# installing new skylight w/curb on top of old and bigger curb



## amakarevic (Apr 12, 2007)

my house seems to have had an old skylight which got boarded up before i got it and the covered curb's dimensions are 28.5 x 30. the box was covered with a sheet metal box which has some coating on it that resembles hot tar (very coarse). the rest of the roof has the same coating but the material under it seems more like thick asphalt sheathing, not sheet metal. not that there is no shingle or roof tile on the entire roof, which is very mildly pitched (10 deg max).

my original idea was to get a custom sized skylight but Velux wouldn't accommodate so i went after their standard 2222 model, whose curb is 25.5 x 25.5 on the outside. now i need to mount the new curb on top of the old curb.

here is how i plan to do it:

1. remove the sheet metal cover.
2. remove the lid from the old box
3. cut a new cover with a 22.5 x 22.5 opening to match the new curb, which would act as an adapter of the new to the old box. this will be 3/4" OSB (unless instructed otherwise).
4. fasten the new curb to the adapter cover from the previous step
5. fasten the new curb/adapter manifold to the old box.
6. wrap the new curb with bitumenous adhesive underlayment and sheet metal flashing from the Velux flashing kit as per their instruction manual

however, what i don't know what to do is how to shield the old box that now has the sheet metal covering as shown in the picture. i have to rip it off to adapt the new curb.

my thought is to somehow fabricate a tub-like sheet metal box to set over the old box. but i do not know how to seal and waterproof the edges if i do.

pictures attached.

thanks


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## tinner666 (Mar 14, 2005)

No velux kit will work on that. Get THAT money back. Assuming the inside frames up good, you just need to make an hour-glass shaped flashing. You'll need to look at my website for flashing details and tin roofing details so you'll know how to fold and double lock the corners into place so they are water-tight. It's doable, but you'll need your thinking/studying cap on!.


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## amakarevic (Apr 12, 2007)

Plan B: what if i scratched the old curb altogether? and put an OSB rectangle lid to cover the opening where it sat. then on top of that set the new curb. the Velux flashing will, i believe, extend to reach to the roofing around, i.e. past the hole. the only question is, does the bitumenous tape and flashing integrate with this type of roofing cause the manual says only shingle and tile.

And then put another layer of the tape over the top flashing to prevent water from coming from underneath it.

what do you call this kind of roofing anyway?

thanks


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## tinner666 (Mar 14, 2005)

The bitumus tape may eat a hole right through that torch down modifed bitumin roof material you have there. You need to torch down new roof material around a new curb. Not a DIY proposition to be honest.


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## tinner666 (Mar 14, 2005)

Don't forget to get the refund for that flashing kit. It will not work here.


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## amakarevic (Apr 12, 2007)

tinner666 said:


> The bitumus tape may eat a hole right through that torch down modifed bitumin roof material you have there.


how so ? what is it about the tape that can "eat a hole" ?


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## tinner666 (Mar 14, 2005)

Mod-bit is particular about the materials it interacts with. Some cements will eat it up. Dissolve it, in other words.


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## amakarevic (Apr 12, 2007)

so Modified Bitumen is what this is called ?


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## tinner666 (Mar 14, 2005)

amakarevic said:


> so Modified Bitumen is what this is called ?


 Yes. And it's not compatible with the self adhered SBS version. It's APP, and no, I don't even remember what the two designations stand for. I only remember not to mix them, nor the various other products each is compatible with.
BTW, that opening looks like it used to be a roof hatch when it began life.


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## amakarevic (Apr 12, 2007)

this is what i decided: i will do it elegantly and remove the old curb altogether, cut out a rectangle around the hole, and patch it with OSB sheathing and make a hole in it to match the new curb.

then i will set the new curb on top of it and spread some kind of temporary water-resisting patch (maybe vapor barrier with gorilla tape) as a temporary fix before i can get somebody or myself to do the hot tar. i cannot really afford now to have a pro do it cause i am unemployed.

i need to do this ASAP because my time and money are running out and i need to finish the drywall on the inside of the bathroom, which includes the skylight shaft. i cannot wait until the outside is all done in its final state.

