Roof Decks – how to waterproof a flat roof deck

Flat roof decks are very popular among homeowners, because you can use your otherwise unused roof space, to create a private retreat for yourself, and to host parties / barbecues and other gatherings outside. Have a roof deck also increases the value of your home, as most people want to have a roof deck / patio when buying a home. Even many condos / apartment buildings have roof-top decks, patios and balconies, for these same reasons.

Private flat roof deck overlooking Mystic Lake in Arlington MA.

However, a roof deck poses certain design difficulties and the waterproofing membrane under the roof decks often leak, which requires taking the deck apart and fixing the roof, which can be a very costly project. It is best to use proper roofing material from the beginning, and to design the roof deck with proper water drainage, so ponding water under the deck does not create leak problems.

Most popular types of flat-roof deck assembly:

Most common roof decks today are floating wood or composition deck installed over waterproofing / flat roof materials – typically a rubber roof (EPDM), but other roofing membranes and materials are also used, such as PVC roofing, Modified Bitumen & Tar / Asphalt built-up roofs.

On top of the roofing material, a floating roof deck is constructed, and laid down on slippers (deck rafters laying on top of the roofing membrane). Railing posts are typically installed around deck perimeter, by ether attaching them to the roof framing/rafters, to the outside of the roof, or built into deck. Second and third methods eliminate roof penetrations (which often cause leaks), but are not as sturdy, as posts mounted to roof rafters.

Issues with flat roof decks and roofing materials under them:

Most of the roofing materials used under roof decks will leak if there is ponding water on the roof. Ponding water can happen due to inadequate roof drainage or roofing material failures. Most flat roofing materials (except for PVC roofing) rely on adhesives to seal the seams and flashing around penetrations, and these adhesives break down with time. Ponding water further increases the rate at which seam adhesive fails, and causes the roof to leak.

It is best to foresee all these issues before constructing a roof deck and to design proper drainage of the roof, such as creating a slope on the roof (typically 1/4″ per 1 foot roof pitch) or installing a center roof drain where design and local conditions do not allow for roof to drain off the edge. Roof drains can be problematic as the can clog, and water will back up and stand on the roof, which will likely cause a leak.

Another issue that you should consider is the clearance between the door sill and the roof. Often, this clearance is just an inch or two and often this is where the roof may/will leak. Architects and home builders should make this clearance at least 4 inches, even if it creates a step-down from the door leading onto roof deck.

Choosing flat roofing material for your flat roof deck:

Considering that most roof decks will leak due to seam / flashing failure, which most often will happen due to seam adhesive break-down, we recommend using IB PVC flat roofing membrane as the waterproofing material under roof deck. PVC roofing has hot-air welded seams, and once the seam is welded properly, the overlapping sheets of membrane become essentially one piece, and these seams do not come apart.

Regardless of the roofing material you choose for your roof deck, proper roof slope should be added either during construction or using tapered (sloped) insulation system. It is also recommended to use the thickest membrane you can get, and to go above and beyond on sealing the seams, and probing them, before installing the floating deck. It is best to use a single sheet of membrane if possible, and to have no seams at all.


Optionally, many PVC roofing manufacturers have developed a special Roof Deck membrane such as IB Deck Shield flat roof decking membrane, which has a special slip-resistant surface that is safe to walk on even when roof is wet, and does not require a floating deck on top of it, which reduces the overall roof deck costs, and is much easier to repair if there is a leak, as roof is exposed. Note, that such roof deck materials are very durable, and extremely puncture resistant when installed over proper substrate, such as DensDeck or plywood.

Roof deck repairs:

If you are building a new roof deck, or using a floating deck – it is best to build it in sections. In case the roof leaks, you can easily remove smaller deck sections to repair the leak, rather than taking the whole deck apart, which will be very costly. It is also “required to have membrane slip-sheets installed under deck rafters to separate the deck from the roofing material.

When roof deck leaks, you should remove one or more sections of the deck, to find the leak, and repair it. Always check the seams to find if there are any voids. Also check roof to wall flashing, and how the membrane is attached under the door sill. If you are hiring a contractor to repair your roof deck, hire only roofing contractors that have extensive experience with flat roofing or specialize in flat roofing. Do not hire roofers who install mainly asphalt shingles roofs or handymen. These contractors will 99% of the time will not be able to properly find and repair your roof leaks.

24 thoughts on “Roof Decks – how to waterproof a flat roof deck

  1. lorie teravainen

    I own a condo in an apt conversion complex built in the late 1970s. The unique recessed balcony construction is causing major controversy within the association now that components are aging. The 2nd floor units have balconies which are recessed over the 1st floor bedrooms, so the construction layers consist of floor/ceiling joists, plywood sheathing, asphalt rolled roofing, and floating or pallet decks on top. They have been loosely called “decks” and left to the 2nd floor owners (who have use of balcony) to repair. My ceiling (and others) have begun leaking because what is essentially a “roof” over that section of bedroom has deteriorated to the point of failure. The dual purpose of a balcony with a deck for the 2nd floor and a roof for the interior space below on the 1st floor is causing debate over repair and ending up delaying repair.
    Looking for suggestions!

  2. Judd Marshall

    I appreciate the article, I just have a quick question? If I put the PVC on my flat roof/deck…over the garage…could I then put like an artifical turf over that? Or even real grass?

  3. James Georgiades

    I am a home improvement contractor and I’m considering building my first flat roof top deck. The roof is a modified bitumen / tar asphalt top with an existing pitch and water drains . My plan is to install a floating wood deck. However, I dont know much about the propper slippers and where to find them. Can you offer me any help and guidance?
    Thank you.

