Converting deck to addition?

scrappy25May 6, 2010


Due to bid coming back much higher than expected for a 30 foot wide, 12 foot deep bumpout, I am working on scaling back the bumpout area to perhaps just a 10x12 foot breakfast room. Right now we have an 11 foot deep treated pine deck that runs across most of the back of the house, between 10-30 inches off the ground. It is 15 years old but looks in great shape since we have it cleaned and stained every 2 years. In the interest of economy, can this breakfast room be built on the existing deck? The deck now sits about two inches below the floor level. This would allow us to not have to tear out the rest of the deck, which could flank the addition on both sides.

Some considerations I have thought of:

1. Can supports/posts be added if necessary to an existing structure to support an addition?

2. Would it be best to remove the decking in the addition area to add insulation underneath??

3. I think that I could add sleepers on top of the decking with 1.5 inch rigid foam insulation, topped by a plywood subfloor, for additional insulation. Or would it be better to remove the decking entirely in the addition area an create a subfloor on the joists?

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Decks are built with sufficient framing for the deck and a maximum load of people/furniture.

Decks are not framed to support walls and a roof.

So, the answer is no, without extensive footing/foundation/reframing. Which would actually be more expensive due to the extra labor needed to remove the deck section.

    Bookmark   May 7, 2010 at 11:47AM
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Building codes usually require a deck to support a live load of 60 psf and the first floor of a house to support a live load of 40 psf. Obviously the perimeter would not be designed to support the roof load.

It would almost certainly be cheaper to tear it down.

    Bookmark   May 7, 2010 at 12:25PM
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Ok that's why I asked here. If all the decking and joists in the area of the addition is removed but the addition is only 10Dx12W, well within the footprint of the original deck which was was 11Dx25W, can the original deck posts outside the addition area still be used to rebuilt a deck on either side of the addition?

    Bookmark   May 7, 2010 at 3:57PM
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You would be well advised to configure the proposed addition to fit within the existing support structure of the deck so that you are not affecting the deck, other than removing that area.

Of course, this all depends upon the direction of the joisting.
Two words; perpendicular and parallel.

    Bookmark   May 8, 2010 at 10:18AM
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Since decks are built to support at least 40 psf or more...the same minimum loading that is required for the main floor of any house...

It is entirely possible to use this deck as a room addition and to support a new roof structure above it without making many or any structural changes to it.

In fact, it's done all the time.

How simply it can be done depends upon the existing support structure.

All things being equal, using the deck as the subfloor system for the addition and roof should be able to be quite easily.

Demolition of the deck with new a foundation normally unnecessary.

Have a professional evaluate the deck.

You'll likley be surprised at how easily the deck can be incorporated into the addition without major obstacles.

    Bookmark   May 9, 2010 at 7:08AM
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