Best type of flat roof?

[GEO-PAR]

Hi guys,

 

Any recommendations on the best type of flat roof to go for? Everyone I speak to has a different opinion so I'm sure it will be no different asking the question on here, but still keen to get people's view and reasons why.  The roof construction will be concrete hollowcore planks, screed to falls on top, circa 200mm insulation, roof waterproofing (no idea what's best?), pedestals, paving stones.

 

I've been looking at the Bauder total bitumen warm flat roof system, but some other people have said EDPM will be better. One person suggested GRP on the basis it's covered and the sun won't get to it so easily (I know they expand and contract quite considerably).  Any thoughts, ideas, etc welcome! 

 

[Onoff]

Pitched 😂

[ToughButterCup]

Slightly, uniformly pitched the correct way 😑

[Mike]

49 minutes ago, GEO-PAR said:

Bauder total bitumen warm flat roof system, but some other people have said EDPM will be better. One person suggested GRP

All can be good or bad - it depends on how well they are installed. I know one high-spec 3-layer flat roof that's still good after 30+ years, and a GRP roof that had to be replaced after 5, so choose your installer with care.

 

Personally I'd choose mastic asphalt. It has a very long history, will last for decades, is readily repaired, with good green credentials too.

[SteamyTea]

Properly done GRP, will last decades.

[SteeVeeDee]

There are two types of roof: pitched or leaking.

 

Epdm is good as long as its not going to walked upon. It can be repaired but the patches look like a teenager's bike tyre after a while. Ideally its for dormer roofs out of sight, or below roof terrace decks where its covered by a walking surface system. 

 

Felt is the industry standard because it's got a predictable life span, it's cheap, companies are always available locally, it's easily repaired, and there are a wide variety of liquid compounds to prolong it's lifespan if necessary. 

 

GRP isn't very good and lasts far less than advertised. Its far too brittle for the task it's given. Fixing it to the substrate doesn't make any sense, particularly when one side is exposed to extremes of weather and the other is fixed down. Differential movement is an actual thing and shouldn't be ignored. Its also horribly toxic and cannot be recycled.

 

 

Edited August 2 by SteeVeeDee
Bad spelling and grammar.

[Nick Laslett]

Other members thoughts on different options from a few years back.

 

 

[Nickfromwales]

Whatever the choice, as there’s dynamic loads from this being a terrace, I’d put 2 layers of decking board under the covering of choice, joints staggered, for bent & braces. Screwed and glued together and NOT nailed.

 

I’d also buy some pedestals with the biggest possible bases, to spread to point load as much as possible. 

[SteamyTea]

2 hours ago, SteeVeeDee said:

GRP isn't very good and lasts far less than advertised

I beg to differ.

There is still GRP cladding  walkways, and roofs I worked on over 40 years ago that are still good.

But note I said 

2 hours ago, SteamyTea said:

Properly done GRP

 

[SteeVeeDee]

1 hour ago, SteamyTea said:

I beg to differ.

There is still GRP cladding  walkways, and roofs I worked on over 40 years ago that are still good.

But note I said 

 

There are always exceptions to the rule. 

[GEO-PAR]

Sorry to bring this up again - I sat on this for a while whilst sorting out other things but planning to get this done over the next few weeks.  I had a quote come back at £15k for 30m2 completely square flat roof.  The quote is based on the bauder total bitumen system https://www.bauder.co.uk/flat-roof-solutions/reinforced-bituminous-membrane/bauder-total-roof-system  Thoughts on this?  Feels very steep to me at circa £500 per m2.

[SteamyTea]

Having done a few GRP roofs, it is the edges that costs the money, not the surface area.

Bitumen based roofs are rather old technology now, polyurethane seems to be the latest technology.

[Onoff]

@MJNewton used Desmopol liquid pu:

 

 

[torre]

16 hours ago, GEO-PAR said:

circa £500 per m2.

Is that just for the roof covering or for the roof layup below plus slabs etc over? 

 

Sounds steep for just the covering. For a simpler roof finished with EPDM we were quoted a bit under 100/m2 (did it ourselves for quite a lot less). It's the edges that need the time and attention to detail

 

EPDM looks a decent choice to me here, it'll be supplied as a single sheet so minimal chance of leaks and very patchable, and you don't need to worry about looks as it will be covered

[GEO-PAR]

On 28/09/2025 at 15:27, torre said:

EPDM looks a decent choice to me here, it'll be supplied as a single sheet so minimal chance of leaks and very patchable, and you don't need to worry about looks as it will be covered

I have looked at EPDM but someone said I'd be bonkers going for this with pedestals? And the cost covered VCL, insulation, waterproofing, edges (incl leadwork). Not the pedestals or paving stones. 

[torre]

2 hours ago, GEO-PAR said:

looked at EPDM but someone said I'd be bonkers going for this with pedestals

Did they tell you why or did they just want to sell you something else?

 

I don't know that Bauder system at all, it certainly looks heavy duty. Bauder also look like they supply pedestals themselves, so if you do go that way I'd try and get those supplied with the rest of the install so there's no question mark later about suitability if you have issues.