View Full Version : What makes a roof bow like this?

rob martinez
06-05-2007, 10:16 AM
I need some help with explaining why a roof may do this. Unfortunatly I had no access to the attic in this area. It was right at where the garage meets the house. Bowed up in this one area. Thanks for any help!

Jerry Peck
06-05-2007, 12:16 PM
Poor framing, but, as you live in CA, maybe earthquake?

See any cracks anyplace?

Chip O'Brian
06-05-2007, 12:56 PM
Any possiblity of truss up lift?

Bruce Breedlove
06-05-2007, 03:10 PM
Movement associated with truss uplift is most noticeable along the bottom chord of a truss, especially where the ceiling meets an interior wall. Truss uplift would cause only negligible movement along the top chords of a truss.

Bill Wieczorek
07-20-2007, 07:35 PM
I hope someone has an answer. My 25 YO home looks like that in 5 different areas of the hip roof and I can't see a thing wrong from the attic area.

Gunnar Alquist
07-20-2007, 08:59 PM

Looks like this is several days short. I guess Bill resurrected this, but I might as well attempt an answer.

There are a few possibilities.

One is that the the roof is not bowed up, but sagging on either side. The wall between the garage and house will often hold up the roof framing while the stuff in the attic and above the garage sags. Garage roofs in my area are notorious for sagging due to undersized rafters and if the room on the other side of the wall is the living room it is likely that it too has an unsupported span. If the house was built in the '50s or '60s there is a good chance that the rafters and/or purlins are 2x4s. Quite often, the ridge is 1x6 as well. I am not terribly far west of you and this was standard framing around here during that time. Probably there as well.

Another possibility is expansive soils. The wall between the garage and house could have heaved upward causing the hump. Since you are in the flats of the valley, I would expect expansive/adobe soils.

Both are possibilities in the Sac area. If I were to place a bet, I would go with door #1.

Jon Randolph
07-20-2007, 09:53 PM
What does your foundation/sill plate look like? Any cracking on the interior walls? Do your doors close or hit the casing?

Matt Fellman
07-21-2007, 10:40 AM
If/when I can't really explain why something is the way it is I resort back to what attorneys and experienced inspectors say.... write what you see. Of course you dress it up a bit more than just saying the roof has slopes. It's nice to always be able to explain what caused something but as you can see there's several possibilities. Nobody can argue with what you see.

Bill Wieczorek
07-21-2007, 06:43 PM
Jon I don't know if your speaking to me, I'll butt in if you weren't. I have an all brick veneer home with a 10 inch foundation. Been in it Four years, its about 25 YO home. There were 2 areas where hydralic cement was used to patch some leaks that happened in the first year according to the original owner, no other cracks. Our soil is mostly clay with some topsoil not fun to dig in. Truss roof, 2x4 24 inch centers with plywood sheeting. When I moved in I didn't notice the rises in the various areas so it seems there getting worse. There was a new roof put on a year before I purchased the home. I posted this question 15 months ago hoping someone could answer it with a definative answer but no luck. Thanks for your input though.

James Duffin
07-21-2007, 07:35 PM
Just curious...how deep was the garage measured from the garage door to the back of the garage? I am assuming the bow is in the garage roof.

Jon Randolph
07-21-2007, 09:22 PM

What about the garage floor, any large cracks or uneveness?

By your last comments I assume that you are on a basement, and all of the foundation walls are visible?

Was the entire home and garage built at the same time or was there additions? The way that the valleys come together at the point where the drop appears to begin makes it look like that has been some adding on in the past. Maybe by a DIY homeowner that made miscalculations during construction.

Is the entire roof trusses?
Are all of the trusses still attached firmly at the gussets?
What about rafter spread (if any are present)?
Am I seeing a hump in the middle of the ridge or does one end drop down? It's hard to tell from the pics, but it looks like it drops down at the garage wall.

Bill Wieczorek
07-22-2007, 07:01 AM
Jon this picture isn't of my home, that's from the original poster. I don't have a picture. Yes, everything was built at the same time, no DIY project here, yes I have a full basement. I can't see any rafter spread. I haven't walked the entire attic space but the areas I can get to where the problem is occurring look OK to me. The tress's look fine and yes attached at the gussets.