*QUESTION: how big of an area should i cut out around the old curb to make it optimal for the mod-bit roofing patch ? i am guessing i need to give some slack around the hole, maybe 6-12" on all 4 sides but i am not sure how much. if you can advise so i can proceed with this project, i would appreciate it.*

thanks


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## tinner666 (Mar 14, 2005)

No slack needed. Cut flush and set the box on top of the mod-bit. Put a bead of geo-cel or Karnak 19 Ultra under the box for added protection.:thumbsup: Use EternaBond for your temp patch-work.


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## amakarevic (Apr 12, 2007)

unfortunately, i do need to cut slack because the 100 y.o. pieces of 1x that were used in place of what today would be OSB sheathing are kind of wobbly and need to be replaces. so i would need to cut at least about 6" on all 4 sides. 

so can i do this and can the new mod-bit that will cover the perimeter be somehow either welded or seamed or taped to the old?


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## tinner666 (Mar 14, 2005)

amakarevic said:


> unfortunately, i do need to cut slack because the 100 y.o. pieces of 1x that were used in place of what today would be OSB sheathing are kind of wobbly and need to be replaces. so i would need to cut at least about 6" on all 4 sides.
> 
> so can i do this and can the new mod-bit that will cover the perimeter be somehow either welded or seamed or taped to the old?


 In that case, cut what you need, and then cut what you remove to fit and lay it in place as a filler. Then use the eternabond over those cut seams too. As to it's longevity, etc, I have no clue.


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## amakarevic (Apr 12, 2007)

OK, so i removed the old curb. the next problem is that i do need to expand the perimeter of the cutout in the old mod-bit roofing. however, the thickness of the cover increases as it approaches where the old curb was, it is close to 2" thick. i need to remove maybe 2-3" on 2 sides so i can fit the new curb (it is not exactly in the center of where the old curb was, i.e. a little lopsided).

so the new question is: how do i make the trim ? it's gonna take a hell of a long time to do it with a utility knife, even a hooked roofing knife. is there an appropriate 4.5" grinder blade that can be used for cutting bitumenous materials? there is also sheet metal underneath.

please see the pictures.


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## tinner666 (Mar 14, 2005)

Skilsaw, disposable wood chisel, straight edged knife and hammer. Any fast blade will gum up pretty good too. They may make a toothed angle blade, but I'm not sure about it.


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## amakarevic (Apr 12, 2007)

actually, i just got done with just some aviator snips and a plain hooked knife. wadn't too bad. pictures to follow as soon as my camera battery charges

thanks


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## amakarevic (Apr 12, 2007)

This EternaBond and Karnak #19 Ultra, do they sell them at the Roof Center or do you have to order online ?


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## tinner666 (Mar 14, 2005)

amakarevic said:


> This EternaBond and Karnak #19 Ultra, do they sell them at the Roof Center or do you have to order online ?


 If the 'Roof Center' is a roofer's supply house, I'd think so. If not, Google Bradco, Allied Builders Supply, ABC Supply of the like. One of them will have it.
I've never shopped for professional stuff at HD, or the other one, so I have no idea what they stock.


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## amakarevic (Apr 12, 2007)

can i use the tape that came with the Velux flashing (i don't think they'll take the return cause i have unpacked it) instead of EternaBond for the temporary patch?


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## tinner666 (Mar 14, 2005)

amakarevic said:


> can i use the tape that came with the Velux flashing (i don't think they'll take the return cause i have unpacked it) instead of EternaBond for the temporary patch?


 It's an underlay. UV will kill it. And it may not be compatible with the APP you have now. I know it isn't meant to be exposed, nor used on an APP mod-bit roof.
My answer is 'No.'


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## amakarevic (Apr 12, 2007)

here are the pictures of the latest developments.

i trimmed a perimeter around the hole (cca 3" give or take on all sides), and cleaned it all down to the wood.

then i made a 3/4" OSB adapter lid on top of which i set the curb. (the metal brackets from the picture are just temporary, will screw it from the plywood into the 2x6).

this does leave some marginal gap between the new OSB and the old mod bit roofing, which i plan to temporarily patch with EternaBond.

i am posting this to get a confirmation from you that it is all done right and that torched tar, as the permanent solution, will be able to seal the gap and keep it water tight. if i need to do something else, like e.g. put some sheet metal in between the old roof sheathing 1x lumber and the OSB lid or something like that, please let me know.

thanks


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## tinner666 (Mar 14, 2005)

1/4" is too much of a gap for mod-bit. It needs to be tight. It would have been better to just reinstall the cut out pieces of mod-bit instead of using OSB to raise the level to match the existing. Am I missing something?


