Got a 100 yr old+ TF house, now how to deal with the roof

[chainsaw_louie]

Hi

What I thought was a simple diy fixer upper has mushroomed into a lot more.  This small 1200 sq ft , 2 story house was built 100 + years ago .  When we started to remove plaster and lathe, we discovered it was post and beam, with clay and straw packed in the walls.  It has now been mostly gutted and its obvious that the multiple layers of cedar + asphalt shingle roof will need to be replaced before any interior insulating or drywalling is done .  My plan is to leave the roof framing intact and put in new 2x8"s sistered against the original 4x4 rafters in the sagged roof section. The sag was due to collar ties being removed at some point .  The original rafters are 30" oc , I plan to put a 2x8 between them.   On the interior side of the roof deck  I thought I would apply 2"-3" closed cell spray foam for a vapor seal and above this a  roof sandwich like this: 

sky
shingles
ice & water continuous  ?  or just on edges
plywood 1/2"
2 layers of 2" rigid Iso board with foil ,  seams taped and offset
plywood 1/2"
spray foam closed cell
drywall
interior

Am I on the right track , or missing something here .

Thank you

Tim


  

 



  


this show the addition, done with dimensional lumber:

 

[BigJim2185]

For what it's worth I have always done ice and water in any areas with the potential for ice dams (edges, valleys), usually two rows, with roofing felt for the drainage plane on the remainder of the roof.  Fairly substantial cost savings over using a product like grace ice and water over the entire roof. I've only put two roofs on, so take it with a grain of salt. 

As far as the rest of your plan, I am in sort of a similar position trying to plan a timber frame shell.  I don't know if you may want to add stringers (run vertically...maybe not still called a stringer?) under your first layer of plywood, over your ISO to create an air gap so that the roof is cold vented and there isn't a possible issue with the sheathing rotting if moisture does find a way in.  The stringers could also be used to create your overhangs.  I'm sure someone who knows a heck of a lot more than me will chime in. 

Sistering w/ 2x8s and adding them between rafters sounds like a good idea.  May be tough to jack the sag out of the roof.  Will you need to add 2x8s across the entire roof, considering plans to sheetrock the inside?  With no plan to show any of the timbers and some of the roof sagging, have you given any thought to yanking the roof and stick framing it from scratch?  Maybe re-purpose some roof timbers as you work on the house?  Just throwing ideas out.   

Good luck with the project.  I spent the last couple years working on an ~1865 frame...and the fact that now I'm in the process of planning to build new pretty much explains how mine is going.  Just be glad your problems are at the top of the house and not the bottom!  :D

[chainsaw_louie]

Thanks Jim.

Oh there is plenty to cry about on the lower end also but fortunately the house is not very large so it can all be dealt with.

Regarding the roof and the different membranes , I was reading on the Building Science site an article that describes 'over roofing' and in that design  he must have 4 or more non permeable membranes :

http://www.buildingscience.com/documents/insights/bsi-063-over-roofing?topic=doctypes/insights


1 complete layer of Grace IWS on the lower plywood sheathing roof deck
2 layers of foil taped on the double layers of iso board
1 complete layer of Grace I&WS on the top below the shingles

Now , he does NOT have a air space under the top plywood layer , maybe this is the reason for the complete layer of I&WS under the shingles .

Oh , and he also has closed cell foam on the interior, under the sheathing .

Do I need to go that far  or is that way way over kill on the non-permeable membranes.

Thanks.

Tim

[BigJim2185]

Interesting article.  A completely un-vented roof like that is over my head (no pun intended)...and it would be out of my price range too I think.  It makes sense to me that if you are going that route, you would want no chance of vapor getting through.  There was a post on here by a member named rooster that provided a good look at a foam board insulated roof he did in a wrap-and-strap style- I did a quick search but didn't find the link, sorry.  I think a thicker layer of foam board (say 8"), in a stick framed structure over the timbers with a vented roof would be less expensive, but it doesn't make sense in your application without plans to show the timbers on the ceiling.  I think it's time for me to bow out and let some folks with more experience take over. 

Post pictures as you go if you can!  Good luck with your project.       

[Thehardway]

Tim,

I'm seeing two different roof structures/styles in the photos.  Are these opposite sides or different sections?  Can you provide more close up photos of what is going on at the eave and ridge and the joinery/connections?

Although sistering the existing timbers might work, there is the issue of what will support them and how they will be installed and fastened in place.  For this to work right they would need to parallel the entire span and have sufficient bearing at both ends.  I may be reading you wrong but if the idea is just sistering 2X8's to the 4x4's in the sagging section after jacking the sag out, you will actually complicate the issue by adding more unsupported weight. The sag will just move or you will cause the rafter to fail as you try to put it back into position and secure the sistered member.

