The Roofing Experts in the Seattle & Eastside Areas
How Pro Roofing Installs a Flat Roof with PVC Single Ply

How Pro Roofing Installs a Flat Roof with PVC Single Ply

Here’s how to roof a flat roof

Flat roofing has many different names.  Low slopped roofing, torch down roofing, hot tar, tar and gravel, build up, PVC, TPO, single ply, IB, etc.  Single ply comes in two main forms which are: PVC (vinyl) and TPO (thermal plastic). We prefer PVC over TPO, but TPO is considered “a greener roof” because it emits less gas when welded together.

Whatever you call it some constants apply.  The slope of the roof is so low that traditional roofing products such as composition and cedar shakes can’t be used.

In our opinion, torchdown is not a good option for Washington rains.  We have seen lots of leaks when too much water sits on a flat roof made from some type of asphalt or tar like torch down.  This is where PVC shines.  It seems like it can have water on it for years with very little effect on it.  This is obviously more ideal for lots of rain.

We looked around for the best installers in the Seattle area.  The flat roof installers we have now are, in our opinion, the best in the nation.  They are very picky about every-single detail and challenge us on improving virtually every job.  They are concerned about venting, water drain off, and all other aspects of roofing a flat or low slopped roof.

Here is how it happens: 

  • We come out and bring all types of samples right to your door. With single ply membranes your color choices are limited to basically gray and white.  Other colors are available as a special order.
  • We have clean-orderly job sites and we tidy up every day before we leave.
  • We will back up a disposal unit and throw all trash and old roofing into it.
  • Typically on a flat roof there is plywood or shiplap under the roofing membrane.  We will almost always tear down to the sub-roof and inspect for rot.
  • If we find rot we will remove it and re-install a new sub-roof to match the existing.

Now comes one of the most important parts, the underlayment.  A roofing company should NEVER install a single ply roof like a torchdown roof.  Asphalt based products are slightly porous and vinyl products are the opposite of porous!  In fact, the good about PVC is also the bad about PVC.  PVC will condensate if not installed properly.

This is where most Seattle roofing companies have had major problems.  They install single ply like they would a torchdown roof and it condensates!  Over time all the moisture from the home such as showers, boiling water, breathing, etc will accumulate in the attic and under the single ply membrane causing condensation. Condensation will manifest itself just like a leak.  BUT your new roof isn’t leaking at all.  It’s just that the water from inside your home can’t get out!

To ensure this doesn’t happen to you it is imperative your roof has the TWO proper layers of underlayment and proper venting. Every PVC roof must have both layers of protection under PVC.

Now let’s walk through some of the important specifics of installing a flat roof.


All flat roofs, in our opinion, must have ICE & WATER SHIELD (AKA: Leak Barrier) under them!  This shield is vital for a healthy flat roof.  Don’t accept fanfold or some other cheap substitute for ice and water shield.  Ice and water can also been known as leak barrier.  It is actually a torchdown-like substance which is porous too.  Small amounts of water vapor may pass up through it, but it will NOT allow water drops (which are larger) to pass down through it.  BUT for the most part it does a pretty good job of sealing most moisture under it.  Your attic ventilation will dry the moisture under the leak barrier.  If you get a proposal with something other than leak barrier it’s because of price not quality.  Don’t sacrifice this step.

Ice and water shield is a peel and stick. One really nice thing is that it will seal around nails and penetrations such as the screws used to adhere the PVC membrane.  A funny visual might be sticking a pencil into pudding.  The pudding is soft and it will fill in around the pencil.  This is essentially what leak barrier does around nails.  This is the 2nd main reason leak barrier is being used.

We have seen roofs with totally rotted plywood and the PVC membrane is still in perfect shape.  The single ply manufacturers won’t cover anything under warranty because their PVC performed like it was supposed to.  It didn’t leak or break down.

The next essential step in installing a perfect flat roof is the separation sheet.  Think of this as a type of shammy.  It will absorb small amounts of moisture under the membrane.  (We will talk about venting the separation sheet later)  It’s a felt-like sheet which comes in rolls and is applied directly over the leak barrier.

Now small amounts of water vapors can come up from the home through the ice and water.  If that happens those will be absorbed into the separator sheet which will keep the plywood dry.  Sometimes water can wick in around the edges of the roof.  The Separation sheet will take care of this as well.  With dry plywood and proper underlayments (and venting) you just have to wait for the single ply membrane to wear out.  That’s a good feeling because it will take a long time for that to happen!

Roofing Separator Sheet under PVC membrane and over Ice and Water Shield will protect against condensation from the single ply membrane.

PVC will also expand and contract.  The separation sheet will also act as a buffer because usually leak barrier has a gravely surface on it and over time it could damage the PVC without the separation sheet between it and the PVC.

Think of the separation sheet and the Ice and Water as a team.  They check and balance each other and work together to keep your roof dry and condensation free.  There are NO other two products that work together like these two do.

After the TWO layers are on the PVC is rolled and fastened down to the plywood.

The following layer is rolled over and heated together.  At this point the two sheets become one.  The term is married or joined together in an inseparable bond or form.  Two pieces of single ply become one single thicker piece.  They will not separate or leak because the seams are welded totally together.  This is one of the reasons single ply is so good against standing water.

The PVC is then run up the walls and over the other side.  This is done so if water does fill up it will still never get into the house or under the membrane.  It’s like a swimming pool liner.

After the membrane is over the wall a fascia cap metal is installed over the top and the roof is now water tight on the edges.  (There are some types of edges which are different and not addressed here)

The chimneys are flashed with the membrane.  It will be adhered to the bricks and new flashing (metal) will be cut into the bricks and flashed over the PVC.  This way water can’t get under the membrane.


There are TWO basic types of vents.  ONE lets the air out of the attic (RVO).  The other pushes air under the membrane and over the separator sheet to dry it (5” One way). An attic basically should have air passing through it all the time.  Some flat roofs do not have attic cavities and only can have one way vents.

RVO High Neck vents are used to let air out of the attic cavities.  The necks are tall because water, in some cases, can build up and run into the house through the vents. RVOs are always outtake vents.  Hot air and moisture should be leaving through the vents and not coming in.

NOTE:  Some flat roofs can have ridge vents on them, but they have to be handmade.  They can be very nice looking and work extremely well if done properly.  They are a bit expensive.  They will take the place of the RVOs and are really mostly necessary if the roof has an attic space.

The soffits, which come in many different forms and styles, are where the cool-dry air should enter in.  Then it will push the air out and up through the RVOs.

Remember the separator sheet?  We want to keep it dry right?  Here is how...

The 5 inch 1 say vents are designed to allow air to flutter in through them and under the PVC membrane.  This will keep the separator sheet dry.  If vented properly it should never feel wet even though it is absorbing moisture.

These vents can be used very liberally.  It is so important to keep the separator sheet dry on and the five inch 1 way vents are the best option to achieve this end. All the plumbing vents are sealed with special flashing pieces and calking.  Sometimes clamps are used to ensure the tightest of fits. When the roof is done it should have a little looseness in it to allow for expansion and contraction.

One thing about all flat roofs is that they are individually unique; more than any other type of roofing.  There are many different scenarios not mentioned in this article.  This is the basics of installation. If these principles are followed you will have a healthy flat roof.