Ponding on flat roof

We have a small 6X16 flat roof off our new third floor addition. Soon we will be building a deck over it. The roof was recently refinished (old roof taken up, new ply layed, and covered with 2-ply membrane) when we added the third floor. Since it has been finished, each time we get a rain, I get ponding on the roof over an area about 2X6, against the support beam of the new addition. The pond has a depth of about 1/2 inch, and will dry up generally over a half week or so of no rain. My contractor is trying to tell me this is ok. I’m not so sure, especially since we will be building a deck on top of it, and therefore will have little access to roof in case of needed repairs. BTW, I live in Toronto so we get plenty of freezing temps over winter.

Looking for advice on whether I should worry about this, or not.

You should get rid of the water if at all possible.

[quote=“yds”]We have a small 6X16 flat roof off our new third floor addition. Soon we will be building a deck over it. The roof was recently refinished (old roof taken up, new ply layed, and covered with 2-ply membrane) when we added the third floor. Since it has been finished, each time we get a rain, I get ponding on the roof over an area about 2X6, against the support beam of the new addition. The pond has a depth of about 1/2 inch, and will dry up generally over a half week or so of no rain. My contractor is trying to tell me this is ok. I’m not so sure, especially since we will be building a deck on top of it, and therefore will have little access to roof in case of needed repairs. BTW, I live in Toronto so we get plenty of freezing temps over winter.

Looking for advice on whether I should worry about this, or not.[/quote]

Most roofing membrane manufacturers don’t warrant ponding that exist for 48/72 hours, depending on the manufacturer. That should tell you all you need to know about whether ponding should be acceptable to you.

It is not uncommon for most flat roofs to have a little ponding water . Generally if the ponding goes away within 72 hours of rainfall it is not a problem .

Since you live in Toronto , You will want a good roof that is capable of holding ponding water, since you have a lot of ice dams and snow .

Snow and ice do not migrate like water does. Get rid of the water or get a true vapor barrier membrane installed. Polyurea does this, but still best to get rid of the water.

As stated earlier by someone else, most roof coatings and membranes will not hold up to standing water. Our Liquid Rubber Liquid EPDM product will, but generally it is used for coating EPDM roofing.

Being that you are going to put a deck over your roof you better be dam sure the roof is installed correctly.
If the roof is still wet after a few days exposed it will take longer to dry out when the deck is installed as the sun will be restricted by the decking.
If possible please post a pic of this ponding water.