if it's out of warranty. Have you contacted them? They may have had the same issue in other places built around the same time. One row of townhouses I worked on had to have all the black iron pipe pulled out...the gasfitters had bought a new threading machine and they hadn't adjusted it correctly...when they pressure tested it, all the joints leaked. It was at least 8 or 9 townhouses, and all the gas lines ran back to one end, so it was a big job, but they replaced it with the stainless stuff I mentioned. Much faster to work with. Only downside is, a nail or screw can puncture it. Black iron's nowhere near as vulnerable. Here is a link that might be useful: Mercaptan

You are right to suspect that liners are for straight runs. They'll need to replace the Ys.

Some good advice above. My thoughts: 1) get that downspout redirected and dumped AWAY from the house. From how it looks, it dumps out right next to the wall and drains right back to the house. That would be like grading your yard to slope toward the house and dumping your downspouts and sump pump right into it. 2) The kick out flashing(on the gutter by the wall) is the right idea, but the end of the gutter shouldn't be right against the wall. The kick out flashing exists so that you can move the gutter trough off the wall but not have water drain through the resultant space. 3) As the others have said, the roof to wall flashing looks very suspect. Looks like maybe some sort of repair to stucco and shingles in that area? If I were to try and address this, I'd also water test that window directly above the door. The issue is more likely at the roof to wall flashing area, but the window is another plausible source. If the window tests fine, I'd do the following: - cut back the stucco the width of a trim board (4 or 6") to pull out all of the janky flashing, including the step flashing. -Tear off the shingles on that small section against the house. Inspect all areas for water damage and repair as needed. - Ice and water that entire section and run up the wall, providing a continuous barrier at that roof to wall joint. Install new step flashing with shingles -Install trim board over the area that was cut back for repairs, OR you could have a stucco pro come out and patch the stucco. -Move that gutter away from the wall, install the new kickout flashing. You may have to replace a few sections of gutter in the process so that it all looks nice and doesn't leak at the seams. Lastly, if you are going this far, replacing the flashing over the door may not be a bad idea either. Similar thought process, cut the stucco back, add new head flashing integrated with the WRB, and then cover and seal with either a trim board or stucco patch. Adding trim boards would change the look a little, but you could minimize that by using a fiber cement board with a similar texture and paint color if you were so inclined. That may or may not look great, something complementary or stucco patch repair would probably look better. There looks to be a seam not that far above, so that may not be all that bad. I would recommend that this all be completed by a professional.