First there is an issue of terminology and function.  The location at which your photo shows the missing members appears to be at the lower third of the rafter but hard to tell for sure.  A tie located here would be called a rafter tie and its function would be to assist in control rafter thrust at the eave acting in tension. If it is located in the upper third it is called a collar "beam" rather than a tie as it cannot effectively operate in tension within the upper third of the roof and therefore "ties" nothing but rather serves in compression to prevent sag and is called a beam rather than a tie.    These terms have been bastardized by framer's over time and used interchangeably.  I only point it out to clarify what purpose they serve and how they function in the roof without adding to confusion. You may have uncontrolled thrust issues at the eave due to members being removed and adding additional weight without rafter ties could cause catastrophic structural failure.

Sagging rafters typically mean that rafters were two small for roof loads when designed, or more roof load was added than the original roof structure was intended for at a later date.  I am not familiar with the homes in your area but I doubt 100 years ago they used asphalt shingles.  It was likely either a shake style roof or covered in tin or copper which would explain the 30" oc.   If it was slate I would suspect a heavier built structure.  Either shake or tin was likely lighter than the asphalt shingles present now or the system you are proposing to add.

Beefing up the roof system to handle the new roof means you need to make sure that the plates, walls and sills can handle the additional load and the mid-span supports you propose.

Are you trying to make use of the attic space?  Is this why the collars were removed?  Does something prevent the replacement of them?  Why are you moving to a warm roof system?  Not saying it's a bad thing just trying to see what the objectives are here to make good suggestions to proceed.

[chainsaw_louie]

I should have added picture that shows how we have put in 2x6 's on the opposing side to meet and carry the 2x8's I mentioned. The attached pic shows the white lines we have put in and the green lines will be put in as new framing when we tear off the roof sheathing. The side with the green lines is the only side I was going to tear off the sheathing because the rest of the house has usable purlins which can be a base for 1/2" decking.  You are correct there are two roof framing systems going on here, dimensional installed in 30's ?  and tf  which is original. The current roof holds the weight of lots of snow and is very solid, despite the slight sag.

I don't want to change any of the interior space from what it had before,  just make it stronger and provide a well insulated roof system that can go for the next 50 years. But as this is a cathedral ceiling, I have learned (after purchasing) that these are not so simple or cheap to insulate, the extra layers required for an *un-vented* roof are not trivial.  It was fine as an unvented space for someone else but used a lot of fuel and was probably not very comfortable, esp in summer.

My real question though is about insulating the roof and building a new roof deck for new shingles because the current shingles are worn out.  So if I went the simplest  and cheapest route then I might cut vent slots in  in the top edge of the top plate timbers (there are none in any of it),  add additional framing where I absolutely have to, then insulate with fiberglass.   If building inspector lets me go with less than R38 ceilings then I am fine, but maybe need some heat and AC to get through extreme cold / hot months.  On the other hand, if I do a 4" foam warm roof with insulations below then I am up at R40 and the top half of the house is more comfortable and cheaper to heat/cool, so I am leaning in that direction.

[Thehardway]

If this will be occupied "conditioned" space meaning heated or cooled, and used for more than just storage,  The best way to go will be with a warm roof system, not a vented roof system.  The 2 layers of 2" foam wrap and strap is likely the cheapest if you are doing a compete tear off and replace of sheathing and shingles. Foil tape the seams in the foam.  No need for foil radiant/vapor barriers.  They serve no purpose if there isn't an air gap.  In my experience/climate warm roofs are not as prone to ice dams as vented roofs so the IWS is not so critical.  May be different in your climate.  Ice damming usually comes from internal heat loss on the upper portion of the roof causing snow melting and then it refreezes freezes when it reaches the cold, uninsulated portion at the eave.  With a properly done wrap and strap roof, there will be very little heat loss through the roof and very little melting.  The overhangs will be insulated as well so the overall the roof temperatures are much more even and consistant on a unvented warm roof.  Be cautious about your snow loads as a well insulated warm roof is likely to accumulate much more snow than the old roof did.  Allow for the additional load.

Apart from that, the easiest would be to use 2" polyurethane spray foam on the underside of the existing exposed roof decking which seals everything and gives you a vapor barrier followed by 2-3" of open cell foam.

I am not sure I understand the cathedral ceiling part but I guess you are saying the interior beams will be exposed?  Installing intermediate span rafters will go a long ways in helping you out and supporting additional weight of the wrap and strap.