The guy behind me just had a roof put on. I talked to to the guy doing to work who said he'd take a look but never stopped by. His opinion without seeing it was possibly the plywood is warping from lack of ventilation. I doubt that I have a continuous soffit vent all around the house with another 10 roof vents. I've been up there on a windy day and it's almost as windy in the attic as it is outside. Great Ventilation.

Jon Randolph
07-24-2007, 07:59 AM
The house in your picture, if there is not any rafter/truss issues, shows some serious signs of settlement. I would recommend a structural engineer and possibly a soils engineer depending on the SE's findings. There is no way that this is warped decking.

Gunnar Alquist
07-24-2007, 10:07 AM

I would help us if you filled out the rest of your personal information. Knowing where you are would give those that know your area more information.

Gary Schutta
07-24-2007, 07:44 PM
Well appears home & garage are on two different foundations where the garage has settled differently than the home. Check permit history for possible addition.


Randy Aldering
07-27-2007, 08:40 AM
What does the framing look like in the attic? How many layers of shingle are on the roof? Has the roof ever had tile?

Mike Truss Guy
02-10-2009, 12:31 AM
It there is not a framing issue...

My first though is differential settling of the foundation, possibly due to poor compaction of the soil under the slab. It's this kind of thing that made post tensioned slabs common here in Las Vegas. It's especially common to have foundation issues where there is landscaping or trees very close the house. The photos do not show this, but there could have been a large tree that was removed.

rob martinez
02-16-2009, 12:45 AM
Thanks for all your input. No signs of movement in the garage floor or sil plates. With all the info provided I would say the cause is related to the framing as suggested.

~rob in sacramento

Brandon Whitmore
02-16-2009, 07:38 AM
I agree with Gunnar's post at the top.
I roofed a duplex this summer that was stick framed, and both units were completely vaulted.
The roof plane was highest at the center between the units, and sagged down away from that center.
There was inadequate ventilation as the original contractors had jammed the batt insulation tightly to the roof sheathing throughout. The rafters were not under- sized, but I believe they sagged due to the high moisture content in the framing. The wall between the units supported that center point, and prevented the sag at that location......

I saw this at another inspection as well. There was a large belly in the framing at the one end of the roof plane, but not the other (same span as it was a rectangular shaped structure). The N. end rafters were sagging, while the S. end ones were not. There was evidence of ventilation issues along the N. end, but not the S. end.......

Ed Garrett
02-16-2009, 11:46 AM
Hi all,

In Davis, across the river from Sacramento, this "sagging" is common in quite a few of our homes.

1) the garage slab is typically NOT tied to the rest of the house foundation. I have seen 1 example where the slab was moving away from the house, but more often, the underlying soil was hastily placed and inadequately settled prior to original construction (compacting is not the same as settling).

A 20 - 25' slab that has settled 4 - 5" at the Garage door will not produce much of visible crack where it abuts the House slab or foundation.

2) Homes in this "ranch" style are either slab or raised foundation depending on year of construction. Raised was the preferred method until 1970's when we switch to mostly slab construction to reduce problems with settling. If the home is on a raised foundation (which I do not see listed anywhere) it is probably a differential settling issue again. Little was done in subdivisions to drain water from the foundation (hey, it never rains in California right?) Expansive soils or low weight bearing sandy soils are our predominant conditions. In either case, alternating wet and dry conditions combined with the extra weight of brick veneer cases soil to be expressed from underneath the exterior footings. Internal piers do not carry as much weight so they do not settle as much. Result is load bearing walls, or in this case, the house foundation between Garage slab and house that have less water exposure result in a "raised" appearance when in fact, you are dealing with a small amount of settling around the rest of the house.

3) Poor drainage that results from initial poor compaction of soils and limited slope away from the foundation is exacerbated by a build up of landscaping around the house retaining more moisture. Again, the exterior of the house settles more than the interior. If the house is slab on grade, this shows up as cracks across the slab (normally hidden by carpet and other floor covering.) My guess is that the doors alternately stick, fit poorly and sometimes work fine.

4) One final, since you say you have no attic access, it is possible interior supporting walls have been removed moving foundation loading from the "uniform" loading expected on a slab to the perimeter. See the above for how this could result in slab cracking and differential settling issues.