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## tinner666 (Mar 14, 2005)

Is the OSB on top of the existing sheathing?


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## amakarevic (Apr 12, 2007)

tinner666 said:


> It would have been better to just reinstall the cut out pieces of mod-bit instead of using OSB to raise the level to match the existing. Am I missing something?


it needed a panel adapter, which is what the OSB does. "the cut out pieces of mod-bit" were like a mess, like 100s of little geometrically irregular shredded or chopped pieces.

can i fill the gap with something else before pouring the new tar? the lines of the cut out mod-bit were irregular, naturally, and the panel had to be an approximation, i.e. had to have some sort of spacing on all sides to make sure it fits.


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## amakarevic (Apr 12, 2007)

tinner666 said:


> Is the OSB on top of the existing sheathing?


yes, it is


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## amakarevic (Apr 12, 2007)

the OSB panel adapter was needed because the old 1x sheathing had warped as it is 100 y.o. and the new curb couldn't really sit on it firmly, it needed something to level it at least to some degree.

man, this is a big time retrofit, not new construction... geometric regularities are rare


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## tinner666 (Mar 14, 2005)

You have a larger piece of OSB you can cut to exactly fit the hole?


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## tinner666 (Mar 14, 2005)

Ideally, if you know the OSB size, you lay it on top of the area to be cut and using a knife, cut an exact matching hole. You may just have to use the scrap and pack it into the gap at this point.


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## amakarevic (Apr 12, 2007)

tinner666 said:


> You have a larger piece of OSB you can cut to exactly fit the hole?


i could do that, however, can't i just stuff something in the gap, like the foamy backer rod by Sika before pouring the new tar.

man, i really believe in providing for a margin of error, especially when you have to cut something to fit somewhere whose shape is not exactly geometrically straight. every time i try to make something fit exact, i have to go back and forth and cut it more, trim it, plane it, sand it, jigsaw it and it is frikkin annoying. so i'd rather make it a little smaller and then fill the gaps.

which explains why i HATE cutting subflooring panels and, to a minor degree, drywall (cause it is easier to cut and adjust).


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## tinner666 (Mar 14, 2005)

If you have the new APP, cut slices to fill the void.


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## tinner666 (Mar 14, 2005)

"i could do that, however, can't i just stuff something in the gap, like the foamy backer rod by Sika before pouring the new tar."
THe new APP will be applied with an open propane flame and that stuff will torch the house when it lights.


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## tinner666 (Mar 14, 2005)

I have to run. Hopefully, Old and Broken or one of the others can also add/give some tips while I'm out.


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## tinner666 (Mar 14, 2005)

The gap could be filled with the 19 ultra and cover with eternabond, as a short term fix. I have no idea if APP can be torched over eternabond. Give a call please, I have to run. It's my cell.


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## amakarevic (Apr 12, 2007)

tinner666 said:


> If you have the new APP, cut slices to fill the void.


what is APP ?


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## amakarevic (Apr 12, 2007)

i mean, how viscous is hot tar? i would imagine it has the consistency of oatmeal. which means it wouldn't straight drip through small cracks and should be able to fill larger gaps ...


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## tinner666 (Mar 14, 2005)

Ok. I'm back. Drop the eternabond. 

Get 4 pieces of OSB about an inch longer than your cut out sections of roof. 

Make them about 6" wide. We're, you, going to make cant strips for the 4 sides. 

Get 18" Peel and Seal and cut off 4 pieces that are about 16" or more longer than your roof cuts.

More........


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## tinner666 (Mar 14, 2005)

In case I'm mistaken, correct me. I've assuming you DON'T have the specialized equipment to apply APP. All my answered are based on you NOT having the equipment, have taken the training courses, gotten experience, gotten the open flame permits, etc? If you have, I'll change my directions accordingly.

