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How To Deck Your Flat Roof Balcony / Patio With Timber or Slabs

Updated on September 15, 2013

How to use your balcony flat roof as a patio to utilise the space!

Do you have a flat roof leading from a balcony? Are you thinking of using the space? If you are then there are a couple of things to bear in mind before you start.

I'm a roofer, so have been roofing balconies for people for years, then installing decking or slabs. These are the things that I have to bear in mind when I'm about to start a job.

Read this guide so you know what you're looking at to prepare a nice area to sit out with a beer on a summers evening.


What sort of material should you use for the roof covering?

The last thing you want to happen is for your flat roof to go down, the decking or slabs to be installed, then get a leak, which means that you have to take it up to fix it. This makes me want to cry just thinking about it! So you need to choose the right sort of material before you start.

Traditionally balconies have usually been covered with felt or mastic asphalt. Asphalt usually doesn't have anything over the top of it, whereas felt often has tiles or sometimes timber decking.

Now personally I wouldn't trust felt under decking. The stuff has been used for years, but that doesn't mean it's any good for a job like this. The problem with slabbing over felt is that the ones I've seen have had the the slabs laid down with mortar, or they're sometimes stuck down with silicon, either way water tends to lie on the surface of the roof as it gets trapped there. Over time this breaks through the felt and then you've obviously got a problem. I've seen it many times and have ripped up many a roof that's been fitted like this.

With Mastic Asphalt you don't quite get this issue as these are usually the top surface rather than being decked afterwards. What happens with this is that over time it tends to bubble up and crack. The surface of the asphalt is generally pretty strong though, so it stays in place for a long time. However underneath that, the timber is slowly rotting as water is seeping through. After a while, this can take years but it does eventually happen, the timber collapses, then the asphalt roof starts to sink. I've put a nice picture of some cracked asphalt underneath..

Failed Mastic Asphalt

Single Ply Membrane

Single ply membrane is generally a better bet underneath decking or slabs than most other materials. There are several brand names out there that supply this, big names in the market are Firestone and Carlisle Syntec, who both supply EPDM membrane. This is a good bet for a flat roof generally as you can usually put it down in one sheet, but for balconies it makes even more sense as you don't have to worry about a failed seam underneath whatever you've covered it with. Something else you often need to consider is that most balconies have railings, so you need a roofing material that will allow proper sealing around the supports. EPDM is good for this, as the flashing material is flexible enough to seal around most obstacles.

The only potential downside over other materials like Asphalt for example is that you typically need a covering over it before placing tables etc down. EPDM is fine for regular foot traffic, but tables and chairs are too much for it to cope with really and if you puncture it then it will leak like any other material.

You ideally need to get a roofer involved that is going to install the decking himself, or at least one that works with someone that does. In my experience then when you get different tradesmen, they aren't always super careful with others work and if nails or screws start being stood on over your nice new flat roof, then it won't be leak free for long.

Timber Decking

If you're going to get decking fitted, then it must be fitted to go with the fall of the roof. This is pretty obvious really, but it is still worth pointing out to whoever is fitting it, you want the joists that the decking sits on to not impede the flow of water to the guttering. With single ply membranes such as EPDM, then standing water is not as much of an issue as with some other materials, but obviously if you can minimise it then that is best.

Something else you have to bear in mind is that if you have patio / French doors leading out onto the balcony, then you'll need to make sure you have decent clearance under the decking. Don't go fitting decking and then find that you can't open the door! This sounds obvious, but it is something to bear in mind before any material gets fitted.

Decking has the advantage that it is nice and solid underfoot and you can walk on it barefoot if you like. I think it looks good, or at least it does if it's fitted correctly and it lasts for years. Disadvantages of decking are that it will eventually rot and need replacing. The timber is usually treated so this will take a while, but it will happen eventually.

Slabs over flat roof

Slabs however are far easier to fit than decking and a lot of the time if people have a roof that has already been slabbed, then the slabs are still available. I've fitted decking and fitted slabs to flat roof balconies in the past. The easy thing about slabbing on top of EPDM for example is that you simply place the EPDM spacers down, then slab over the top of them. It's sort of like when you're setting out a room for tiling, but once they're down, that's it!

The photo shows a slab sitting on EPDM spacers. You fit these down wherever you have a corner then put the other slabs in place. It's fairly straightforward really. These spacers are easy to cut with a sharp blade so you can space out the edges of the slabs.

The advantage of slabbing is that slabs are extremely hard wearing and long lasting. When fitted they basically don't move or rot, which is something that will eventually happen with decking. Disadvantage is that the finish isn't quite as appealing to the eye, although this does depend on the slabs used obviously.

This is a nicely done flat roof with slabs over the top

What other options are there?

You don't have to stick with slabs or decking, although they are the most common choices. Something else that people do sometimes use is astro turf. This has got pretty good nowadays, it doesn't look quite so fake. Of course it is still fake grass, but it almost looks like a lawn.

Now I've never used this on a flat roof before, although I have offered it as an option over the years. I'm yet to have anyone take up this option though, so you at least know that not many others will have it!

It's perfectly fine to fit this onto EPDM roofs and to clean it you just get the pressure washer out. Nowadays as the technology has come on then the modern systems show the grass in various stages of growth so it can look like a real lawn. Pretty cool really.

Whichever option you go with, you want to make sure that you've had a decent roof fitted with ample drainage before you even think about decking it. Get yourself a decent roofer in who knows what they're doing and you'll reap the rewards, rather than misery if the whole thing leaks!

Thanks for reading, if you have any comments then please leave them below.

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      5 years ago

      Great ideas, I especially like the astro turf idea! The wood decking is looks great but what a concept to have a yard on a roof!


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