The picture is helpful in understanding the different roof sections. I would need to look closer at joinery to be sure but it looks to me more like a "balloon" frame than true  timberframe.  Looks like you have built up plates and multiple structural wall studs though slightly oversized both typical of balloon frames.  Most timber framing was replaced by balloon framing by the early 1800's. Platform framing and stick built evolved around after 1900's.    Still a very worthy restoration project.

Does this help?

[chainsaw_louie]

Yes, very helpful , thanks.

It will be a conditioned space .

The old original rafters are joined at the peak with a true pegged tongue and forked joint.

Is it not advisable to do away with the straps and screw the top deck of plywood directly to the rigid foam?

Now I can see that the use of straps provide multiple features :
        provides a solid base for the screw heads , and hold the foam down solidly
        provide a solid base to attach the top layer of plywood to 
        provides a airway for drying and cooling

There is probably a lot of discussion on the details on doing these roofs here already....got to have a look.

...did some more reading and thinking (dangerous)

I saw your post , Thehardway, posting[ https://forestryforum.com/board/index.php/topic,67714.msg1033845.html#msg1033845] about wrap-n-strap roofs in which its said 
"The wrap and strap method is much better suited to a asphalt shingle or wood shake style roofs as they are not as prone to condensate buildup for many reasons."

But do I need that air channel provided by the strapping , doesn't it also bring in humid air that will condensate in there, as in the case of metal roofs, just less .  It seems that there should be some drainage plane under the sheathing  esp as its also mentioned :
"I doubt you will find taping seams on polyiso a suitable means of creating a waterproof barrier.  Condensate will find its way through and you will end up with that pretty decking messed up from water stain, warped, and likely rotting on the back side.  If nothing else, the moisture will migrate down through the foam around the strapping screws or nails.  "

So based on these real world experiences,  I cant see the advantage of the 'strap' without some addition of a drainage plane other than the foil face of the iso board .  On the other hand , if a layer of IWS is put down over the entire sheathing and under the shingles there should never be any infiltration of moisture from the sky side .



Tim

[jwilly3879]

If you frame the whole roof with 2x8's from the inside you can spray 7" of closed cell and eliminate all the poly iso on top. It should work out cheaper and will give you about R-45. You will need to use 1/2 sheetrock as a thermal barrier over any exposed foam. This will meet code for most of NY, climate zone 6 requires R-49.

[timberwrestler]

I would definitely go with the foam on top of the sheathing.  There's no dew point and no thermal bridging.  BSC sets the standard for how to detail homes all over the country.

[Thehardway]

Tim,

There are lots of ways to skin this cat.  In the post you mention the OP was about putting a metal roof on a barn with minimal insulation. 

Metal, being very conductive is much more likely to cause condensate issues than asphalt shingle or wood shake.  Using minimum insulation under a metal roof increases the likely hood that warm, moisture laden air will contact a cold metal surface and condense. 

In your case you are not using metal.  You are also insulating a great deal more than you would for a barn and your indoor activity will be much different. 

There are different methods of wrap and strap which have been done with great success in applications like yours.  I don't think you will have any condensate issues as you have no hard thermal breaks.  What type of foam are you planning to use?  Polyiso, EPS, XPS? 

What your recent post suggests is in essence a site built non-structural insulated panel without adhesive.  Using a 15" oc spacing as you indicated, this should be fine but it will take lots of screws and it will be a bit tricky getting the foam to stay in place while you layer it and put the sheathing over it and screw it down (this is one of the reasons for the straps). The upside is that you will have no thermal bridging other than the screws.  Come to think of it, you may want to check on the price of sip's.  Using sips, you could likely eliminate a lot of the intermediate rafter's you proposed as they can span a fair distance. This would eliminate having to install lumber, labor, time, risk of things getting rained on etc.  There are some that come with a T&G style or T111 style skin on the underside

http://www.sipsproducts.com/t111/

This would give you a much cleaner and look while meeting all the objectives. You could tear the roof down to the original beams and put the sips directly on them with no need for repair or modification.  Cost might not be that much more. I stole the following images off the Fisher Sips website so you can get an idea what I am talking about.



  


 

[chainsaw_louie]

My architect friend also mentioned 'why not use SIPs' and to be honest, I dismissed the idea due to my imaging that they are too expensive or complex or hard to find etc , but now that I've heard it a second time from you, I have to consider it  and check out cost and availability.  I like that they could go on the existing framing and it would save many steps if I installed the pre-made SIPs instead of building the foam core roof in place.

One side of the roof does have a sag in it so I'm sure that I would have to correct that before laying SIPs on top of the existing rafters.  But I might be able to do this with sistering 2x6 or 2x8s on the existing rafters alone.

Is it typical that  the top surface of the SIP the nailing surface for the shingles and with subsequent tear off and reshingling this top surface can be layered over if necessary?

