Cape Rafter Sizing

[Dave]

Just purchased house plans for a 1-½ story cape by a well-known architect and have concerns about the size of the rafters. Except for the vaulted ceiling over the great room where the rafters are 2x10’s, the plans call for 2 x 6’s. Main roof pitch is 12/12 with maximum rafter length 22', and the porch roof pitch is 3.5/12. The porch wraps around the front and both sides, but along the back the living space extends into the space under the lower pitch roof, with multiple gang lam. beams supporting the main rafters since they don’t extend all the way to the back wall. The only major change we plan to make is to use a metal roof instead of shingles, but according to the metal roofing supplier a metal roof will be lighter than the 240# shingles specified by the architect, so it doesn’t seem we’d have to beef up the rafters for that reason alone.

My question is whether I should trust that the structure will be strong enough if I build it as designed (a rep of the firm assured me all plans are engineered), or whether I should play it safe and modify the plans to use larger rafters, which the rep told me would probably also necessitate re-engineering of the plans because of possible effects on beam sizes, etc. I’d rather not waste wood by overbuilding, but at the same time want to make sure the house is strong enough to withstand our occasional snows and windstorms.

Would greatly appreciate any insights on this.

[Mike Sloggatt]

Dave -
Where will the house be built ?
That will determine your snow load -

The sapan charts allow a steep pitch roof to go to 18' SPAN on a 2x6 - 16 oc. rafter
So although it does seem light - depending on the design - it would likley be ok. (remember thats span - not rafter length) If they are engineered that's what they get paid for - I personally would want something a little heavier so I would get less spring in my Rafter -

MPS

[kcoyner]

Dave,

Here are the new span tables for 2x6 #2syp and #2spf 30lbs L.L. and 7lbs D.L. with light roof covering. #2 syp 2x6 16"o.c. max span (that's horizontal distance) is 12'0". #2 spf 2x6 16"o.c. max span is 10'7". 2x8 syp 16"o.c. can span 15'9" and 2x8 spf 16"o.c. can span 13'4". Also consider how much furring you will have to install to get R30 insulation w/air space above. It may be more cost affective to go with 2x10s depending on design.

[Dave]

Thanks for your responses. Some additional info: checked our almanac and found that we average 13" of snow/year here (Tennessee); by my experience we usually only get 2-5" at a time but there have been exceptions. The longest rafter span that I can see in the roof plan is 15’, and a note indicates the design is based on 15 psf dead load and 20 psf live load. Not sure how that relates to potential snow load.

Because I’ve read so much about the advantages of insulating the rafter bays on a cape to avoid having to insulate and air seal the knee wall, insulate HVAC ducts in the attic, etc., I called the architectural firm before ordering the plans to make sure the rafters were sized to allow this. They misled me by saying yes, so I’m pretty disappointed that I’m now going to have to pay extra (have no idea how much extra) to re-engineer the plans with 2/10 rafters. If I stick with the 2x6 rafters as designed, the only simple way I can see for fitting adequate insulation under the roof deck is by using a spray-in foam like Icynene (recommendations for our area are 5 ½ " in the roof), and then put furring strips on the roof deck before installing the metal roof so there’s a ventilation space directly under the metal. Would this be a workable solution?

If not, may just bite the bullet and go with 2X10’s.

[Jim]

Dave,
With a roof rafter at a 12/12 pitch and a span of 15' with a snow load of 20#psf & a dead load of 15#psf with l/240 live load and l/180 total load a 2x10 spf #2 @16" oc. or 2x12 spf #2 @24" oc. will work. This does not include any wind loads.

[DaveA]

Dave:

Since the architect or whoever you got the plans from says the roof system (and remaining structure) has been reviewed by an engineer, have them send you a set with his/her seal and signature on them. If they wont do it my guess is they are just blowing smoke about the review. Once the plans are sealed either by your architect(which BTW I would insist on) or an engineer, they take responsibility for the structure. They sure as hell are not going to take it if they havent done the review.

And my opinion on your 2x6 rafters is that they are too light to carry the potential live and dead load combination.

Dave

[Dave]

Sounds like good advice about getting a set with the architect's seal; the plans have the company letterhead on them and a copyright stamp, but no "official" architect's or engineer's seal.

However, I suspect that the architect will assume responsibility only if plans are not modified, so if I happen to shift a short non-bearing wall a few feet, they're probably legally off the hook, right? Based on the responses here it seems wise to upsize the rafters, so will consult with a local engineer in case doing so requires changes elsewhere. Appreciate the suggestions.

Dave

[Dunk]

Is there any room in your attic to install a purlin brace. this is a perpendicular board that is braced to a bearing wall below. It is done to reduce the rafter span and allow a smaller size rafter. The rule of thumb for a purlin is to size the board to a 2x8 or 2x 10 and brace with a two by four or six every 8'. the brace must be notched to carry the purlin board not just be face nailed. If you can put in one this will help offset the additional cost of larger rafter. It is of course absolutely necessary to brace to a bearing wall. A bearing wall will have a concrete footing underneath it.