Parapet Walls - Are They A Good Idea? 

Great explanation of "thermal siphoning" - most of the houses here in Ukraine have no insulation, so the little house we bought got an upgrade - first the ceilings got two layers of rock wool, about 30cm, then the sides of the house were covered with 10cm foam and stucco, then I extended the roof by one meter all the way around, so I could wrap the corners with foam and cover it with under roof rock-wool. There are no cold spots, therefore no condensation. For winter windows I added a third layer of clear plexiglass with wooden frame to the outside of the double glazed windows. Little condensation around the glass edges in the kitchen and bathroom. Sure keeps from sending gas money to Putin. Love to watch your videos, you get to the point without much ado and tell it like it is. Thought about heat pumps, watched your videos and will hold off. Best investment were some solar panels, a low frequency inverter (6KW) along with 800AH LiFePo batteries - reduces my utility bill by half, except for winter months. Thanks for your videos!

Perfect timing! Roofer asked me today if I wanted to retain the parapets. He explained the situation exactly as the video. I now feel knowledgeable.

I knew nothing of parapet walls. Although I have seen them I never knew the nomenclature. You pointed out some very informative problems. Thank you. 👍👍

I have a couple of neighbours with the same story. First the roofer replaced the leading (sloping warm roof with parapet) then new coping, still condensation. Massive job to fix, so they have to live with it.1960s build, so originally it did not have heating. Cavity wall retro-fill. You have nailed the reason.

It needs the insulation standing up on the inside of the parapet wall and then the roof membrane covering extended up and under the Cap stone. Will require extra width on the cap stone to accommodate the increased depth of the parapet. Excellent rant Roger.

That's a clever way of solving the issue, similar to my old builders which follow Joe Lstiburek's building science "perfect wall". As long as the Water > Air > insulation barriers are all joined and layered in that order it minimizes and in most cases removes the issues.

Roger is correct about taking insulation up the parapet wall, I always make sure I take the membrane up , over and down 20mm . So as the roof is water tight without the cope/capping.

I like your solution Roger and it's pretty much how a competent Architect/Designer would normally detail it. In Australia we don't have as many dampness condensation problems as you have in the UK due to our warmer climate - we certainly have them but not as often. Over here, concrete roofs are not all that common, we certainly have a few parapet walls but usually in conjunction with metal deck roofs. Before I retired I worked in the Architectural profession and I have detailed many parapets in my time. I am pleased to say however, that I can't think of a single instance where they gave a problem. I would say that trying to flash into the face of brickwork at a DPC is always fraught with danger and we always detailed our parapets with the roof flashing carried up for the full height of the parapet and over the top and almost always in conjunction with a stud frame as the inner part = exactly as you explained.

Excellent stuff! I've been a plasterer, tiler, plumber, electrician and carpenter for many years now and consider myself to be experienced, talented and knowledgeable but I still learn something new from all of your videos. Keep up the good work!

Good stuff. Glad you mentioned roof overhangs. The first thing I look at on a building is whether the roof overhangs the walls and how far out it goes. The overhang is an enormous asset in preserving the condition of walls.

That was a very good explanation Roger. As a bricklayer I would like to add to this, the increased specification of concrete blocks rather than thermalite blocks. Yes they are stronger but they create a real problem with cold spots. Your example is a classic case, if the inside wall is made of concrete blocks the cold spot would be far worse than if the internal wall was built out of thermalite. I suspect dot and dab is also far worse than a light weight plaster too.

They've been the rage here Down Under for the last 20 years! So there's been plenty of leaks in that time, few guys know how to construct them properly and even fewer plumbers know how to flash them and sort out correct sized drainage. We don't/won't have them on our houses!! Keep up the good work Roger.

I lived in a 60's low rise block, with a flat roof. This was a concrete slab construction, with brick piers. This was before expansion joints as well. The flat roof always gave problems for the top floor flats. Block spent a lot of money on re-roofing with a specialised covering. I did suggest that the block get planning permission for a butterfly roof constructed on top of the old flat roof, since the flat roof sloped to the middle where all the drainage was. But the block went for the cheaper option.

