Sloped standing seam metal roof parallel to and over existing steel joists and metal deck 1

[MWPC]

All,
The owner and architect want to place a standing seam metal roof over an existing conventional bar joist and metal deck roof system. The slope of new roof would be parallel to the joists. Joists span 45', are spaced 4' oc and have panel points about 4' oc. Narrow rib metal deck with 5" of insulation and a BUR. I am looking for framing suggestions.

On past projects with shorter spans and bearing walls, I have used light gauge trusses at 8' oc and hat type purlins at 4' oc or so to support the standing seam roof. In this case, providing a structure over the existing structure seems like overkill. The existing structure is adequate for the loads. I have considered running beams perpendicular to the joists at 4' oc with stub columns down to the joists. Lots of little pieces and cutting holes through the deck to weld the columns down for up lift. Architect suggested bearing walls at 4'oc cut down to the deck but there would still be point loads at each joist and uplift connections to deal with.

I assume this has been done many times before and I would like to avoid reinventing the wheel. Making budget would help too!

Thanks

Ken

 

[BAretired]

Why not rest the standing seam roof on the metal deck and ensure the top chord of the joists are adequate to sustain the full load? That way, no additional framing is required.


BA

 

[MWPC]

Sorry I forgot to mention that the existing roof is flat / nearly flat. New standing seam roof requires a slope.

 

[CANPRO]

what would be the total difference in elevation you would need for the new slope? You could put spacer clips (light gauge "hat-sections" or similar) in the low flutes of existing deck, fastened to the joist below. Then run new purlins over top of the clips - purlins would run perpendicular to the existing deck, then you could lay the new standing seam roof over the purlins which would place the new roof parallel with the existing deck. Depending on your required change of slope, the spacer clips might get too tall, but at some point along the new slope you could also adjust the depth of the purlins to maintain the slope.

 

[KootK]

I'd give Nucor a call for some free recommendations. They have some proprietary systems that are quite similar to what you're likely to end up with (below). This has come up here before and, if I remember correctly, building envelope concerns come into play. Ventilation, condensation etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MWPC]

CANPRO,
I can check but new slope should be about 1:12. For 45' span, mono pitch, 45" of slope. Slope will be parallel to the existing joists for the main part of the roof. Ends will have a hip but whatever solution we use for the main part can be adapted for the ends.

 

[jayrod12]

I think with that kind of slope required, the architect is probably on the right track.

 

[MWPC]

jayrod12,
Neglecting the temporary waterproofing issue of a slot cut through the roof every 4', I still have structural issues with the bearing wall. A strip of deck 6" or 1'-0" wide will not support 4' of trib roof therefore I would still need something to span between joists, a deep leg track might work. The point load would need to be transferred down to the joists. Cut out deck and extend a couple studs? Attach for uplift? We can't cut through the deck for the full length or we loose the diaphragm. A bearing wall might be the best solution, I am just not there yet on how to make it work.

 

[jayrod12]

I think the deck probably can support the line load considering it is done with Z-girts all the time.

Also on the waterproofing aspect. Why the standing seam roof if the flat roof is expected to be the actual building envelope layer. Why not just replace the flat roof and leave it?

The owner has to accept that this is not a cheap modification.

 

[jimstructures]

Contact Butler Manufacturing Company and ask about the Reroof product. Basically a standing seam MR24 roof panel with insulation and a truss-like framework underneath. It weighs about 3 to 5 p.s.f., including trusses, with up to 9" of fiberglass blanket insulation ~R-30.

Jimstructures

 

[MWPC]

Thanks jimstructures. I will check their website first and if I don't see anything, I will call.

 

[structSU10]

you could nest channels into the deck, welded to the joist thru the deck (create panel points as required) and build off of that - an angle frame, stud walls, etc.

 

[hokie66]

The architect, jayrod12, and jimstructures are on the right track. The "bearing walls" need to be trusses, spanning between the joists.

 

[MWPC]

Thanks guys/gals. I was just hoping someone had done this and had an easy solution. Narrow rib deck makes nesting anything into the deck difficult. The architect did not like my idea of core drilling holes down through the deck but I still think a series of stub columns hitting the panel points on the joists and welded down will be quickest and easiest to waterproof around. Pipe boots are cheap. Almost any member will span the 4' between joists. Spacing along the joist will be determined by max span of standing seam roof. Just need to keep laborer on the core drill from ruining the joists!
Thanks again.
Ken

 

[jayrod12]

In order to give you somewhat of free-reign and to not worry so much about waterproofing the penetration work, why not provide a new water barrier under the new standing seam roof. I don't understand the intent.

What happens when the roof actually starts to leak? They've got to rip off the standing seam roof and your support structure to make repairs? That seems ridiculous.

 

[hokie66]

Agree with jayrod. Core drilling, welding in columns onto skinny bar joists...that all sounds too tedious to work. And maybe you are just talking temporary waterproofing, because the metal roofing provides the waterproofing. Probably best done bay by bay on fine days, so you can keep the building dry.

 

[JedClampett]

I've run into this and have used nailable insulation. The standing seam can be attached anywhere. The nailable insulation is a twofer, insulation and attachment medium in one. I know Atlas makes it.

 

[Ron]

Several metal roof panel manufacturers have framing subsystems to go under their roof panels. I designed a roof system about 20 years ago for a school library building using a Fabral panel and subsystem. Worked well and we were able to get the required slope for the metal panel warranty.