I have to span a square of about 50 by 50 feet with a roof. The roof
should have a cupola in the middle.
I am wondering if I can simplify construction of the roof and reduce
cost by putting up four columns arranged in a much smaller square in
the middle of the larger square. These columns would form the base of
the cupola and be tied together on the top by a sturdy wooden square
What I am hoping to do is to eliminate large and complex trusses by
having rafters running from the exterior wall to the columns and tying
into the frame on the columns directly (somewhat like in a shed roof).
The cupola will then be erected above the square frame on top of the
columns. The rafters would have to be about 25 feet long
and span empty space.
If simple rafters cannot span this, would it at least result in less
complex and smaller trusses
that are easier to transport and handle.
Can I do this and achieve significant cost reduction in the
construction of the roof.?
If this particular construction does not work can columns be used to
simplify the construction of the roof in some other way?
What do you think?
Well, you've left out a whole lot of information, like what general
shape you want the roof to be, how big the cupola is, where you're
located (snow loads, hurricane, seismic?), what the structure is going
to be used for (affects code requirements), etc.
Generally speaking 50' trusses are going to be a problem to transport
and you'll certainly need a crane. The four columns in the middle
would result in shorter, lighter, less deep, more transportable and
easier to handle trusses. Cheaper to buy and most likely cheaper to
install. It's hard to say without more information, but most likely
the four columns and their required footings wouldn't entirely offset
the cost savings of the shorter trusses, so you'd probably come out
ahead in the construction cost.
You might want to take some sketches to a local truss supplier and have
them use their software to work up a couple of variations so you can
get a more precise idea of what you're dealing with price wise. That's
usually a free service, so there's no reason to delay contacting them.
After that, it's time to talk to a local engineer, architect and/or
contractor to determine the rest of the design and get some budgeting
Many neophytes assume that all columns are bad, but they can actually
help organize a plan. I'd suspect that might be the case in your
situation, but without knowing the building's purpose it's not possible
Your "design" does not appear to have any provisions for dealing with
lateral forces. I think you should hire a local engineer to investigate
this. The structure you propose is way too complicated to discuss in a
Bob Morrison, PE, SE
R L Morrison Engineering Co
I believe that the OP is asking for a design. That too cannot
be done in a newsgroup.
To the OP: Try the least expensive route first; Call a truss
company and talk to the truss designer. Discuss what you want
and ask what can be designed for your project. He can give
you some details without actually designing the project. If
you can't live with what he can do for you, then you should
contact a structural engineer to design it for you.
The truss manufacturer will not charge anything to tell you
what he can do. He will not charge you for the design of the
truss system if you buy the trusses from him. The engineer
will cost you money right from the start.
On the other hand, the truss manufacturer will assume that the
building that you set his trusses on is designed to withstand
the forces that will be placed upon it. That includes not
just the weight of the trusses, but lateral forces, wind
uplift, etc. If the building is not engineered for that, you
may have to contact an engineer anyway, to make sure that it
will all work together.
Thanks for all responses.
The roof would be a pyramid with a roof pitch of about 4 in 12 along
the shortest rafter
(parallel to the exterior walls) and less elsewhere.
This is for a house to be built in Florida ("Cracker style") so no snow
I am in the early stages of design.
The columns would be masonry 2 by 2 feet fully rebared and grouted and
anchored to the foundation slab with rebar.
The columns would be about 16 feet tall and form a square of 11 by 11
If these are connected rigidly at the top they should be resistant to
I agree that this has to be worked out by an engineer though.
I am just soliciting opinions to see if such a design makes sense from
a cost perspective.
I want to have some columns in any case but having them exactly in the
middle makes the interior design somewhat harder.
In general, the material for a truss roof is more expensive, but there
are usually substantial labor savings. About the only time I have used
rafters is when the design calls for a vaulted ceiling that can't be
accomodated with a scissor's truss. In your case, setting monstrous lvl
hip rafters and hand framing all the jacks sounds like a ton of labor.
I think trusses are going to be the way to go.
That's a big if, particularly for site built block columns. R/C
concrete or steel would be my first choice.
If you're thinking that you're going to have a 50' x 50' open space
with only four columns and no other walls to stiffen the structure to
resist the lateral loads that Bob "If I Design It, It Will Stand"
Morrison referred to, you might have a hard time...errrr....a too easy
of a time getting it off of the ground!
First step. Truss manufacturer in the area where the house will be.
Next step, if you can't accept the truss manufacturer's "can't be
done", is an optimistic engineer.
Report back here when you get the results from the truss manufacturer.
I forgot to say that there will be load bearing perimeter walls around
the 50 by 50 square (it's a house), so the space is not open. Nothing
in between would stiffen the structure though.
I'll report back if I get the opinion of an engineer but it will take
quite a while.
OK, I get what you are saying: the perimeter walls have to be stiffened
against lateral forces.
I did not think about that and actually planned on having more than
half the space without walls of any sort. Thanks for pointing out that
this is not possible. If I were to stiffen a 10 foot high perimeter
wall with a perpendicular load bearing wall, how long would that wall
have to be about: 5 feet, 6 feet, 7 feet?
How many such walls would I need along the 50 foot length: More than
Obviuosly nothing exact can be said without having all the details but
if it's pretty clear that I need two such walls I might have to
rethink the design in general.
......as you said
..."Obviuosly nothing exact can be said without having all the details
the details & requirements are coming out slowly.....I thought this was
a giant outdoor / patio / bbq area at first...like in a public
park....not a house
If you're really serious out this potential project I would suggest
getting (paying) a home designer to bounce some ideas off of (realtime)
so you can arrive at "doable" design starting point.....details &
changes / tradeoffs can be made from there.
otherwise I can see this getting a little frustrating for the guys in
......"If I were to stiffen a 10 foot high perimeter
depends on the number of stiffner walls you have & what they're made
out of....brick, concrete, timber?
If the perimeter walls were r/c & thck you won't need to stiffen
The perimeter walls would be autoclaved aerated concrete block mortared
and possibly surface bonded in addition. Interior load bearing walls
would be standard concrete block surface bonded only (no mortar) but
rebared as needed and fully grouted (for thermal mass).
The need to stiffen these walls does put a crimp in my plans since I
was hoping to generate a very large open space. I might have to rethink
this but once the structure is properly stiffened the idea of using
columns to simplify roof construction and installation.
I'll draw up several designs and then pay an architect to talk over how
realistic these are.
Thanks for all the advice, it is certainly extremely helpful.
Don't bet on that. If you have way more money than you know what to do
with, you might be able to make it work, but the walls and roof
structure would have to be so heavily built that it would be
Post this in alt.architecture. Put "Ping Don" as the subject line.
Don is a designer who worked in FL for 20 years and will know exactly
what you are up against with code and hurricanes. He also used to work
for a truss company. Tell him he still owes me twenty bucks.
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