Puddle in the centre of a flat roof

Standing water on a roof is not good, if it freezes it can expand and blow the roof apart this is known as freeze thaw weathering. The lifespan of a roof can be drastically reduced. All flat roofs should be build with a slight fall so that the water can drain, over time settlement can eliminate the fall and you can get standing water. On this roof there seams to be some sort of design problem and the supportive timbers running through the centre of the roof and sagged to such ash extent water is pooling above

[@theroofmonkeys]

Great help!

[@pauldavies5655]

as a roofer for 30 years plus , i ve done many types of flat roofing , the old hot pour bitumen flat roofs , which were actually flat (no slope ) covered in gravel , which held water , moss etc lasted a lot longer then roofs with a slope.
if you put a piece of bitumen in a glass of water it will stay the same for 30 years , if you put a piece of bitumen on a road in summer , it ll be gone within a week !!
water does nt destroy flat roofs , the sun does !!

[@pauldavies5655]

off course today we have different systems , epdms , grp , liquid rubber etc, water does nt affect then either , but the suns uv rays do !!

[@pauldavies5655]

paul davies i m about to change a hot pour flat (4 mm mineral top coat ) roof that i did 27 years ago,the customer put wooden decking on top of it which held back the water and kept the sun off the surface of the roof ,
i ve tried too get him to have a different covering but he was so impressed with hot pours durability that he wants the same,
ps ,the decking has been changed 3 times in those 27 years ,,lol
pps , the germans design flat roofs to hold water also !!

[@patterdalezipsuzilil]

I fitted my own flat roof 33 years ago torch felt smooth 2 layers it was like 70 a roll in the 80s 4mm thick 2 layers paint it every couple of years with alluminium reflective bitch paint still going strong

[@giotto4321]

It looks to me that the timber you're calling a 'trimmer' is actually the primary support onto which the new extension's roof joists are hung. (doesn't look like a flitch beam either to me) It looks under-sized for the structural loading it's being asked to accommodate and should be doubled up, rather than being a single member that's the same section as the other joists. The load is causing the deflection as you suggest, resulting in the top-side ponding.
The exterior of the roof also looks a bit strange to me, it appears that only the external brick leaf is extended up to form the parapet; also the upstand on the rooflight seems a bit lower than I'd expect. Must admit, I'd also have had a board below the insulation rather than having it bear straight onto the joists. Depending on the spec. of the insulation and how compressible it is, that might be exacerbating the problem. It's usually fine for a UDL but not so good for point or line loads. Just my tuppence worth!

[@SteveRoofer]

Yes you are correct its not a trimmer its the main support your obviously an engineer?

[@giotto4321]

@@SteveRoofer No, an architect actually but I really enjoy your videos and learning from articulate specialists like yourself who physically build things on site. It's fine reading about or drawing things, but until you see how elements are put together in the real world, you don't really learn. That's why I much prefer being a site architect and not just a CAD monkey!😉 Keep up the good work. 👍

[@mattthomson1689]

This construction all looks so very new, I wonder if that central “sinking trimmer” is a normal flexure? With everything exposed below the roof, now may be a perfect time to very closely inspect each joist hanger for proper installation with proper nailing patterns and the use of manufacturer’s specified length and size of nails or other fasteners, especially if this is a DIY project. Pulling a few nails if necessary to check lengths is not unheard of, even amongst licensed contractors. Simpson Products is extremely good with recommending appropriate installation methods, such as metal roof joist hangers. As long as my own flat roof puddles dry out within 24 - 48 hours of rain stoppage, I remain happy.

[@SteveRoofer]

Yes, very good points it's quite normal that the joist hangers haven't been put in correctly. There's always a chance that there will be some movement that's why with flat roofs you are supposed to do 1in 40 to achieve 1 in 80 when finished and settlement has occurred

[@petesims]

I reckon they let the timber drop when putting in the sky-lite

[@jemedemande5166]

Some roofs in warmer climates are designed so an inch or two of water is constantly lying on their roof for insulation and to prevent the recooking of the roof covering. IMO "Water lying is very incidental"

[@SteveRoofer]

thats interesting I was only talking to a customer the other day and he was telling me in Africa they have water free days, days when the people of the villages go round emptying all containers of standing water this helps brake the life cycle of the mosquito.

[@leahmay1011]

Hi is pooling a concern if it's 5-10 mm

[@SteveRoofer]

Yes as the roof will normally deteriorate quicker at this point

[@leahmay1011]

@@SteveRoofer what sort of pooling is acceptable on a flat roof befor the evaporation drys it out