building a flat porch roof

huangyhg

building a flat porch roof
that will be in couple years will be converted to a balcony  with following spec...
- approx. 5ftx 20ft
- snow bearing

using engineered i beams for the 20 ft spans   and   4x4s for 5ft spans ;  what i-beam thickness/quality should i use to support the balcony and can i use  two  4x4 posts to support the front i beam and if not what about 4x4 or 8ft aluminum posts that are available for fence construction.  do i need to support the frontal i beam in the middle by a post or not..
your post starts out in the middle of a thought so it is a little hard to follow.  you need to find answers to the following questions as a start.
what is your snow load?  snow loads can vary from 25 psf to over 400 psf depending on where you live.
how are you attaching your 4x4s to the building? are the 4x4s being used as joists which will overhang the 20 ft girder with decking above them?
how long are your columns?  are your columns supporting a deck and a roof?
what are you using for foundations under your columns?  what are you using for connections?  are your columns continuous for more than one story to support a deck and a roof?
once you have answers to these questions you best answer will be found by consulting a local licensed engineer who will look at the whole project.
assume
20' dim for beam
joist 4x4 - 5' oc
ll=50 psf
dl=10 psf
sl=25 psf (wild guess)
deflection limit l/360
wet service
no overhang
attachment to house ok
beam size (don't recommend i-beam)
3"x14" glulam or 3-2x12 syp #2 (deflection limits design)
joist
4x4 ok but use 2' oc
post
check bearing limit
i would use a center post and 2-2x12 syp joists and 2x6 joists.
boo1 and rockengineer
thanks for the replies... i do appreciate it.
- attachment to the house is ok ( it is brick wall with cement blocks as  backing!   )
- likely that few bricks will be removed to embed the 5 ft beams
- the snow load is about the average in new york area..
- post length -  approx 8ft
- no overhang
- the be a  deck
what is it about the i beams that will make them a bad choice..
if you are talking about a steel i beam, you have the problems associated with connecting your wooden joists to the steel beam and then if you want to use wooden posts you have the connection problem again.  the steel beam is only an advantage if you have really limited head room.  do you like the looks of a steel i beam for your deck or are you going to cover it up with something else?  
if you are talking wooden i-joists, they lack the strength you need and are not made to be exposed in the weather.  any exposed wood should be pressure treated or of a naturally decay resistant material.
you mentioned the possibility of using aluminum posts.  again you have the change in materials and the interface between steel and aluminum.  where i live on the west coast there is often a salt spray environment and when you have a steel/aluminum interface you can have exfoliation of the aluminum at the steel/aluminum interface.
your post is entitled "building a flat porch roof" so i assume you are trying to make this deck weatherproof to provide a roof for the area below.  if you are making the deck watertight you need to look at drainage and ponding and how you are going to protect the wood.
thanks again for the reply...
i was refering to the currently popular wooden i-beams - i  will look into gluelam ( i  believe it is 5x more expensive and i havent seen them around here, we will see)  and yes i understand that aluminum and steel act like a battery - thanks for the reminder - regardless, i did look into finding  steel beams in the local stores not too long ago  with no luck. the  aluminum posts that i saw in home depot looked very strong and attractive, size of 4x4s but i have yet to find their spec.   what is the required bearing limit for the posts in this case, is it 3000 pounds? or more. the regular wooden posts dont seem to  have any specs that i could find on the net.
as for waterproofing, there will be 3 degree ( 4 inches for 5 feet) downward slope to this so called flat roof and and there will be standard flat roof water proofing..
ps. right now i have a damaged aluminum roof  for the porch that is poping out and will not stand the next winter -  i am surprised to see how expensive it will be to modify it.

check with your local building code department to be sure all code requirements and snow loadings are satisfied within your porch design.
for two post configuration:
r1=r2=2125# for 25 psf snow load
     =4000# for 100 psf snow load
i would prefer to use three 6x6 post (knotch for bearing) if i were designing it.
see connection details at:
thats a very good document and i believe that i have most the info required and will make sure that the code is complied with.
thanks again.
liketools
your original post states the roof will be converted to a balcony in the future.  what will the balcony be used for?  will it in turn have a roof?  is your primary concern cost or appearance?  are you in the mountains, down wind of a great lake or near a large city in ny?(snow load)  these all affect the structure's design.  what about the foundation?  do you know the soil conditions at the foundation locations?
the code will probably give you maximum spacing for standard building materials.  you may need a design if other materials are used.  good luck.