I am a newbie to the forum keen to receive advice relating to the correct level of insulation and/or cooling/reflective measures for a totally flat roof - which will be used as a roof terrace in Malta. This being Malta the island off Africa/Sicily not in the US! I have put in hours reading forums and gone through the learning curve to the point where now I just know what i dont know....so any help with what we should be asking our contractor to go with would be great. Sorry can not be more specific in relation to roof construction as it is a part built building we have recently bought and I just dont know what the roof is made of..

I will take a stab at the insulation question.
Your best solution if available, would be to use a closed cell foam insulation sprayed under the deck surface. You are in a tropical location and I would assume that humidity is an issue. The closed cell foam would have an added bonus. It can also act as a vapor barrier. Cost is the highest but once installed its done.

The Mediterranean isn't a tropical location- it's ~30 degrees north latitude at it's southernmost, on the Egyptian coast- about the same as Mobile AL or New Orleans LA, not the Yucatan or Venezuela.  But the humidity in the Mediterranean is much lower than the US gulf-coast- it's mostly semi-arid since prevailing winds cause ocean moisture to fall in the northerly parts Europe or western Africa.  Malta is at ~35 degrees north, about the same as Charlotte NC, not Havana.

Spray foam would be fairly expensive for this application, and under the roof deck is the wrong side if you go that route.  A waterproof insulation under the roof deck can mask roof leaks until the roof rots and fails.

Sheets of expanded polystyrene (EPS, or "Styrofoam")  ABOVE the structural roof deck, with a thinner non-structural nailer-deck of plywood or OSB above that to provide a walking around structure/nailing surface to the upper deck will be much cheaper, and if the roof ever leaks, you'll know about it long before rot sets in.

In flat roof construction it's best if the roofing material itself should have both high solar reflectance and high infra-red emissivity to reject & re-radiate solar heat gains. (google "cool roof")  This will reduce air-conditioning requirements during the hot seasons.  EPDM membranes are popular for waterproofing flat roofs in the US, but membrane roofs should be coated/painted with high reflectance high-e materials after it's installed to keep it cooler inside & out.

Thanks so much for each of the replies. They are very helpful. Dana1 I have seen many of your informative posts on various topics and appreciate both you and renergy trying to help with such little information to go on. I will take both views into account and continue my research on cool roof issues. There are no buiiding code standard requirements for insulation I am told in Malta so it is a bit of an uphill struggle to persuade developers any is needed! Any advice re the wall insulation which is basic single brick breeze block construction i.e. bricks with internal air gaps not usre if you use the same name in the US - would be very much appreciated too. We have renewable energy firms looking at the house for us re insulation and solar etc etc but it seems to me from the forum that you guys on the forum know more than many and it would be great to have an unbiased view. thanks again to Dana1 and Renergy

I assume from your location that you will use masonry and concrete constuction and that your roof structure will be a concrete slab.  I will also guess that the most readily available insulation materials will likley be styrofoam boards (EPS and XPS) and rockwool batts.

If you are going to use the flat roof as a terrace, you might consider doing an extensive green roof, with part of the roof in pavers.  It will help with solar load and runoff.  At 35° 50' latitude Your solar load will be about the same as Raleigh, NC and Tulsa OK, so it is hot, but not extreme.

My preference is also to keep the concrete structure inside the thermal envelope.

How you do the roof will depend somewhat on the type of roofing material you use and local practice.  For a green roof, however, I prefer an "inverted" roof where the roofing membrane is under the insulation and the pavers/green roof provide ballast for the insulation.  In this way the membrane will well protected from thermal cycling and physical damage, although it is a bit harder to get at the membrane for reparis, if needed.

Do a 24 hr flood test on the roof before any additional work and remember to have overflow drains or scuppers (to prevent overloading the structure if drain gets clogged).

Bruce

Thank you for your reply Bruce it is very helpful I think all your guesses are correct from what I have seen of the shell. I will do some further research on using a green roof - it is difficult as you point out because I dont yet know what is available locally. If you have any further advice re wall or roof insulation please feel free to post or contact me thanks again

I will again guess that the most widely available roofing material will be some sort of modified bitumen "torch" roof.  My view is that a roof is usually more sensitive to the flashing details and installation workmanship than the actual roofing material.  If you are going to use part of the roof as a terrace, I would suggest HD extruded styrofoam instead of expanded because it is stronger in compression.

Roof slabs are usually poured flat and slopes are provided by pouring a screed layer of lightweight cementious material over the structure and insulation or by using tapered insulation (if the the roof membrane is placed on top of the insulation).

