Building a wood deck over a concrete slab

Factual Questions
1

So we have a slab with a huge crack as a patio. I’ve been looking online and it doesn’t seem that hard to put a wood deck on top of it.

Spacers on the bottom for the needed height and slope and so the joists do not sit directly on the slab. Spacers cut from joist wood.

Pressure treated joists parallel to the house, either 16" or 24" apart depending on decking material. Joists glued to spacers and attached to the concrete. Also a barrier between the deck and the house.

Decking perpendicular to the joints (and house for water runoff). May need to be stained and/or watersealed.

Am I missing anything?

Also, this guy is doing the same thing but call them sleepers instead of joists. Is there a difference?

2

Just last summer I considered doing the same thing but I decided I didn’t want a concrete under the new deck and paid a guy to break it up and haul it off ($500). My neighbor did not, and built his deck over the top of the slab. Now several years later the slab is grungy and moldy.

Bottom line… remove the slab if you can.

3

As far as I know a joist or beam supported by the ground or a slab is called a sleeper.

4

You may need a building permit for a deck. Check with your local authority.

5

Probably not. The action limit for a permit is usually around 30", but varies by community/county. Same for hand rails.

I’d seal the concrete slab really well, maybe even with an epoxy to retard the growth of nasties under your deck.

I’d also use metal, concrete or synthetic material for your spacers/sleeper supports. Wood, even treated wood, will eventually rot, especially if water pools on your slab.

You said you’re putting in a wood deck. Have you looked into composite decking instead? Both will fade, but the composite won’t rot. In either case, make sure you can easily replace deck pieces when necessary.

6

From the guy who built a huge deck, singlehandedly, partly over existing concrete and partly over a new raised extension…

You can probably leave the slab in place but you might want to drill it full of drain holes. Rent a 2-4 inch concrete drill and have at it. Try to hit all the low areas you know collect puddles in the rain.

Use ceramic tile, brick or another indestructible base for your pilings, topped with good pressure-treated timber. That half-inch to a couple of inches of zero-rot will keep the timbers high and dry enough to greatly extend their lifetime.

Use composite for the decking and railings. Wood is expensive if you use second-rate stuff and crazy-spensive if you use first-rate (no knot, heartwood) stuff. Composite looks good and lasts forever. (I used Trex, in the era before it got a bad reputation from a huge bad run that rotted in place. Still like it better than most of the alternatives - it has a new name. It has a nice woodgrain side and a smooth side, giving you more alternatives than smooth-only or the kind that has only one usable side and “tiling” ridges underneath.)

You don’t need a permit if it’s not attached to the house structure and doesn’t exceed a certain deck-to-ground height anywhere - typically 30 inches, but check your codes.

Railings have to meet code no matter what, mainly in that there’s a minimum spacing of balusters to prevent toddlers from catching their heads in them. It’s a standard distance, 3-3/4 inches IIRC, and there are plastic gauge tools that make the job easy. (I used one multi-block that had spacers for two different deck board spacings and was the baluster distance long - two of those and I never needed a measuring tape for laying board or checking railings.)

I also sank full, footed posts for the outer perimeter, rather than half-posts on footings and then bolted half-posts for the railings.

A laser level system is the gods’ gift to builders of this kind of construction, especially if you’ll be doing a lot of it without help. Even a cheap one saves vast amounts of time and reduces stupid, expensive errors.

My deck was over 1000 sf, wrapped around the corner of the house, dovetailed into an existing raised porch, went over an old concrete porch with swoopy curls like something from a 1960s James Bond movie, and was built like a stretch of the Maginot Line. And I really did it 100% myself - one of the last things I did at that house was walk out back and say goodbye to it.

Happy to answer any questions, now or as you get along.

7

if you are in a freezing environment then footing will keep the deck from ripping itself apart.

the slab is not a level stable base.

8

First: is that a real slab, or just an inch of concrete used as a weed killer?

Second: Since the big box “Home Improvement” places have started selling brown pressure treated (goes well with their green (undried; will warp, twist, and split as it dries) dimensional lumber) crap - find a real lumber yard and get green pressure treated wood.
Green - rated for structural use
Brown - not so rated.

Third - slope structure away from house.

9

Both true. I’m assuming it’s at least a 3-4 inch pour, that it’s been there long enough to be stable and that you don’t have a huge temperature cycle, especially deep freezing. In any of the above, either remove the slab or cut footing holes in it and build on concrete footers. If you’re in a more temperate zone with only brief dips below 30, you can probably use it as an overall footing as long as there’s no super-rigid point that a small heave will shatter. (Like, between a tree or rock and the house foundation where Ma Nature’s leverage can push things around.) If the deck can flex and move an inch or two in any direction, you’re probably safe. Might have to resink a few deck nails from time to time.

Brown PT isn’t worth buying for any purpose. At all. Ever.

Not as critical as with any kind of solid floor or deck - assuming there is plenty of drainage space between the deck boards, including at least a half inch between the innermost ones and the wall or house foundation. But yeah, it’s not hard for a few years’ inattention, followed by a messy fall, followed by a heavy rain to pour a lot of water in towards the house and foundation.

If any of that’s a possibility, a little wider on the board spacing and a tilt away from the house - maybe an inch in ten feet - will be a good thing.

10

save yourself the trouble and fill in the crack and paint the surface nice. Much easier

11

If the slab will be visible:
Take that “looks like crap” comment to heart.
An epoxy coating might hold up, but: dirt doesn’t show dirt or discoloration (unless something starts growing). Concrete will - that squirrel that crawled under it to die is going to be there for a while.

Either solve the problem - raise the deck high enough to allow access or coat or remove concrete or install decking with screws and station a pressure washer nearby.

12

If you are attaching to the house there should be 6" galvanised flashing that sits on the joists under the decking and runs up the wall under the house wrap. This is often missed.

If you do not attach to the house you really do not need to worry about deck movement for a simple deck. It will float with the slab if there is frost heaving. If the house and slab are properly drained frost heave should be negligible anyway.

Here (Calgary) ACQ pressure treated is pretty cheap and really not that bad a product, Cedar is more expensive but still both are much cheaper than composite decking. Cedar is rated a 30 year product above ground, PT about 25, that’s unpainted. Composite should never rot but it will probably fade a bit after that length of time. It looks ‘woodish’ is obviously not; this can be a good or bad thing aesthetically - depends what you are looking for.

Any wood touching the slab should be rated for ground contact (treated is called PWF). The joists and any beams should really be PWF as well. Fasteners must be ACQ rated if using treated wood. Regular nails and screws degrade quickly.

13

The only problem with raising the deck for access is that it will likely then require a permit, which means a design has to be submitted that conforms to the governing entity’s notion of being structurally safe. My deck builder spent two days at the county office trying to get his permit. It will also increase the cost, probably significantly.

14

Will we just had a major freeze here in Northern Colorado so I think once the snow and ice clears off, I’ll take a look at the crack and see if one side is raised up higher than the other one. The crack does worry me a little and it sounds like that the best solution may be the more expensive one and take out the slab and build the deck the conventional way.

15

If you decide to put in pilings why not just break the concrete up where you are augering the holes. Hard to beat concrete as as a ground cover for under a deck - it drys well, water mostly runs off it, much less likely to get musty than plain loam or even clay.

I will put road crush under a deck when ever the budget allows for the same purpose. Concrete would be even better.