Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Concrete roof forming

Concrete roof forming

Concrete roof forming

I'm planning on building a couple of underground rooms with
concrete roofs. I was thinking that small steel I-beams spaced  
2 feet apart would work. The rooms would be 8' wide x 30'. The
concrete will be about 4 inches thick. Between the ibeams would be either 1/2 or 3/4" plywood. Does this sound resonable? the beams would remain in place. I was hoping to get like 4" or 6" I-beams.

Any better way of building a concrete roof?


RE: Concrete roof forming

Methods like this were used a long time ago for small concrete slabs and culverts.

They work, but have
some disadvantages :
If the steel is not covered by at least 1'' of concrete, it will rust if exopsed to miosture.
The strengths of the steel and concrete are not used to their full efficiency.
The mass of steel is considerable, and so the construction is more expensive.

Conventional reinforced concrete is cheaper and more efficient.

RE: Concrete roof forming

Are you planning on leaving the wood in place? That's not allowed in most areas. You should be able to determine the weight of the deadload (concrete, beams and plywood) and determine applicable live loads (wind, snow, etc) and design for the beams from there. From a formwork standpoint, the spacing of you beams may be determined by how you intend to support you plywood. If the plywood spans over the beams, 2' may not be sufficient depending on the slab depth. Get an engineer.

Unless you can get steel beams for free, I too would expect a Reinforced Concrete (RC) roof slab to be cheaper and more efficient, even easier to build.

Assistance from a local engineer is the appropriate route for the slab design. Assistance from a local contractor or formwork engineer would be helpful for construction of the slab.

I design and build RC structures formed off wood daily. The formwork would not be too difficult, although it all depends on the slab depth and your location.

Good luck,


RE: Concrete roof forming

ok. Correct me if I'm wrong but isn't RC simply concrete with rebar in it? If this is the case then I still need to form the slab....i.e. I need to support the concrete while it dries. That is why I was going to use steel beams. Do you think a 2x4 or 2x6 form would be sufficient? wouldn't need a ton of support under the form! I have not checked into steel beams as of yet but are they that expensive?

RE: Concrete roof forming

I think the above responses thought you planned to use I beams for reinforcement, as opposed to a support system for you formwork deck.

Can you give me some more information on the I-beam?
Dimensions and weight per unit

But my first reaction is that steel I-beams would very heavy and cumbersome to use.

There are a couple material options for a slab like this,

But the system is generally the same

First Set up primary beams, depending on what material you use for your primary beam this will dictate the interval of the props required to support your formwork if using      6"X 3" timber I would suggest no more then 4'intervals
Again depending on what material you’re going to use for you secondary beams this will dictate how far apart you span your primary beams if you are using 6"X2" timber 4' intervals again will suffice based on a 4" thick slab

Start at the wall and keep going until you reach the end of the room you can alter the span up or down a small amount so your intervals are spanned equally, my preference would be to put the primary beams in the 8' direction.
To hold your primary beams in position you can use lengths of 2"x1" timber or 2" rips of plywood and simply nail one on each end of your 1st primary and then nail it to your 2nd primary at the correct interval repeat this sequence for each primary beam until complete.

The next step is to put your secondary beams on top of your primary beams these should go in at 20 " intervals if using 3/4 inch plywood. I wouldn't recommend using 1/2 inch as you would have to reduce your intervals by at least 25%.

The next step is to sheet out your deck, and again start at the wall and work way down to the other end with your room being 8' wide and a sheet of ply also 8' wide put your sheets in to the 4' orientation as if you put them the other way it may cause difficulty's when striking.

I hope this is of some use to you

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close