Building deck over uneven concrete slab

[beisizhao]

My family and I are looking to build a low lying deck over our concrete patio but we've run into a few problems with the idea.

The first is that the concrete slopes down and there's at least a 20cm difference in height between the highest and lowest point. We were unsure about whether it would be a better idea to try and level the ground out first and if so, what the best way to go about doing so would be. I thought about using leveling compound and my mum had suggested using wedges to offset the slope to keep the timber level.

The second is that there is less than 10cm of clearance between the floor of the house and concrete outside, and even less at the highest point. I presume that to build a deck it would be higher than the floor of the house which is fine, but just wondering if that would present any additional issues.

Another point of note is that there has been a small raised structure built alongside the house to level out the slope on that part of the patio in order to place things on it, have attached an image that shows it along with the slope. Was thinking of using that as a guide if I were to use leveling compound.

Been doing a lot of browsing in these forums and have gained a lot of insight but still very unsure. I would really appreciate any assistance in clearing up my confusion and I give many thanks in advance :))).

[Jason]

Hi @beisizhao,

Let me extend a very warm welcome to the Bunnings Workshop community. It's great to have you join us.

I'm sure our resident Bunnings D.I.Y. expert @EricL will be happy to assist as soon as possible, but in the meantime, have you seen the helpful resource How to build a low-level deck by experienced member @Adam_W? Let me also tag helpful members in @bruce93, @Chiren_J and @TedBear to see if they might like to share their thoughts.  

You should also get ideas, information and inspiration by searching for the term "low-level deck" and checking out out Top 10 most popular deck projects. Low-level decks are certainly a very popular topic on Workshop and have their challenges. 

Looking forward to seeing how your new deck takes shape (with a bit of help from our clever and creative community members along the way). 

Thanks,

Jason

[TedBear]

Hi @beisizhao ,

I will leave it for EricL or others to go into the how-to of deck building, but I need to mention that your local council will probably be horrified at the idea of building an external floor surface higher than the the internal one. (i.e. not a legal option, water entry issues,..)

Even though they won't know about it now, you may have to remove the decking if you ever wish to sell the house (or an inspector ever visits).

It may be wiser in the long term to break up the existing concrete floor, (at least the high end and leaving any existing posts supported) thus negating the slope issues, and build your deck at the correct level, which will be a bit below internal floor level (check your local council regulations on-line for the correct floor level differences. It's probably the height that the existing concrete is now near the doorway).

[EricL]

Hello @beisizhao

Thank you for sharing your question about building a deck over an uneven concrete slab.

It's great that you've received some advice from @TedBear. The points raised in the recommendations are worth considering when planning your deck. How big will your deck be? Decks of a certain size sometimes require a permit. How tall will your deck be? It is important to consider material size and ventilation when constructing your deck. If there is not enough space, levelling the existing area by removing the slabs may be necessary.

Looking at the picture you've posted your current cement floor outside is not too far from the indoor level. This may present an issue when building the deck in front of it as it will be taller than your interior floor and present a tripping hazard.

I suggest reviewing the options for this build. The deck can be built away from the slab. This will make it easier for you to plan a regular deck and not have to deal with the uneven slab. The uneven slab can be removed and the area levelled. Please note that even by removing the slab there may not be enough space between your interior floor and the outside area. I recommend considering other possible locations or reconfiguration of the deck layout that avoids the cement slab.

If you need more advice or information please let us know.

Eric