Can a flat and level roof be waterproof?

Question

I would like to build a 6 x 8 ft playhouse with a deck covering the top. Is there a kind of roofing material that can (1) withstand a floating deck, (2) is DIY friendly (no blowtorches, hot asphalt, or custom pouring), (3) is reasonably cheap, and (4) will keep the water out even with a completely flat roof, or with a very minimal slope?

Modified bitumen is said to be great for building deck on top of. The GAP SBS Liberty system is DIY-friendly, but the instructions call for a slope of at least 1/2" per ft. If using only the cap sheet, it's about $1/ft².

Building a deck on top of the roof is much easier if the roof is completely flat. Is that asking for trouble, or will it be OK for my application?

Are there other roofing materials I should consider?

Answer 1

EPDM rubber roofing might work for you. It can be installed without any heat or fancy tools; I'm planning to use it for my flat-roofed concreted detached garage. It should withstand ponding water, and because your project is small and your playhouse probably doesn't have plumbing, you should be able to avoid seams, complex flashing, and pipe penetrations. You can install the rubber using an adhesive directly over clean plywood.

It can accommodate a floating deck. I have the book Roofing, Flashing & Waterproofing (For Pros By Pros), which includes a nicely illustrated article on installing EPDM with a floating deck. The method they recommend is to use a non-adhered strip of EPDM underneath the decking sleepers, so that any motion in the decking doesn't stress the roof membrane much. You'll still want some slope in the roof for better drainage, but you can compensate for that in the sleepers, allowing you to have a level walking deck.

Answer 2

No such thing! All roofs need a slope. What I've seen is that the roof slops the normal rate (1/2 inch per foot or whatever) but then the deck is sloped in the opposite direction so it ends up level.

Answer 3

No such thing, not even for a playouse! Even if it's flat the day you install it, it will cup over time.

This is why all floating deck systems accommodate slope. Slope your roof, slope your floating deck the other way. No roofing material, not even seamless EPDM survives standing water (even if you can stand the mosquitoes and rotting debris that would collect). If you can build the playhouse to fit a single sheet of locally available EPDM, that would work well.

Answer 4

I've walked around on a ton of flat roofs (some with walkways/decks) - but they were all on commercial buildings (or large multi tenant residences). However, they all involved some kind of torch-down install with the roof material being contoured to direct towards roof drains.

I don't know how well the material would support a floating deck, but you can use an EPDM liner for a flat roof. EPDM roof installs are common on older motorhomes (thankfully my roof is aluminum) so a roof with minimal penetrations should be a piece of cake!

Answer 5

Flat roofs are simpler to build but require a lot of maintenance and inspections to make sure that everything is functioning as it should. One major drawback of flat roofs is ponding of water. Liquid EPDM rubber can withstand continually submerged water year round. Liquid EPDM rubber is designed to use on flat roofs.