Interesting video. We built a large extension about 20 years ago with a parapet wall. It's still on its original felt roof and we never had any issues with moisture ingress. We keep thinking about replacing the felt because it's technically at the end of its design life, but we inspect it every summer and it's completely fine. I can't even justify changing it to GRP, since the felt has lasted so long. I think your opinion is a bit too harsh on parapets, but I hear you.

Whilst I have no need nor likely never will need a parapet wall, this was a very good explanation of what to consider should I ever have the chance to have anything to do with one (hope not). Thanks.

There are new load bearing insulation blocks (like Marmox Thermoblock) that removes the thermal bridging and is rated up to 9Nm. Currently using it for an extension project below DPC and parapet.

Steve Roofer's videos are the absolute best when it comes to all things flat roofs. Great if you could get him on for a discussion on flat roof construction techniques.

"If you don't know what a parapet wall is, then why are you watching this video!?" Roger, you are a legend.

Thank you for doing this Roger and the consensus view of the comments thus far is that you have nailed it. However, I am surprised that you did not consider adding a heat pump to sort out condensation problems. Taxi! 😉👍

Really great video to better understand the vast world of parapet walls. Many times in my life I’ve tried to make some quality parapets, unsuccessfully. Now, with your great explanations, i will be able to do so. Again, thank you very much. I Love you Roger. Deeply. I would like to feel those steady hands on my body.

Excellent as always. Last winter I found a leak into a shop in a 1970s property which no one had been able to trace for 25 years. Turned out it was because it was on a parapet edged slanted roof above an entirely dry flat above the shop. Despite roofers and architects and surveyors over the years it had never occurred to anyone that it might be the UPPER roof, not the walkway outside, which was part of the shop roof. Why? Because it was following the cavity down and bypassing the upper flat. leak was a combination of poor flashing and damaged tiles.

Good opinion, I will use this when I build my house in the pacific, parapet walls are good for cyclone proofing.

I'm going to put down prestressed slabs and put a mass concrete parapet wall and a railing. For a balcony. Obviously I'll have to put a screed over the slabs, some form of damp proofing which will run under the mass concrete...I'm trying to work it out. Make sure it's waterproof, I will have issues with thermal bridging...there will be a ceiling below the breton slabs, and lots of insulation, and the internal wall will be well insulated too..I'll work it out

Great demo. Absolutely gem of a video. I wouldn't of learnt anything today without that video👍👍. Top man rodge.

Thanks so much for explaining this Roger! I've had this EXACT problem in the garage of an extension we finished recently and couldn't work out what the f#%king hell it was!!! Well at least I now know how to rectify the problem. Will be looking at this detail a lot closer next time when building / going through the architectural drawings. Cheers from Australia mate, Nick

Great video and explanation. I'm building my extension now and am debating going with a parapet vs not. The top will be a walk out balcony so I figured the parapet would act as a nice bit of privacy from neighbors. The points you make about straight walls and how they are great looking when they're first done but over time they begin to stain and other problems are created, I could not agree more, I see this all the time in Brooklyn. I do like the overhang roof because when it does pour you do feel a sense of relief knowing that the water isn't coming straight down as soon as you open the door or running down the wall. That sense of relief is reason enough to go with an overhang versus a parapet.

Hello from California. Good content. I'm used to seeing them built in commercial multi-tenant buildings and row housing developments, they're usually used as firebreaks between the units in the building by extending firewalls 2-3 ft above the roof surface to keep flames from jumping along a roof. I've never seen a house with them as a decorative element.

Roger, I love parapet walls when they are built correctly. Especially when built on a rear extension as a party wall. It allows the neighbour to abut the wall at a later date at perhaps a different height from the first neighbour and without interfering with their roof covering. But they MUST be built correctly.

I just love Rogers humor! As always a good video, educational and funny.

it all makes sense roger, you've got a knack of explaining things so that anyone can understand them

Great video, saved to my house design playlist for reference!

Makes sense,thermal bridging in that area is easily missed,and missed diagnosed. Good video cheers

Roger, thank you so so much, you have given me a lot of inspiration and key things i need to take into account when designing a parapet wall for my Masters in Building Surveying coursework!!

So right about stains appearing a few years later down the front of the building due to no overhang in the first place.

