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A home in Salt Lake City that I am considering purchasing has the following condition regarding the roof structure.
A. The outside walls appear to be constructed of 4” high by 4” deep by 16″ long cinder block, doubled so that the total thickness of the bearing walls is about 8 inches. The house is essentially a rectangle 37 ft front-to-back, 68 ft side-to-side.
B. The orignal flat-roof is comprised of 2 x 10 joists set on the walls front-to-back 12″o.c., 1 x 6 docking, several layers of paper and/or tar and one layer of gravel on top. THIS ROOF HAS BEEN LEFT IN PLACE over which has been constructed a large hip roof with trusses spanning the entire 37′ distance front-to-back.
My question is whether I should be concerned by the weight of a “defacto” double roof on the bearing walls? Should the decking, tar/paper + gravel have been removed before the trusses and roof were set?
Replies
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Seems like a bad idea to leave that old roof as is. First,
the weight issue is a valid point. Second, it makes for an
awfully strange attic floor. Third, very complicated system
regarding insulation, venting/moisture control.
On the plus side, it'll be quieter in there, and if your hip
roof leaks, you may not know it until you have an indoor
swimming pool.
So then they go with 37 foot trusses without removing at
least the gravel? Who are these guys, Butch?
A couple sticks of dynamite should be enough
MD
*James,I've trussed many flat roofs, and it is not necessary to tear off the old flat roof. Unless this house has a problem with the original footers(not to code or poor soil conditions), or the cinder block is disintigrading, then you should have no problem at all. Trussing a flat roof is a great way to eliminate the problems associated with flat roofs, and make a dramatic improvement in a building's appearance.If you are in any way leery of your footers or block, then have an engineer inspect them. This would be true whether the trusses are on or not.Hope this helps, John
*James,The tar paper & gravel should have been taken off before the roof trusses were installed. Who knows how much water or other damage was under that built up roof before the geniuses framed over it? Did they at the very least cut back the perimeter down to the joists, put some solid plate material around the edges to get above the roofing, and set the trusses on top of the new plates and not on the roof surface itself? Mad Dog makes some very valid points about the insulation, ventilation, and moisture control. If there is a typical bearing wall running down the middle of this thing, weight does play a part. Even if the trusses are bearing on the plates and not the middle of the joists, 2 x 10's framed flat and spanning 18' would have trouble over time not sagging from their own weight, not to mention the weight of a built up roof.I would agree that trussing a flat roof is a good way to eliminate the problems with a flat roof, but take the roofing off first and check out what you're working with. Unless you're stealing the house, I'd look for something else. You will have more problems than it will be worth. Red dog
*Red Dog stated my thoughts almost exactly. When it warms up in the spring, I have a roofing job on a flat roof ahead of me. From what I understand, underneath the gravel is about 10 years worth of driveway sealer or the equivalent, one layer applied each year over an original failed membrane, and who knows what else. The last thing I'd do is think of just throwing a new roof over all this stuff without taking some of it off first, but that's just me, probably erring on the side of caution.It just seems like there could be a whole lot of added weight stressing out the original framing. It is possible that the only reason the hip roof was added is a design consideration, but that is far less likely than the fact that the original roof had problems. It needs to be checked out before you buy the house.MD*******
*James stated the 2x10 joists are 12" OC,(I don't see any mention of them laying flat) and that the roof consisted of 1x6 decking, several layers of felt(no mention of roof insulation-this is what what soaks up water in flat roof leaks), and 1 layer of gravel. This roof is not that heavy, unless there is an unusual amount of gravel on it. Granted, any loose gravel should have been swept off-that's SOP on any truss-over jobs that I do.This house now has a ceiling of 2x10's 12" OC spanning less than 18' (when you deduct bearing wall thicknesses-and I am assuming there is a center bearing wall. If there wasn't I am sure there would be a noticable sag that James would have mentioned). 2x10s 12" OC, even carrying the weight of the old roof described, is plenty strong, and meets code for ceilings in an attic that is not to be used as living space. And if the gravel was removed, it may even meet code for a floor.Trusses spanning from outside wall to outside wall should not be adding any weight to the existing roof/ceiling and all loads exherted on the truss will be transfered to those outside walls. That's why I mentioned making sure those walls were up to the task. If installed to code the truss bearing will be on a plate installed on top of these walls and not on compressable material such as rigid roof insulation. But James makes no mention of this type of insulation in his description.I won't argue the fact that entirely removing the old flat roof may be the best way to do these jobs, just that it's a very expensive and impracticle option on an already expensive project. Also, most of these jobs are in occupied homes or buildings. By completly removing the old roof, you expose the living area to the weather. Anyone who has tried to "dry-in" a flat roof knows exactly how tough this is.In any case every job is different, and there are always unique situations that must be taken into account.John
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James,...There are always IDEAL solutions to problems."By the book" answers to the way things "SHOULD" be done.Those perfect world answers usually ignore real world finances ,weather realities,and assorted other factors which will influence a projects eventual scope and shape.
If you read enough posts here on BT you will be led to believe that all of us do flawless work,textbook examples of the way things should be.Of course this is baloney,since all of us have to make compromises and trade offs every day.
Your roof situation involved one such set of trade offs and compromises.It is pretty unlikely that any benefit resulting from the proposed removal of the flat roof would have been worth the money and work involved.IDEALY it would have been done,but that is different than saying it NEEDED to be done,or it should be done.In fact ,in my limited experience with this topic it is rarely done and the buildings suffer no ill effects.
If you are really interested in the building,why not consult with a local guy experienced in this type of work.He can evaluate the workmanship and materials involved,discuss any code matters and in general give you the reassurance you want to close the deal,or give you a good excuse to activate a "weasel clause" .
Good Luck, Stephen
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Excellent advice, JRS and Stephen. It is true, the real
world compromises. And you know, the roofers there had a
much better look at it than the impression we get on the
screen. Chances are, it wasn't the roofers/framers who made
the decision anyway. When someone changes a flat to a hip,
you'd think they would have plenty of experience, I just
tend to be cautious because of all the nightmares I've
uncovered.
MD
b "Perception by cyber space can equal distortion of
reality"
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*Hi Mad Dog,There certainly is no shortage of nightmares out there, and when someone finally gives up on a flat roof the nightmare ratio goes way up!Good luck on your up coming roof, John
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A home in Salt Lake City that I am considering purchasing has the following condition regarding the roof structure.
A. The outside walls appear to be constructed of 4” high by 4” deep by 16" long cinder block, doubled so that the total thickness of the bearing walls is about 8 inches. The house is essentially a rectangle 37 ft front-to-back, 68 ft side-to-side.
B. The orignal flat-roof is comprised of 2 x 10 joists set on the walls front-to-back 12"o.c., 1 x 6 docking, several layers of paper and/or tar and one layer of gravel on top. THIS ROOF HAS BEEN LEFT IN PLACE over which has been constructed a large hip roof with trusses spanning the entire 37' distance front-to-back.
My question is whether I should be concerned by the weight of a "defacto" double roof on the bearing walls? Should the decking, tar/paper + gravel have been removed before the trusses and roof were set?