What is Post and Beam and Why would your Deck care?

Have you seen that impressive framing in some church ceilings?

Gorgeous eye candy.  Displayed before you, intentionally to inspire awe.  How do they build that, anyway? There’s not a screw or bolt visible.  All wood connections — they use mortise and tenon joints, tongue and fork, dovetails, trunnels,…designs and techniques from centuries ago.  This construction is called “Timber Frame”, …

Hey, what’s all this fancy talk?  you ask.  Sounds like dinner with forks and doves and morsels and tongues….What’s this got to do with decks?

OK, I’m getting to deck construction.

….So, the primary, underlying element here is Post and Beam.

Post and Beam is simple, right?  There’s a post and there’s a beam.

No, that would fall down.

          

                              You need 2 posts.

OK, I see.

So, how does Post and Beam apply to decks?

Backyard decks want an airy feel, they want to bring you close to your yard, close to nature.

Houses, on the other hand, need to protect you from the elements – rain, cold, snow, wind,… So houses need solid walls (and a solid roof), and they commonly use conventional lumber framing – “stick framed” it’s called – with thin studs covered with plywood.

 

Plywood (or OSB) sheathing reinforces and ties the stick framed walls together.  And there are variations: Balloon framing, Structural Insulated Panels (SIPs), even modular houses built in a factory….

There you go again, getting way too complicated.  Cut out this extraneous stuff about SIPs and modules and balloons.  You’re supposed to be explaining deck construction – remember?  

Sorry.  Instead of lots of skinny sticks and plywood panels, decks use thicker 4×6 (and 6×6) posts and stronger beams that span openings 6 or 8 feet wide and 8 or more feet high to create an open frame.  And most of that frame is below the deck floor, so you get great views from your deck.

  

Here’s the frame of a simple Post and Beam deck:

Now, I am about to use some technical words;  this may get scary.  (For a primer on deck construction terms, see my article,  “Can’t Understand your deck Contractor? Here’s Help.”)

To understand more about Post and Beam construction, let’s examine what it is not.

1. THROUGH-POST

 First, Post and Beam is not Through-Post construction.  Here’s a Through-Post deck:

Note how the support posts run from the footing at grade up through the deck frame to become the rail posts.

That’s ingenious!  I bet those rail posts are strong.

It is not ingenious.  It is seriously flawed.  Let me explain.

Rails posts are normally built with 4x4s, which are strong enough for rails and look appropriate.  A Through-Post builder runs a long 4×4 post from a concrete footing up 4 or 6 or 8ft to the floor frame and then 3 ft higher so it becomes a rail post.  Seems good.  HOWEVER, a 4×4 is too small to be a primary deck support.  It’s ok for a rail post, but it lacks the lateral strength that a deck needs.  A deck support post has two  tasks:  to support the vertical load of the deck (often 1,500 lbs or even 2,000 lbs when the deck is full of people or snow ) and to secure the deck frame against horizontal forces (wind load and, potentially, 750# lateral load of people moving around).¹ Realistically, these primary support columns need to be 4x6s for low decks (up to 5 ft above grade) and 6x6s for taller decks (6 ft or higher).²

And Through-Post has an even worse flaw:  its connection.  To fasten the deck’s rim  to each post, the builder has two choices – one bad and the other worse.

A. He can bolt the structural rim to the side of the 4×4 post:

This is inherently weak.  Those two through-bolts must now transfer the deck load (hundreds or even thousands of pounds) without wobbling to the 4×4 column, which runs down to the footing.  That load may not shear ½” diameter bolts, but it can force the bolts to crush the wood and sag, which loosens and weakens the joint.

OK, that’s bad.  You say the alternative connection is worse?

Yeah, the other option is worse.

B.  Some builders notch the 4×4 post so part or all of the rim joist sits directly on the post.

Here the carpenter can cut his notch in two ways.  First, he can cut a 3” notch to fully support the rim joist (a double 2×8 or larger).  This connection supports the vertical load well but leaves only ½” for the “tongue”, which is far too thin for lateral support.  Someone leaning on the rail will break it.

Or, he can cut a shallow notch so the tongue is thick enough to resist the lateral load – at least 2” thick — but then half the rim joist hangs in the air with no direct support.  Its vertical strength is seriously compromised.

Choose your poison:  The full notch is strong vertically.  But it is horribly weak laterally.  The shallow notch is stronger laterally, but fails in its primary purpose — supporting the weight of the deck and people.

Furthermore, the Building Code and many Building Inspectors will not allow notched rail posts.

If this weak connection is not bad enough, Through-Post hides a nasty surprise.  It allows the rails to dictate the locations of the support posts.  Rail posts at deck corners require support posts be placed exactly at those corners.

Aren’t corners good places for support posts?

Sometimes.  Unless large rocks underground block the footings.  Then the carpenter has to dig out those big, obnoxious rocks (ugh) or, if the rocks are impossibly large, he must move the footing.  And move the support post.  And change the deck size.  Do you really want hidden rocks dictating the size of your deck?  Can’t you outsmart dumb rocks?

Actually, you can.