Now, Cut the bottom sides of your OSB strips at a 35 degree angle all the way down their lenght. The plan is to lay then along the cut roof edge, with the top edge lying tightly against the SL curb. Then mark the top corner at each end, and cut the angle, with the saw bevel set at 45 degrees.
If the centers feel flimsy, just add pieces of underneath wood in the middle of each leg to firm them up. 
Screw or nail them all the way around the box.
Now is the time to find out if the wood and/or the APP needs to be primed!
Starting at the lowest side, take the first piece of P&S and starting at the top edge of the curb, press into place as you rotate it gently and smoothly down the sides, over the cant, and onto the roof. At the apexes of the cant, make a simple straigth cut 'outwards, and press the P&S into place. THis is called wrapping the corners. Now, do the same for the two ends, and then the highest side of the box. If your cuts are exactly on those apexes, marked in black, you might not even need to caulk the thing. 

You could wrap it twice and I'm sure it WILL be free of leaks! Might have to prime it to do so!


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## tinner666 (Mar 14, 2005)

amakarevic said:


> what is APP ?


 Mod-bit.

There are two main types of mod-bit; APP and SBS. Neither is compatible with the other! Nor are their other componets.


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## tinner666 (Mar 14, 2005)

I drew in the area covered by one piece of corner warp P&S. It's marked in green on one side of the box. The cuts I mentioned before go about like I marked in yelow. Each succeeding wrap will overlap each corner about 8" AND cover the cuts in the prior piece.


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## tinner666 (Mar 14, 2005)

Once you've done all that, cut some strips of P&S about 5" wide and ?maybe?? 8" long. Measure first! start at the bottom of each corner, about 2" onto the flat of the roof, and run them up the ridges of the cant strip. I doubt that they will flex well enough to go all the way to the top in one run. If not, just go past each apex marked earlier, starting from the bottom, about 2" out, and overlap at the apexes by about 2". It won't leak now.


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## OldNBroken (Jun 11, 2008)

Sorry I haven't been able to help Frank, I've been doing some Forest Service jobs and no phone or internet for the last few weeks. Looks like you are doing just fine for a broken-down tinbender though. :thumbup:

If he hasn't cut the wood cants already and has a local roofing supplier, I would recommend seeing if they have some wood-fiber cant strips he can just set in mastic around it. Instead of torching the new curb he could also get a compatible cold-process and cap sheet to flash the new curb. I believe GAF matrix may be fine for that application. Lessens the chance of burning the bldg down too. A hot-tar will just shock that existing system and he will end up with a huge mess and failed roof on his hands.


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## tinner666 (Mar 14, 2005)

Is there an 18" eternabond available to you? It's possible the job can be done with either product. I'm not familiar with either though I see others using them both around here. I guess either could last a year or more.


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## OldNBroken (Jun 11, 2008)

Eternabond is a great product for emergency repairs but should never be considered permanent unless your roof is on wheels.


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## OldNBroken (Jun 11, 2008)

btw, very nice explanations on the cant strips and cutting and wrapping the flashings.


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## amakarevic (Apr 12, 2007)

tinner666 said:


> Now, Cut the bottom sides of your OSB strips at a 35 degree angle all the way down their lenght. The plan is to lay then along the cut roof edge, with the top edge lying tightly against the SL curb. Then mark the top corner at each end, and cut the angle, with the saw bevel set at 45 degrees.


i am very confused by this, both how to actually execute, what to accomplish and why it is needed. i don't doubt you are right, i am just confused


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## tinner666 (Mar 14, 2005)

To start with, doing a box with perpendicular walls like it was before is complicated and prone to leaking, but I was going to stay on that bent.
When you cut the large irregular opening, I had to switch to plan B, then plan C, and so on. I finally settled on Plan R.

Cut the OSB to the 8" by lenght of the cut, + an inch. Lay it against the box and you see the light. It won't fit!:no: But, you will see why you need to camfer the upper and lower edges, and 45 the ends. Get that done and we'll go from there.


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## tinner666 (Mar 14, 2005)

Google can be your friend. http://www.answers.com/topic/cant-strip


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## OldNBroken (Jun 11, 2008)

Amakarevic, Any type of asphalt or modified roofing systems requires there be no right angles on the roofdeck in order to avoid stressing and cracking the system. Cant strips provide a way to break a 90 into two 45 degree angles relieving this stress. 

What Tinner is trying to explain to you is how to make cant strips out of osb very simply and inexpensively. Once that is done, you nail a 40# base to all of the framing and decking and install your curb flashings over that. His other drawing was explaining how to cut and wrap the material around the curb once the cant strips are installed.


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