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Can I fix my patio deck?
November 9, 2010 9:50 PM   Subscribe

DIY Advice: paving over wooden decking, can it be done?

The house I'm about to take possession of has a patio in two parts, being a 3m x 8m concrete and brick structure, and a 2m x 5m wooden deck, with (I think) concrete stumps.

I have to get rid of the deck, it's ugly and it's a fire risk, which is important. No idea what timber is used, lets presume it's durable.

In an ideal world I would replace it with a concrete and brick structure similar to what's already there, and I would then put pavers/outdoor tiles over the lot to integrate it.

However, that would be time consuming and costly (particularly since it's on quite a slope at that point), so I wouldn't be able to get round to it for several years. I wonder whether I could do something a bit more dodgy, but still successful: put down two layers of cement sheeting/tile underlay straight on top of the decking, then pave over it, and pave the concrete section at the same time to make it look integrated.

Two issues I can foresee: timber rotting due to lack of ventilation, and shifting/cracking of the pavers on the deck as it settled under the new weight.

Apart from an expansion joint where it met the concrete patio, could I do anything about cracking? Reinforce the deck with crossmembers underneath? Could I waterproof treat the timbers (bituminous substance) or do something else to avoid wood rot?

Any other issues, and is my dodgy plan feasible?
posted by wilful to Home & Garden (16 answers total)
I reckon you could do this, but your concerns about rot and the pavers cracking are fully justified.

The pavers also risk cracking the deck itself, either from their own weight, or once the extra weight of furniture/people/barbecues/whatever else is added. I'd suggest weighing a few, then trying to test the deck by lumping a pile of them (or something else) of calculated weight onto each unsupported looking spot. Then stand on the pile. Then jump up and down..

With regard to the rot - sealing the timber won't necessarily stop it. If the timber's been exposed for a few years when you seal it, the rot may already be in it, and it can just keep going. No way to really tell except up close inspection by someone who knows what wood they're dealing with, knows your climate, and knows your local types of rot. The likely outcome also depends a bit on whether you're talking about sealing all sides of each board, or just the top.

Also, from the point of view of fire hazard, I'm thinking a bitumen seal may actually worsen your problem, not make it go away. I'm not sure of this, but I'm thinking that it would reduce the risk of fire taking hold, but increase the burn temperature if it does. Interrogate your supplier about this one if you go ahead with the project. And whatever the eventual answer, bear in mind that throwing pavers on top will definitely make it much harder to put a fire out.

And then there's termites. Have you got them where you are? If so, you need to be able to inspect - preferably from as many angles as possible.

posted by Ahab at 12:44 AM on November 10, 2010

Rather, feasible, yes. But a bad idea because when it starts to disintegrate in ~three years, you are going to have a huge pile of cement pavers and rotten timber and etc to get rid of before you can do anything else.
Get bids on doing it 'right' - removing the wood and then leveling and paving. And consider (If fire is a big consideration) using a non-wood decking. They make all kinds of not bad looking 'composite' materials that are fireproof and long lasting and relatively easy to install.
posted by From Bklyn at 12:46 AM on November 10, 2010

how does this plan (including "bituminous substance" treatment for the existing timber) change the fire risk status of the decking? I'm no expert in fire dynamics, but isn't that just trading a quickly-combustibe deck with through-flow of air for a longer burning substrate under the concrete deck top?
posted by russm at 12:52 AM on November 10, 2010

No, not feasible. Dodginess level very high. Decks are typically built to support the weight of people and a barbecue. They aren't rigid enough to support all that extra weight without flexing (which would crack the stone), and the footers aren't big and beefy enough either (likely to sink into the ground). Very likely the footers don't go deep enough to provide the stability needed for stone (frost heave moves things around in winter). Furthermore, the wood used for decks (treated southern yellow pine) is dimensionally unstable and likely to warp with changes in weather. As soon as cracks develop, water will infiltrate and be trapped against the wood, which may not rot it immediately if it's treated lumber, but will cause it to swell, which will widen the cracks and worsen the problem.

The hoops you'd have to jump through to make a paved-over wooden deck more durable than your average theater set would be just as much trouble as doing the job right.
posted by jon1270 at 2:18 AM on November 10, 2010

russ, it's less fire risk because there's no air flow (hence, also, rot happening) and no place for embers to lodge and smoulder. I would also be enclosing the sides of the deck (with colourbond). Definitely less fire risk.

jon1270, thanks for the advice, sincerely, however note some context, we certainly don't use treated southern yellow pine, and dont get frost heave, we're not all ridiculously north of the equator. But yeah you're right, chances are excellent that the footings would be inadequate.

I think the answer is, as From Bklyn says, fake/composite wood. Though aesthetically unpleasing. Hmm, or quite a few metric shitload tons of rubble. Which then will stop airflow under the house too...
posted by wilful at 2:54 AM on November 10, 2010

Oh, Australia. Yeah, your deck won't be SYP.
posted by jon1270 at 3:15 AM on November 10, 2010

If there are concrete stumps, just rip the timber off them, and pour a concrete slab supported on the stumps - you will probably need to extend the piers to compensate for the substructure of the timber floor. Use the right steel formwork, and it should be fairly straightforward. Assuming that the stumps are concrete, and that they will support the slab and all that goes onto it.