Since this home was built, we have increased footing dimensions under brick veneer to handle the extra weight. I think we have also increased overall footing dimensions and require better ties between garage slabs and house footings.

In most cases I have seen working on remodels for the past 30 years, there is no one thing but a combination of smaller inputs that lead to these conditions.

If you want to confirm settling as a culprit, do a quick elevation survey of the top of the foundation all the way around the house. I would start with the Garage foundation though and expect to find at least 1" of difference front to back rather than 0.

Ed Garrett

Ed Garrett
02-16-2009, 12:09 PM
One other note since you say "garage floor".

The slab in the garage is typically "floating", poured inside the garage footing. It is expected to move independent of the footing.

Check the footings themselves. Even a cheap laser level should show any drop in the "footing".

I'm familiar with the condition where the planting area between garage wall and sidewalk allows moisture under the garage foundation. If you look at the front of the garage, you will note there is no tie between the two sides. Your roof load is concentrated by the single wide door opening on the front corners of the garage, often where the downspout (if any) drops the water from your roof.

Keep in mind you are dealing with geometry and changes in footing elevation as little as a few fractions of an inch can be compounded "normal" movement of framing members by the time you get to the ridge.


Dennis Webber
02-17-2009, 08:03 AM
I need some help with explaining why a roof may do this. Unfortunately I had no access to the attic in this area. It was right at where the garage meets the house. Bowed up in this one area. Thanks for any help!
I read the what the other responders wrote, and other one thing seems to come to mind.
I would tend to suspect a very thin roof deck, fastened with staples and warping (possibly due from leaking flashing or improper underlayment.)

Ed Garrett
02-17-2009, 10:23 AM
Hi Dennis,

If you look at the cracks in the drive leading up to the garage, you will see the primary culprit, expansive soil conditions that promote uneven settling.

Also, there is a second trim piece on the roof edge (now that I have found I can enlarge the attached photos) that indicates the home was originally roofed with Cedar Shakes and later re-roofed with Comp shingles. The practice here is to tear off down to the skip sheeting and nail on 1/2 inch roof decking of plywood or OSB. This raises the roof edge requiring the extra molding to cover the raised deck edge.

The roof sheeting under the comp roof (looks like at least 2 roofs) bends, but it is reflecting the elevation changes of the foundation shift rather than a roof frame issue.

I suspect the 1x4 skip sheeting under the roof deck is working like a set of tension members actually keeping the deck intact rather than letting it separate as it would if the deck was loosely nailed to the framing. The "nail through" of deck to frame with skip sheeting in between does produce a bit more "float" in the deck itself so in part, you are right about it not being fully tied down.

Since I do not see foundation vents, I suspect the house was built in the years when we were transitioning from raised foundations to slab on grade. Subdivisions were being developed extremely rapidly and soils were not adequately settled to reduce shifting. Add to that "Most" subdivisions like this had 0 or negative slopes in the back yard and the floor plan that puts the house on the side of the garage (more compact than previous garage in front of house) and you get this type of settling issue with a bow along the house to garage junction.

As stated in an earlier post, the Garage slab is not tied to the foundation. This lets it ride up and down.

The house side garage footing extends from the house with NO tie to the other side. The rest of the garage footing is essentially this same type of "flyer", a grade beam extending across the back of the garage around to the front....you will see a sagging beam over the 2 car garage door and when you add in the expansion cracks in the drive, you know the "free" side of the garage footing is shifted away from the house resulting in a lowering of the Garage Ridge that includes the valley tie into the house roof.

Again, this is geometry in action where small movements in the foundation are magnified by the triangulation of the roof structure. There is probably a gap under the garage door on the right frther indicating foundation settling.

Here in CA, home building is much much different than across the country. There are limited numbers of hand built homes and those are often produced using independent subs that practice "production" methods.

When you part out the construction process, there is limited connection between trades and problems with site development (my speciallty) are blamed on the concrete forms, the rough carpenter and finally covered over by the sheet rockers and roofers.

Homes built prior to 1955 tend to be far higher quality since we had fewer tools to move massive amounts of soil in order to prep subdivissions. As those tools became readily available in the late 50's and 60's, we forgot about "not building in the swamps" and simply graded and filled until we had a slightly elevated level spot to plant houses on.