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

amakarevic · 06-10-2012, 03:27 PM

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

tinner666 · 06-11-2012, 01:33 PM

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!.

amakarevic · 06-11-2012, 01:59 PM

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

tinner666 · 06-11-2012, 02:19 PM

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.

tinner666 · 06-11-2012, 02:22 PM

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

amakarevic · 06-11-2012, 02:25 PM

Quote:

Originally Posted by tinner666

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" ?

tinner666 · 06-11-2012, 02:32 PM

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

amakarevic · 06-11-2012, 03:14 PM

so Modified Bitumen is what this is called ?

tinner666 · 06-11-2012, 03:31 PM

Quote:

Originally Posted by amakarevic

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.

amakarevic · 06-13-2012, 01:43 PM

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

tinner666 · 06-13-2012, 03:29 PM

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.

Use EternaBond for your temp patch-work.

amakarevic · 06-13-2012, 03:45 PM

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?

tinner666 · 06-13-2012, 03:57 PM

Quote:

Originally Posted by amakarevic

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.

amakarevic · 06-14-2012, 02:56 PM

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.

tinner666 · 06-14-2012, 03:53 PM

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.

06-10-2012, 03:27 PM	#1
amakarevic Member Join Date: Apr 2007 Posts: 2,083	installing new skylight w/curb on top of old and bigger curb 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 Attached Thumbnails

06-11-2012, 01:33 PM	#2
tinner666 Pro Slate Roofer Join Date: Mar 2005 Location: Richmond, Va. Posts: 1,427	installing new skylight w/curb on top of old and bigger curb 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!. __________________ Frank To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

06-11-2012, 01:59 PM	#3
amakarevic Member Join Date: Apr 2007 Posts: 2,083	installing new skylight w/curb on top of old and bigger curb 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

06-11-2012, 02:19 PM	#4
tinner666 Pro Slate Roofer Join Date: Mar 2005 Location: Richmond, Va. Posts: 1,427	installing new skylight w/curb on top of old and bigger curb 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. __________________ Frank To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

06-11-2012, 02:22 PM	#5
tinner666 Pro Slate Roofer Join Date: Mar 2005 Location: Richmond, Va. Posts: 1,427	installing new skylight w/curb on top of old and bigger curb Don't forget to get the refund for that flashing kit. It will not work here. __________________ Frank To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

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