Technicalogical Logical…Technically

Building a Deck, Day 3 – Mapping Out The Foundation Piers

DAY 3 – Mapping Out The Foundation Piers

Luckily, you don’t need an engineers degree to layout the foundation piers for your deck, but you might need at least a little help from outsides sources (aforementioned books or the internet) to determine their position.  Here’s what I’ve gathered so far:

• The easiest option for post placement is at the extents of your deck.  If posts are on the corners and along the edges, you’ll only need one set of measurements to determine the outside edges (rim joists) of your deck and where the posts will fall.  This will create a more boxy-effect when the deck is complete.
• I don’t like to do things the easy way (it bites me every time…)
• You can set your posts back from the edge of your deck to create a cantilevered area over the posts, both front to back and side to side.   This requires two sets of measurements however as you need to find the edges of the deck and then determine how far in the posts will fall from that edge.  This will create a more floating-effect when the deck is complete.
• The maximum overhang for a beam to cantilever past a post is 12″.  This is what my beam will be doing.
• The maximum amount of cantilevered distance you can have for the joists that overhang the beam is roughly 40% (unless you’re Frank Llyod Wright, or one of his students).  To be even more safe and to allow squeamish in-laws to use the deck, make the overhang no more than about 24-36″, regardless of framing lumber used, to prevent a bouncy feeling when standing near the railing.

With all of that in mind, I decided on 4×4’s for my posts and 2×10’s for my joists (all Pressure Treated Pine – PTP).  My beams will be built-up beams of 2-2×10’s sandwiching 1/2″ treated plywood.  The total width of the built-up beam will be 3-1/2″, the same width as my 4×4’s.  I wanted to do the deck in cedar, and most likely still will use cedar decking, however PTP has is stiffer and stronger than cedar, although it “moves” more as it ages, so I can create longer spans and use fewer post (less digging, less cement – something you can appreciate if you’ve ever done either before).  Since this will be a built-up beam, that means I’ll be mounting it on top of my posts, rather than bolting it to my posts.  This seems to be the safest option as you’re not relying on the aging ability of your posts, as you would be if you drilled holes through them for carriage bolts to bolt your beam to your posts.  Drilling those holes slightly weakens the wood, increasing the chance for cracking or checking.  I’m not saying that doing it that way isn’t safe, it is.  There’s just a greater potential for future problems.  For lower decks, it’s actually a necessity as there may not be enough room to have a beam mounted on a post and joists mounted on top of that.

As you can see in the chart above, the maximum distance I can have between my posts is 9 feet.  I am using a built-up beam, which is 3×10, and my joists are 11 feet total in length, however the beam sits at 9 feet with 2 feet cantilevered (so the greater of the two supported lengths is 7 feet – 9 divided by 2 – beam one one side ledger on the other – is 4 1/2, plus my cantilever – 2 feet, is 6 1/2 feet, rounded up to 7 feet).  Confusing?  It really isn’t, but as I type I realize how hard it is to explain in font rather than a diagram.