Flat Roof Options / Advice...

[bjferri555]

I need flat roof advice / options. I’m purchasing this home and there are minor issues with the flat roof. I never wanted a home with a flat roof but in any case…there is living area beneath this roof.

I don’t want to be the homeowner that has flat roof leaks that either goes unnoticed or that require constant maintenance. I’d love to convert it into a deck – or am I looking for trouble? I don’t know what’s out there that is tried and true, any ideas? Am I better off having it pitched and using regular shingles or just keeping it as is? I believe it is painted on tar substance – it’s not torched.

[jadnashua]

The best flat roof protection is probably a continuous membrane. If you decide you want to tile it, there are several methods that also would waterproof it. The roof needs some slope so stuff doesn't pool and if it can't slope to the outside, needs a drain of sufficient size, with the area sloped to the drain (similar to a shower). www.schluter.com (http://www.schluter.com) makes a system that is designed to tile an outside roof deck, but if you don't want to tile it, and the area is walked on, you need something else. Most membranes are not designed to be walked on, but many can support an applied, floating wear surface on top of it. Flashing is critical on any roof, but even more so on a flat one, along with proper drainage.

[Spaceman Spiff]

I'd go with a continous membrane, adhered to at least 2" of foam insulation. That way if there is a cut in the membrane it still won't leak. Also, there are special pads for flat roofs so you can sit pavers on top of the membrane safely. They will protect the membrane and allow water to flow underneath, and have shims so that the paver surface is flat. I do this in commercial architecture on a regular basis.

[bjferri]

Can you be more specific about the membrane to use and 2" of foam insulation? Something I can suggest to a flat roof specialist. Thanks.

[jadnashua]

Sorry, but a flat roof specialist that can't offer these suggestions, is not a real specialist! Flat roofs are more of a commercial thing, and maybe search in that area verses residential purveyors.

[Dana]

Can you be more specific about the membrane to use and 2" of foam insulation? Something I can suggest to a flat roof specialist. Thanks.

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/images/figure_12_unvented_flat.jpg

Membrane roofs are exterior vapor barriers, so the foam helps protect the roof deck & rafters from rot by keeping it above the dew point of the inteior space air in winter, avoiding condensation at the roof deck, making it resilient to air leaks from the conditioned space. But the amount of foam required is climate-dependent, as well as the R value of the insulation in the rafter/joist bays.

The dew point of 70F 35% relative humidity air is about 40F, so if you keep average temp at the structural roof deck in January stays above 40F with a 70F interior it's enough prevent moisture accumulation at the roof deck. In say, Gaithersburg the outdoor mean temp for January is ~30F so if you have R30 batts or cellulose in the rafters/joists (2x10s, nominally), you need at least R10 in foam above the roof deck to prevent condensation. That would take 2.5" of EPS (bead-board), or 2" of XPS (extruded polystyrene), or 1.5" of polyisocyanurate. In Baltimore the mean January temp is ~35F, so with R30 in the rafter bays you'd only need a minimum of ~ R5 above the roof deck, so 1.5" of EPS or 1" of XPS or 1" of iso would be enough, but you'd have more margin with 2".

If you had R38 in the rafter bays (2x12s), that's about 25% more R, so you have to scale up the foam by ~25% (3" of EPS or 2.5" of XPS/iso for Gaithersburg, or 2" EPS or 1.25" of XPS/iso in Baltimore.)

If the rafter bays are empty or don't have full depth batts/blown fiber, measure the full cavity depth and multiply by 3.5" to come up with a number to approximate a full fill of cellulose for calculating your foam-R, then retrofit it with cellulose. That way convection loops impede any interior air from finding it's way to the roof deck in the first place, and during the hours where it's colder than 30F outdoors and potentially condensing at the roof deck, any condensation that forms is wicked up & safely redistributed by the cellulose to dry seasonally toward the interior. If you don't want to mess up the interior with blowing holes it can be drilled & filled from above prior to laying down the rigid foam.

[bjferri]

Ahhh - commercial is perhaps the way to go. Providing I can find a commercial company that does residential. Thanks for the above technical information.

[Spaceman Spiff]

Yes, Commercial roofers do it all the time. I'd go with a TPO 60mil min. membrane over blue ISO board mechanically fastened to the deck, and the membrane adhered to the foam. They might have a piece sitting around that would cover your entire roof... These are the type of paver pedistals I was talking about... http://www.appianwaysystem.com/ appian way system. There's other companies out there that do similar stuff but this can do extra height because you can use PVC drain pipe as part of the stand.

[chefwong]

The best product I know of is Kemper. Deep pockets required.

[hj]

quote; Flat roofs are more of a commercial thing, and maybe search in that area verses residential purveyors.

Maybe in New England, but in this area residential flat roofs are the norm, not an exception.

[Dana]

Flat roofs would also be primarily found in commercial construction in bjferri's mid-Atlantic region though.

Flat roofs are generally a bad idea in residential construction anywhere snow can accumulate (even in areas where deep snowfalls are a once-a-decade event.) That doesn't stop people from building that way, but it does keep it from being the norm.

[johnfrwhipple]

When doing a flat roof you need to help of a pro roofer with great insurance. The best flat roofs are pitched so there is no standing water and pitched enough that the water does not pool even near the seams in the waterproofing products.

A torch on roof is well proven in the field and can give you years and years of service. You can't tile it though without other methods.

Jim mentions Schluter for a tiled roof and this is possible but remember Schluter does not make a primary roofing membrane, only accessories (Ditra Drain, Troba, Edging etc.). Noble Company makes a product called Noble Deck which is what I used for my own exterior deck on my home. This product is nice to work with but not approved in Canada as a primary roofing product - I believe it is in the USA but you should contact Noble Companies technical department to check if it is in your state. Email Eric for advice. Eric Edelmayer <eric@noblecompany.com>

Most torch on tar roofs will not be prepared for tile anyway, so if your planning this you should plan on a large remodel.

It might be wise to get multiple opinons. Try calling on a few roofing supply stores and asking for the names of the top crews shopping there. Get them all out and give you a best case and worst case scenario.

Good Luck.

JW