2. GIRDER

Consider Girder construction.  Here the builder separates the rail posts from the deck frame:

Girder construction allows the main supports to be larger and stiffer — 4x6s or 6x6s.  We’ll call them “Columns” and the double rim a “Girder”  — to distinguish them from Thru-Post members.

Steel joist hangers fasten the joists to the Girder.  The Girder transfers the deck load to the Columns.  But now the Columns are large enough to accommodate full 3” notches, which properly  support the vertical load, and still have 2½” tongues to bolt securely to the beam and accommodate the lateral load.

That’s smart.

Yes, it is smart.  

And Girder framing offers another advantage: because its Columns are independent from the rail posts. the rails do not dictate where the Columns and footings must go.  So, if a hidden rock forces the carpenter to move a footing, he need not change the deck size — he just slides the footing over.

I like this.

And you say Post and Beam is better?

3.  Post and Beam

Yes, it’s better in several ways.

Simplicity.  Post and Beam puts a double (or even a triple) beam under the joists, where it supports the joists directly.  Joist hangers become unnecessary.  Wood joists on top of the wood beam are inherently strong.  A carpenter can screw each joist to the beam below, so it cannot slide.

That seems strong.

Yeah, it is strong.  And it gets better.

Deck Size.  Note how the beam is set back from the front edge of the deck.  Setting the beam back like that allows you to build a larger deck.

Really?

Hey, I wouldn’t lie to you.

Deck depth (the distance a deck extends out from the house) is limited by joist length.  Based on actual breakage tests, the Building Code specifies how far a joist can span out from the house to its support beam.  Thus, for example, #2 Pressure Treated 2×8 joists, placed 12” apart, can safely span 13’ 6” out.³ To make your Girder deck larger, say, 16 feet deep, you must upgrade your joists to 2x10s, which adds considerable cost.  But the joists in Post and Beam sit on top of the beam, so you can keep the beam at 13’ 6” and extend the joists 2’ 6” further to reach the 16 feet you want.

Now, that’s really ingenious.

Now you get it.  Extending those joists past the supporting beam creates an overhang, which builders call a cantilever.  (For more about cantilevers, see  “What is a Cantilever and Why would your deck want one?”.)

Greater Independence.  Notice how Post and Beam frees column locations from the deck frame.  The columns can slide left or right along the beam, as they did with Girders.  But the entire beam can also move in towards the house or further out (within limits).  It can even angle to avoid rocks or other obstacles.

Efficient Construction.  For some deck shapes, Post and Beam construction can eliminate some footings.  Consider a 12ft x 16ft deck with angled corners:

Thru-Post or Girder frames require 5 footings here, but Post and Beam solves this configuration with only 3 footings.

Which technique would you prefer?  I know my tired back votes for Post and Beam.

Design Freedom.  Consider the beam’s location.  It can be at the deck edge, or it can be further back.  That independence allows you to efficiently build some interesting designs.  For example:

Nice.

Code Compliance.  Do you need further convincing?

Consider this:  The Building Code mandates that decks use this Post to Beam connection, or “other equivalent means capable to resist lateral displacement”.^{4 5}  What does “other equivalent” mean?  In practice, it means you’ll have to convince the Building Inspector that your Thru-Post or Girder construction is as strong as Post and Beam.  Indeed, he or she may require you have a Structural Engineer review and stamp your drawing.  Think $$$.

So now you’ve reached the end of this little treatise.

What structure will you choose for your deck?  Thru-Post? …Girder?…or Post and Beam?

I think I know your answer.

And why should your deck care?  Your deck wants to be Strong.  And Big.  It craves Flexibility and Independence.  It wants a long life.

Decks have feelings too, you know.  Your Deck wants Post and Beam.

We at Archadeck of Suburban Boston offer professional design and build services for clients west and north of Boston.  Over the past 27 years we have designed and built over 1,000 projects.  We have enhanced the depth of our expertise by limiting our work to decks, porches, sunrooms, and patios.  To view some of these projects, visit our website.  To learn how we treat our clients, check on us in Angie’s List or read an article about us in Remodeling Magazine.  For a free design consultation and a relaxed and rewarding design and construction experience, contact us via e-mail, subboston@archadeck.net or by phone, 781-273-3500.

© 2019 Advantage Design & Construction, Inc.

FOOTNOTES

1. International Residential Code 2015, (IRC 2015), para R507.2.4.
2. Technically, the current building code, IRC 2015, allows 4×4 deck support posts. For now.  I expect that will change in the future.
3. Per  IRC 2015, Table R502.3(2), live load 40 psf, dead load 10 psf.
4. IRC 2015, para 507.7.1 Figure R507.7.1.
5. The Residential Building Code provides limited construction details.  It sets general load standards, safety requirements, and occasionally some specific dimensional limits (e.g., rail height), but is silent on many specific details.  Some jurisdictions (states, counties, and even some towns) reference other documents that prescribe deck construction more explicitly.  The Design for Code Acceptance 6, “DCA-6”, is the most popular.  Because its detailed connections and configurations must apply to a wide range of deck situations, its author, the American Wood Council, made it very conservative.  Its section on Post Requirements implicitly outlaws Thru-Post construction.