Alternatively, clear everything out, pour concrete pads with reo for future piers (step 1). Step 2 - get some steel pipe (or plastic), sit it over reo, cut to even heigth, and fill with concrete. If steel, you can weld in bracing to ensure stability, otherwise you need to make sure the slab (see step 3) is braced or otherwise will not move. Step 3, get some of the specific steel sheeting designed as base/reinforcing for suspended slabs, lay on piers, make up edging, and pour your slab. This is the short version, it is a little more involved that I have set out here. One thing to remember, wet concrete is bl**dy heavy, and if your formwork is not properly supported and braced, make sure you are not around it when it comes down.

You might like to try some of the owner-builder and home reno sites, for ideas too. I am sure you will find that you are not the first with this sort of issue. They will be able to help out with dimensions and placements for stumps, slab thickness and concrete specs. And think about how much you want/need to involve the local council ...
posted by GeeEmm at 3:51 AM on November 10, 2010

cheers GeeEmm. The council would certainly not be involved.

It certainly looks like starting again is the option... :( I hate doing everything properly. It'll take forever to get that sorted.
posted by wilful at 4:17 AM on November 10, 2010

Elevated concrete slabs are not a job for the layman. Mixing wood and concrete is only really appropriate when you're using it as formwork, and that's not at all what you're talking about here.

So, possible, yes, advisable, no. Can you get a building permit for it? No way.
posted by electroboy at 7:57 AM on November 10, 2010

I would consider a third option. Does everything have to be at the same level? You could pour a new section on-grade. If the deck (as they often do) abuts to a doorway, you could simply have stairs. How high is the deck, or rather how high is the difference between grade and the existing concrete?

Oh, I missed the part where you said it was on a slope. That rules out everything except moving earth or making a deck. Is the fire risk due to the exposed decking material, or is the understructure considered a risk? If the latter, synthetic decking won't help. If the former, you have a number of options.

If you (can) reuse the existing framing your life will be easier, but using any other material for the deck means you're probably going to reinforce things. Synthetic decking is notoriously noodly and will almost certainly require joists at closer intervals than what you have. Anything like tile will probably require more joists/blocking and more framing for the load. Synthetics are often heavier, too. Finally, in my experience decks are not usually engineered as solidly as interior flooring; more flex is permissible outside.

Another possibility: if framing doesn't permit slapping tile on top of the existing structure, you could possibly rip off the decking , reinforce the framing, and reinstall (exterior) plywood, cementboard, and tile. This combination would also let you keep closer to the same level, although plywood + backer + thinset + tile is probably about 6cm thick, and your decking might only be about 3.5-4 cm or so. Actually, this might work out well if you can work it out, since the concrete portion won't need backerboard.

As always, the John Bridge Tile Forum is the best place for tile questions. I would recommend asking your question there, possibly after you have more detailed info about your substructure.
posted by RikiTikiTavi at 10:09 AM on November 10, 2010

How is the deck any more of a "fire risk" than anything else in a modern structure? Just sounds rather stupid to use that and an excuse. But not nearly as stupid as trying to put incredibly heavy masonry products on top of it. You'd then have an out-of-sight wood structure ready to break or rot out and fall out from underneath anyone or anything on top of it. That would certainly be infinitely worse than notions about fire risks.

If you want to be rid of the wood structure than remove it entirely. Replace it with a properly engineered wall. Then extend the existing concrete surface onto the filled part of it. Modern interlocking masonry products are great for this sort of thing. But you should not do it without consulting with a building engineer or the local permitting department first. It would be a bad idea to just slap up a wall on top of what might be unstable or poorly drained ground.

But nowhere near as bad an idea as laying anything else on top of the current deck.
posted by wkearney99 at 11:31 AM on November 10, 2010

wkearney99, that was a bit of an aggressive tone you used. If you don't understand fire risk factors in bushfire zones, it's OK to just admit it.

electroboy, I agree that an elevated concrete deck is beyond me.
posted by wilful at 6:08 PM on November 10, 2010

here's the plan now, if anyone's looking back - I'm going to remove the entire wooden structure, and maybe put a few more stumps in, then use steel bearers and waterproof ply, then cover with tiles. Thanks folks for your help!
posted by wilful at 8:42 PM on November 12, 2010

If you're going to tile over wood, outdoors, you might consider using a decoupling membrane between the tile and wood deck. Also, be sure to slope the surface so rain runs off quickly.
posted by jon1270 at 6:04 AM on November 13, 2010

Cheers jon1270.

Here's the instructions from the manufacturer.
posted by wilful at 6:21 PM on November 13, 2010

If you don't post information about living in a brush fire zone it's OK to admit you don't give people enough information to work with.

Have you spoken with the local housing authorities (zoning, permitting, etc) for their advice on what would work best in your area? Presuming they exist, of course. It would seem rather stupid to plod blindly toward making a whole other set of mistakes.
posted by wkearney99 at 6:37 AM on November 25, 2010

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