Re Roofing under a roof mounted AC Unit


#1

Hello,
I am looking to re roof my house from wood shakes to asphalt shingles. Currently there are slats under the shakes. After pulling up the shakes I will be replacing the slats with OSB sheathing. My question is what is the best way to install the sheathing under the AC unit without taking the AC unit off of the roof. I have roofing experience but have never dealt with roof mounted AC units before. If someone could shed some light on this situation it would be greatly appreciated. Thank you in advance!


#2

Use a jack (bottle jacks work well) and blocks of wood. Jack one side up, block it and change the decking under the unblocked side. Rinse, repeat… You can usually run your plywood directly over the top of the existing skip sheathing also if conditions permit.


#3

Just before the roof is started is the time to do yourself, the roofers and your roof a big favor. Come off the extra cash and get that AC unit off the roof.

I was called out to estimate a replacement a few weeks ago where it had been leaking around the AC unit for so long that it had caused so much rot that it was sunken in and about to fall through the roof and then the ceiling. I told them that I would not do the job at all unless the AC unit was re-located to the ground first.


#4

[quote=“LuckyChucky”]Just before the roof is started is the time to do yourself, the roofers and your roof a big favor. Come off the extra cash and get that AC unit off the roof.

I was called out to estimate a replacement a few weeks ago where it had been leaking around the AC unit for so long that it had caused so much rot that it was sunken in and about to fall through the roof and then the ceiling. I told them that I would not do the job at all unless the AC unit was re-located to the ground first.[/quote]

This is the best advice. When we see AC units on the roof, we usually inform the customer in the proposal; “Purchaser to disconnect and remove compressor units from the roof before the start of any roofing work.” If we are going to work around the units, “Although the utmost care will be taken when working around the compressor units, we can not be responsible for kinked cracked or otherwise damaged lines. If this is a problem, please contact your HVAC technician to remove and/or recharge the units from the roof.” Everything is negotiable.

Also do yourself a favor and use CDX. 4 ply 1/2" will be adequate since you are going over spaced decking.


#5

Hello - I am looking for similar advice. I am applying asphalt shingles over an existing asphalt shingle roof and have a roof mounted AC unit. I used the jack idea to lift the side of the unit supported by angle iron on the down slope side and it worked great. The other, opposite edge of the unit sits on the roof with duct work coming out that same side. How do I fasten shingles under the unit? There is no room for a gun or hammer, or even me, where the roof meets the underside of the unit. Ideas? It is a large and heavy unit, I would prefer not to remove it if I can avoid it. I live in the foothills where we have blowing snow that could get under there and thus the melting snow would run under my new shingles as it sits today. Thank you!


#6

Post some pics if you want good advise.


#7

Best advice I can give you is have the unit disconnected and doit right. It is much cheaper and easier to do It that way in the beginning rather than have to come back in 4 years and have to remove the unit to fix the leak.


#8

Hello IslandRoofing. I am working to up load a photo.

!


#9

Thank you MPA. I am not opposed to disconnecting the unit to roof under it, I just need to figure out a way to lift it and support it while I do that. I am sure I can open the unit up, find the appropriate screws to detach the ductwork, and then reattach and seal.


#10

As for nailing them you can get a cheap ($30ish) palm nailer and nail as far under the unit as you can and simple tar the remaining shingles in place. However there is a much bigger issue that you will not properly be able to flash the shingles against the wall behind the unit. That AC unit has to go to do that unless you want that area kept waterproof with caulk and a prayer.


#11

Nailing th he shingles under the unit doesn’t look like a big deal. As island said a palm nailer will help. Might get shot on here for saying this, but in that situation I have set the shingles in cement and used some lath screws with big flat heads and a right a right angle drill.

I would be more concerned with how to properly flash the duct work where it penetrates the Shingles. Right now it looks like the shingles are butted to the side, is there a flange on duct that runs under the shingles?.

You want to make sure the bottom horizontal of the duct is flashed properly. The bottom corners where the water runs down the side of the duct and can hook underneath ate the most common place for a leak to happen.


#12

I was wondering about just using roofing cement to fasten shingles in tight areas. Today I beat some nails in under another tight situation using the side of my hammer - but since I just read your advice, Amazon has been called into action and a palm hammer is on the way! Thank you both for the suggestion on how to get nails in, and more importantly, the need to check the flashing associated with the duct. I would have never thought about the downhill side being an issue, but I see your point. I will give it a hard look, and with the palm hammer and cement idea, I think I will disconnect the duct from the unit, raise the unit up enough to get a shingle under it using a jack, then reconnect it.

Islandroofing and MPA, I deeply appreciate your taking the time to help me.


#13

You always want to try to get some fasteners in thlle at ngles. Not sure what part of country you l are, in but petty cold in pa Right now. Might be fine with no nails through the colder months but first 95 degree day next summer when the roof is 140 degrees the whole mess could start sliding down.


#14

Hey guys,
Wondering if you can help. My roof started leaking and it appears the issue is directly under my AC. I’ve called a few roofers but most are either busy (post monsoon season) or don’t want to bother with just a patch job. My choices are wait a few months, get my whole roof done, or it myself.

I’m handy buy have no roofing experience. Do you guys thing I’m crazy for attempting to jack up the AC and replace then busted up shingle? Would using wet patch be a better idea, even as a temporary measure before a professional can come and fix it properly?

Any help would be appreciated.

Thanks,


#15

Close up of damage.


#16

You should be able to lift up the unit and slide In some new shingles as a good temporary fix. No amount of goo is going to replace the missing shingles and cover that hole.

When you do go to replace the roof you will need to do something with the stand the ac unit is sitting on. Probably talk with your roofer and have a new one fabricated or have the existing one modified while the unit is disconnected.


#17

Thanks! I’m going to give it a try!