Using Deck Foot Anchor for Deck with part of deck over a patio stoop
I want to build a 12' x 20' (or, possibly 13' x 23') deck over my concrete patio stoop - that is 4.5' x 8'. Can I do this using the deck foot anchors? (the height would be between, roughly, between 14" and 20" off the ground - slight slope to back yard)
I live in Colorado - where there is frost heave (I assume) and, of course, snow.
There is nothing wrong in theory about building a free standing or floating deck so that a portion of the deck covers an existing concrete stoop. However there are some things to consider to avoid problems later.
First off you should think about whether you want the new deck to be supported partially by the stoop or completely by the footing system you use so that none of the deck framing is in contact with the stoop.
This latter concept may not be so easy to do if the clearance you have between your door sill and the surface of the stoop is very low. In fact it may force you to have to use the stoop as a support. Furthermore, if the clearance is too low you might have to notch some of the joists to fit over the stoop.
So without the details and the height differentials I am just raising the issues as I see them. Let's take a look at this second idea, using the stoop to partially support the deck and using a floating footing system like the deck foot anchor...
If the soil you are building on is undisturbed and not subject to any more settling and if you use enough footings in order to distribute the load of the deck without exceeding the soil's bearing capacity, then resting a portion of the deck on the stoop should present no problems and allow you to get the deck low enough to fit nicely below the door sill.
What Can Go Wrong
Let's assume the soil can safely support 1800 psf but you use too few footings and a portion of the deck above a footing (tributary area) imposes more than 1800 pounds of weight onto the one square foot load plate of the deck foot you can expect that portion of the deck to compress the soil and settle. Then the deck is going to tilt because the stoop will not ever move.
This is a bad thing. But fortunately it is very easy to avoid by using enough footings rather than too few.
A non-frost footing like a concrete block or the deck foot anchor will be subject to the effects of frost. When soil freezes it expands if there is any water in it. The soil acts like an elevator platform. While the deck foot differs from a concrete block because it is firmly anchored and connected to the ground, the size of the load plate is so big that it will rise with the surface expansion. And so the entire deck supported by those footings will rise.
The amount of rise depends on the amount of water in the soil and how deep the soil freezes. Assume 1/4" to 1/2" in a good cold winter with three feet of frost.
Then in the spring the soil will thaw and the soil will contract lowering the entire deck back to it's resting position. You should expect this annually and account for it so the deck has room to move up and down. You will not ever perceive this movement but it happens.
I thought I would just quickly mention the opposite of frost because there are soil types which actually shrink during period of drought or dryness. So the very reverse is true and would apply with a deck supported by a floating footing system.
Mind you, you would very likely see the same thing occur with a traditional concrete footing system as well unless the underside of the footing is very deep so as to give you enough vertical distance that the shrinkage is negated.
So for this to work properly, use enough footings so you do not overload the soil and account for any upward movement in the winter. Know the soil type you are building on and get a sense of what the soil can bear. Your building department will have rules of thumb in your area for soil type as they approve building plans for residences and have to know the common footing sizes and soil bearing capacity.