Thread: Slope of deck

Does the IRC specifically address the slope of a deck surface if the deck has a sealed membrane on the deck flooring. A client asked today and I'm trying to find an answer. I know it's common sense but.....

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I always carry a marble in my pocket to check the slope of drainage on a deck, porch, or balcony if it ever comes into question. As long at the slope is sufficient to carry run off away from the structure, but not to exceed an inch drop every 10 feet...you want it to be safe.

Dub, did you mean an inch every four feet? That's what I thought was the minimum slope required for drainage. A max of an inch every ten is quite a bit flatter.

Originally Posted by mathew stouffer

Does the IRC specifically address the slope of a deck surface if the deck has a sealed membrane on the deck flooring. A client asked today and I'm trying to find an answer. I know it's common sense but.....

The minimum slope will depend on the roofing, and the minimum slope for most roofing types is 1/4" per foot.

Here are some examples:

- R905.12 Thermoset single-ply roofing. The installation of thermoset single-ply roofing shall comply with the provisions of this section.
- - R905.12.1 Slope. Thermoset single-ply membrane roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage.
- - R905.12.2 Material standards. Thermoset single-ply roof coverings shall comply with ASTM D 4637, ASTM D 5019 or CG SB 37-GP-52M.
- - R905.12.3 Application. Thermoset single-ply roofs shall be installed according to this chapter and the manufacturer’s installation instructions.
- R905.13 Thermoplastic single-ply roofing. The installation of thermoplastic single-ply roofing shall comply with the provisions of this section.
- - R905.13.1 Slope. Thermoplastic single-ply membrane roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope).
- - R905.13.2 Material standards. Thermoplastic single-ply roof coverings shall comply with ASTM D 4434, ASTM D 6754, ASTM D 6878, or CS GB 37-GP-54M.
- - R905.13.3 Application. Thermoplastic single-ply roofs shall be installed according to this chapter and the manufacturer’s installation instructions.
- R905.14 Sprayed polyurethane foam roofing. The installation of sprayed polyurethane foam roofing shall comply with the provisions of this section.
- - R905.14.1 Slope. Sprayed polyurethane foam roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope) for drainage.
- - R905.14.2 Material standards. Spray-applied polyurethane foam insulation shall comply with ASTM C 1029.
- - R905.14.3 Application. Foamed-in-place roof insulation shall be installed in accordance with this chapter and the manufacturer’s installation instructions. A liquid-applied protective coating that complies with Section R905.15 shall be applied no less than 2 hours nor more than 72 hours following the application of the foam.

- - R905.14.4 Foam plastics. Foam plastic materials and installation shall comply with Section R314.
- R905.15 Liquid-applied coatings. The installation of liquid-applied coatings shall comply with the provisions of this section.
- - R905.15.1 Slope. Liquid-applied roofs shall have a design slope of a minimum of one-fourth unit vertical in 12 units horizontal (2-percent slope).
- - R905.15.2 Material standards. Liquid-applied roof coatings shall comply with ASTM C 836, C 957, D 1227, D 3468, D 6083 or D 6694.
- - R905.15.3 Application. Liquid-applied roof coatings shall be installed according to this chapter and the manufacturer’s installation instructions.

Also keep in mind that any slope *over* 1/4" per foot is not allowed for landings, treads, etc., which means that is a good maximum slope to use for decks too.

Jerry, I'm confused which is easy to do. So if the deck floor is sealed it falls under the category of roofing? The area under this deck was open exterior space.

the main issue was the potential buyer noticed areas of ponding water on the deck after a storm and he is freaking out.

We are talking about a deck that you walk on, not a roof or the slope required for landscaping. A slight gradual slope of a walking surface is sufficient for drainage. Gravity is always predictable.

Traveling Gnome, yes you are correct, a deck for walking. The client is asking if the IRC specifically addresses a required slope of a deck surface (for walking), if the surface has a sealed membrane. Jerry posted info refering to a roof, this is why I am confused.

I know there's spec for shower pans as far as slope... I want to say no more than 1/4 per foot to prevent slipping but I may be incorrect. I also don't know if a deck surface is held to the same standard.

I was thinking the deck was over an enclosed living space. Since it's not, I would think that it would come back to the manufacturer's instructions for the membrane. I don't think a deck is required to have any slope, since normally it wouldn't matter.

We did an aluminum water shedding deck over a lower patio once, and there was a minimum slope in the instructions, but I don't remember what it was.

Originally Posted by mathew stouffer

the main issue was the potential buyer noticed areas of ponding water on the deck after a storm and he is freaking out.

If it has water that is "ponding" or standing on the decks surface then the deck has a problem.

What is type of flooring is on the deck? Tile, Stone, Pavers, nothing?

You want a slope so that water does not stand. 1/4" per foot would be great, but if it is a large surface say 12' you are going to end up with a pretty darn good slope. Most of what I find is 1/8" per foot. Depending on the membrane that was used to waterproof the deck area, you might find some different requirements.

The attached diagram is for HydroBan membrane.

Originally Posted by mathew stouffer

Jerry, I'm confused which is easy to do. So if the deck floor is sealed it falls under the category of roofing? The area under this deck was open exterior space.

Originally Posted by Jerry Peck

The minimum slope will depend on the roofing, and the minimum slope for most roofing types is 1/4" per foot.

Here are some examples:

Mat,

Allow me to change one word, maybe that will make the difference for you.

Change this "The minimum slope will depend on the roofing," to this "The minimum slope will depend on the MEMBRANE," ...

That puts me back where I was with my examples: to the manufacturers minimum slope requirements.

There *ARE* some heavy duty, commercial use, waterproofing membranes which are designed for dead level surfaces. I have inspected many during their installation (always done wrong until you take them by their hand, open the installation instructions, and page by page, detail by detail walk them through it ... and even then they still do not do it right!

When I was applying Duradek on apartments and condo's it was required to be pitched 1/4", Common sense would tell you to slope. Who wants to walk out in a pond. Duradek - Duradek Waterproof Vinyl Roof & Walking Deck Membrane. Durarail Aluminum Rail & Fence. Tiledek waterproof underlayment and anti-fracture membrane.