  4. admin Post author

    Hi Jummy,

    Just use pieces of rubber mats to separate the rafters from roof.

    I recommend you thoroughly inspect the roof before building the deck – if it leaks – it will be big mess taking the deck apart to fix a leak.

    Better yet, install a low-slope system like PVC, before building a deck. I know this may not be realistic (budget-wise), bit it will be cheaper than building a deck, taking it apart to fix leaks and then putting it back together.

    If you still plan to build it on current roof, put in your contract that you are not responsible for future leaks, say after first 3 months of you installing the deck (if it leaks in the first 3 months – it is probably you puncturing the roof).

    Also build deck in sections, if you suspect it may leak soon – it will be easier for you to take it apart later.

    Good luck

  5. admin Post author

    Hi Judd,

    What you are referring to is “Green Roof”. PVC roofing is perfect material for that – I do recommend going with thicker 80-mil membrane and using modular green roof such as Green Roof Blocks (

    You can then grow Green Roof plants / grass which can grow in as little as 4″ of specially designed for low weight soil.

    This is an expensive “garden” though – so do your research first.

    Good lick.

  6. admin Post author

    Hi Lorie,

    Rolled asphalt roofing is the cause of your problems, and if it started leaking – it cannot be fixed. This product does not last mor than 5-7 years.

    I recommend doing either IB PVC roof and floating deck or IB Deck Shield, wich is also a PVC membrane, but can be walked over, and will be less expensive in the end, as you do not need to build a deck on top.

    You will also need to consider drainage issues, to get water off those roofs.

    Where are you located? I can help put you in contact with certified IB Roof Contractors, or if you are in New England, we can help you out.

    Best of luck.

  7. Jill

    I have a fiberglass roof deck but the pitch runs all the water off in one corner. The builder tells me you cannot put a gutter under a roof deck but I am now having problems with window leaks and siding stains below the area where the run off is located.

    My question is why can’t I install a gutter up on straps up under the lip of the fiberglass or some kind of flashing to divert the water away from the siding? As it is now, even when he fixes the window problem the water is still going to be running right down in the same place.

  8. admin Post author

    Hi Jill,

    I think you absolutely can put gutters below the roof deck – there should be some sort of drip edge, and tucking the gutter under it, is the best ides – this is how we do it most of the time, of course dependent on actual roof conditions.

    I think it would be best if you could provide some pictures of this troubled spot, so I can see and tell you what I think – upload them to Flicker or Picasa, and post a link in comments here – I mill take a look and tell you what I think.


  9. johanna broeksma

    my condo is about 35 years old.
    We had no trouble with leakage on my Roof-deck. there are 3 patio’s on there.
    I have a grass cloths(carpet)on the pation so not to see the nails popping up.
    Now strata wants to repair one of the decks as water is collecting. I am in the middle and because of my grass cloth they will not redo my deck.
    reason there is not much wrong with it???? but I feel as the decks are redone 19 years ago it is time to do all at once to avoid leakage.
    Please advise.
    sincerely Ann Broeksma.

  10. Chris

    How do I know if my roof of my single family brick house can support the weight ? My house was built in the late 1800’s but obviously has a newer roof that is rubber. Can it support 20lbs per sq foot?

  11. Andy

    We have a rolled roof and want to build a moveable deck in sections. What can we put under the 2×4’s to protect the rolled roofing?


  12. Amy

    I am a designer, and I was approached by a contractor to draw his plans for a rooftop deck. Where can I find codes that need to be followed? Really where should I even start? I can envision it, but I don’t know specs. Help please!!!

  13. admin Post author

    Amy – you should start with state building codes.

    Talk to an architecture student to get their help – for them it can be a school project.

    Good Luck

  14. admin Post author

    Andy, rolled roofing (asphalt) lasts 5-7 years. You are better off installatin a single ply membrane roof my choice is PVC roofing ( )

    As for what to put under the deck sections – it should be strips of roofing material used on your roof.

    Good luck

  15. admin Post author

    Chris – probably not – that is too much weight for a typical roof construction, when you add snow load.

    You should get structural engineer if you are serious about it.

    I would instead recommend IB DeckShield ( )> – it’s a light-weight option for deck waterproofing.

  16. Larry

    I am in lower westchester,ny and need to replace 2 walkout roof decks on a Tudor home.
    I am going to put the new plywood decking material and posts in myself and need a qualified contractor to install the ib deckshield membrane. Could you recommend someone?
    Thank you, Larry

  17. Cool Flat Roof Post author

    Hi Larry,

    We service Westchester. I will have our foreman email you.

    If you want, email me your contact information and some pictures if you have them to


  18. metal roof supplies

    Alternatively, a Zinc-Aluminum coating (of the kind sported by galvalume shingles) will not only have this effect, but also make your roof far less likely to corrode – the only thing you’ll need
    to do for up-keep is hose it down occasionally.
    Instead of the giant towering metal antennas of yesterday, you can get even better
    performance with a smaller digital antenna that mounts on the side of your
    house. If your roof is leaky or you’ve recently
    had a storm blow through and damage the shingles, consider making the change to a metal roof.

  19. Avinash Prasad

    I have a roof deck which leaks water over the wall of the underlying garage when there is heavy rain. Can you recommend a contractor who can look at it.

  20. Avinash Prasad

    My deck roof needs repair because of water leak during heavy rains. Can you recommend a contractor.

Leave a Reply

Your email address will not be published. Required fields are marked *