An added  bonus is that I'm getting an education in some important aspects of the modern TF building , which I have been interested in learning more about but didnt know how to get started !

-Tim

[chainsaw_louie]

I spoke with two  two SIP manufacturers and here is what I learned :
     cost - SIP with wood on just one side are $4-5/sq ft,  with osb on 2 sides are $5-8 sq ft  - R30 sip
                for $8 / sq ft they will design and custom build the sips and include all fasteners etc
               
      inter sip panel joints are the most critical part and if not done correctly can allow moisture up into the panels

      require a crane to lift for the bigger panels

      can be made so they dont require any rafters on a 12 ft span

      require installation by someone with prior experience

Speaking of foam as a sealant,  yesterday I was looking at a closed cell foam job and I could see where the foam had shrunk a bit from the framing.  This might not be the end of the world but if the foam is being relied on for the seal between the sip panels , its possible that it could fail after many years of expanding and shrinking . 

Based on this , I think the site built , iso foam board roof with a layer or two of IWS can be built to have a high resistance to failure and provide a high R value. The thinner foam 2" in multiple layers with staggered joints should ensure a tight seal.   The materials are all available locally and the skill / workmanship required for such an install seems to be anyway straightforward .

just thinking ...

[Thehardway]

Tim,

There are lots of different styles and methods for joining sips.  Some use T&G, some use splines, some use gaskets, camlocks, and others just fill the gap with expanding foam and roll on.

It's your time, effort and budget, and ultimately only you can decide what is best for you.  I believe if your sips were laid out right, you could actually pull the rafters back up using SIP screws a jack.  Go an inch, let it sit and hour or two and then go another inch pulling them up to the bottom side of the sip. No sistering needed.  Sips are exceptionally strong and stiff.   You could do this after the crane was gone and while you work roofing the other side.  The look on the inside would be much better and the condensation concerns would be a non issue.  If you use a good quality architectural shingle over 30# felt, I doubt that you will ever need to re-deck the sips due to tear offs.  Maybe your great grandchildren might?

I used jumbo sips (8"X 8'X24') on my roof and put corrugated galvalume over it.  I put them on with a 40' straight stick all terrain boom lift and some big ratchet straps for lifting. Boom had a man cage on the end. It worked pretty slick.   Lower the boom over the panel, attach panel to end of stick, pick it up and pivot into position using chock stops at the foot of the rafters Release straps and screw it in place using Sip screws and an impact driver.  No need to pay a crane and you can use the lift for roof tear off and replacement as well.  Probably need a week of boom rental to complete the whole project if you have things lined up and a good helper.  If I did it again, would probably use smaller panels just for ease of handling. If you are on flat ground you could likely use a smaller cheaper lift as well.  My site was fairly steep.

Only issue I have had with SIPS is snow and ice sliding off the metal roof.  Sips don't provide a great means of attaching snowbirds or ice breakers.  For that matter neither does wrap and strap.  Something to think about but with shingles it shouldn't cause a problem.

There is absolutely no discernible heat/cold transfer through my roof.  On the hottest summer days the inside of the panels is the same temp as the air inside the house which is usually around 72F.

[witterbound]

If you frame the whole roof with 2x8's from the inside you can spray 7" of closed cell and eliminate all the poly iso on top. It should work out cheaper and will give you about R-45. You will need to use 1/2 sheetrock as a thermal barrier over any exposed foam. This will meet code for most of NY, climate zone 6 requires R-49.

Have you considered this idea?  We sprayed foam in our walls, and did a wrap and strap on our roof.  But if I had already had the rafters up and didn't want them exposed, spraying the roof would have save time money and a lot of work.  So, I'd urge you to explore this possibility. 

[chainsaw_louie]

Hi Witterbound,

Yes, to frame/re-frame out the bad sections where I need to and spray foam from the underside with closed cell .  This is simple and should suffice esp if ccSPF has R6.5/in then  5.5" has  R35.5  which should be enough for my area.
The question is whether I want to spend another $4000 for putting on  a foam core roof .   
I could always do the simple roof now with the ccSPF and then add the foam core roof 20 years from now.

Tim
   

[Holmes]

 I have 5" closed cell spray foam on the underside of my roof.   30" half tree rafter spacing , original board roof ,plywood, some ice and water shield [ 6' wide ] then asphalt shingles. The only ice dam problems were at the lower ends of the rafters where they met the exterior walls. The spray foamer missed some small areas and  that allowed the warm attic air to escape to the enclosed overhang and create ice dams. I fixed it by redoing the outside trim and using cans, I sprayed foam into those gaps. I have had no other problems with my spray foamed roof, 12 years now.