If you are not familiar with the civil code in Malta, it may be worth looking into.  The civil code of most of southern European countries has a "decennial liability" concept, which means that the contractor (and in some countries the architect, developer and maybe the works director) guarantees the structure to be free from defects for 10 years and is usually interpreted to include the roof and facade (because if there are leaks, the structure will be damaged).  Owners can purchase Decennial Liability Insurance which provides funds to repair the problem while you (really the insurance copmpany) try to collect money from the contractor (et al).  Smaller contractors seem to go out of business periodically and reorganize under a different name to escape decennial liability obligations.  In some countries, developers are required to buy Decennial Liability Insurance for residential projects.

Bruce

Can anyone please advice regarding whether the membrane layer should be over or above the insulation? The contractor installing the insulation (50mm on roof and 24 on walls of Extruded PS (i think)) was initially intending to put the membrane over the insulation but is now suggesting that he lay it under so that the structure is water tight for the main contractor to get on with the build. Can anyone please advise urgently if possible if this is ok? I dont want to encounter problems later due to this issue. The main contractor says it would be better for the membrane to go on after as then any leaks can be identified earlier. Any comments on level of insulation als appreciated! Thanks for your help

Flat, concrete roof slab, correct?

Are you present at the site or are you doing this by "remote control?"

An important item is that there is positive drainage to the roof drains or gutters.  This is done either with sloping the structural slab (unusual in Euro markets) or with an additional "screed" of lightweight concrete like material (the more usual way).  Another "usual" method for larger commercial projects is to use tapered insulation (in this case the roofing membrane should go on top).

Assuming that there is positive drainage,  where the membrane goes depends somewhat on the roofing membrane material. Insulation used for roofing is typically XPS.

If the membrane goes on top of the insulation, it must be rated for exposure to the elements.  I suspect the roofing material probably comes in rolls about 1m wide and looks similar to a regular USA style asphalt shingle (this has a fine, colored sand impregnated in to it).  It is applied by heating the underside with a torch as it is rolled out to adhere it to the substrate.  If this is what you have, it can go either on the slab or on the insulation because it is rated for exposure.

If you put the insulation over the membrane, ballast is needed to keep the roof insulation from blowing away and as a UV shield.  Ballast can be gravel or something like concrete pavers.  If you are going to use the roof as a terrace, you will probably want pavers or tiles in any case.  While ballast is not a large load, it should have been considered in the structural design.  Just to further complicate matters, insulation used in this situation usally had rabbeted edges to promote drainage, although it is not fatal if it does not.

If you use gravel for ballast, it should be washed and there should be a layer of filter fabric under it.

Another potential consideration is the roof drains.  The location of roofing membrane must be compatible with the drain configuration.  Some drains are adjustable for high level or low level drainage and some are not.

To summarize:  If I were my job and if I were going to use the roof as a terrace, I would put the roof membrane on the slab with insulation and pavers over it.  This provides a good terrace surface and protection for the roof membrane.  Pavers and insulation are not too hard to removed if needed.  If you are not using it for a terrace, then I am neutral as to membrane location (as long as the roof material is appropriate).

If it were mine, I would like at least double the amount of inuslation.  You will get significant solar gain through 50mm of roof insulation on a hot sunny day, although 15cm or so of concrete will buffer it.  This may also complicate flashing and drain details if the membrane goes on top.  If the membrane goes on top, a lighter color is better than a darker color.....same with paver color.

If you can, make them do a 24 hour flood test.  Make sure there is protection for any work that goes after the membrane.

The devil is always in the flashing and drain details. If you have roof drains, make sure there is a place for water to go (scuppers over the side or overflow drains)  if a drain gets clogged.  A heavy pool of water can cause structural failure.

Good luck,

Bruce

 

Hi Bruce Dana et al.

I am trying to figure out a flat roof in Jersey City, NJ (also posted a thread on the design forum but saw this one and figured I'd chime in....). I have a 2100 sq ft torch down, don't have the money to do a complete tear off, the deck is fine, and I want to have cathedral ceilings under the roof...

I'm looking at open cell spray foam between the rafters (rough cut 2x8s). I am also looking at a foam roof. Advice?

Thank you!!

Sorry....I am a bit confused.  Is this a new build or a renovation?

I have no firsthand experience with foam roofs, although I have read about them.  It seems that they are "molto brutto"  ugly and sloping to roof drains is a bit hard to control.

Bruce

Sorry I'm being confusing. This is another user...and I've added a question to your thread.

I've got an existing building with slightly sloped flat "torch down" roof, which drains to one gutter in the rear of the building. No one will see this roof except for me...

Any thought on the under roof deck foam?

Bisseti