Your channel is the best building site on YT. Love it. You are a great instructor.

Great video. Makes perfect sense to put a thermal break in the masonry. Personally, I insutlate everything on the inside e.g. 50mm EPS under the plasterboard and XPS for the floors.

You hit the nail on the head, I'm going to renew and insulate my flat roof, and I was thinking how to construct the edges of the roof, in combination with exterior wall insulation. My idea from the start was to stay away from proper parapet walls, which means I'm not extending the inner leaf of the walls above roof level. My idea is to have a bit higher raised roof edges with insulation against the inside of the outer leaf which extends above the roof, the top is covered with a thick plywood sheet which extends enough over the insulation on the inside, and outside the outer leaf, both 70mm. The roofing membrane is covering all of this and finished with a trim piece. Zinc sheeting with rolled over edges is used to finish the top of the edging. This will give the idea of a parapet wall from the outside, but in a more classic way and without any cold bridging and leaks, as long as the fasteners for the zinc sheets are sealed well where they penetrate the membrane.

Thanks for sharing your knowledge and opposition on this subject 👏🏿👏🏿👏🏿

Thank you for our content. I would use external insulation on the inner layer of the parapet, which will stop any cold bridging. This method works perfectly in countries where the winters are very cold, and the autumn is very humid. Your solution is very good, but it makes the detail too complicated.

Absolute Genius. Many thanks for that clear and concise explanation.

Roger is spot on again great video

I’ve got that problem, we had Roy Rogers and Billy the Kid build our extension (badly as well) they took all the cash and don’t answer the phone now! At some stage I will need to get it sorted, but you are 100% correct in what you say Roger, great video as always, thank you.

Cheers Roger, very helpful buddy

Excellent explanation Roger, I definitely prefer a roof with a good fall to shed the water and channel it away safely. Wish you were in business near me.

Steve Roofer/London flat roofing is an absolute Jedi Master when it comes to flat roofing, top bloke too.

That is a great solution, very well explained, thanks Roger.

Good vid Rog but I wouldnt mind some photos of one you done earlier. Got to do some 1200mm high PP walls on my back extension if I get it started this year as a wind break for a first floor balcony. 😁

US builder here Roger... Great fix for a poor design.... nice to see a no nonsense info vid about this design. On a parallel thought concerning wasteful/design/building etc.. We've built 4 plexes in Fla. where the entire design is about eliminating waste, for instance the rooms were designed so that you could hang full sheets (using 12 footers) where the rooms were 8' or 4' or 16' or 12' ... the outside of the building was the same..... the scrap pile was (for a 4 plex mind you) fit in the back of a pickup truck.... I love that concept.... another great design is a rectangle building (so the roof's cheapest design's been achieved), with a 9ft ceiling downstairs and and 8 ft. upper.... have an overhang coming out all the way around the building from just under the windows extending out 6 ft. all the way around the building which overhangs a 6 ft. deck. This design gives one that solar heating in the winter and cooling because of the over hang. The look's usually dolled up with the hand rail etc... the economy of this design can be further extended by consolidating the plumbing by running a common plumbing wall with the bath and kitchen below and the baths above. anyway... always appreciate your videos Skill Builder... Jon O @ Idaho

Very interesting Roger, when we had ours done we noticed almost immediately after the first heavy rain, water coming in to one side of the bifold doors, on closer inspection the existing rain water down pipe from the original hip roof that was now impeded by the extension was discharging onto the new flat roof but through an extended piece of down pipe now laying flat to the roof, massively directing the contents of the main roof rain water along and under the flashing. We are in a small terrace and three roofs collect into a joint guttering at my house. In future I’d do what you suggested to avoid the cold bridging and direct rain water either off the roof entirely or a small boot into the centre of the flat roof

There is a really good video by stewart hicks on flat roofs that goes into quite a lot of detail. The problem here doesnt seem so much exclusive to parapet walls as a conflict of older and newer construction where insulation and air tight design mixed with older methods often leads to problems like condensation, mold, siphoning and trapped water in the event of leaks.

Thank Rog! That's a fantastic explanation of the issue. Why is it not built correctly in the first place to remove thermal bringing? Also why not run the membrane all the way over to be sure there won't be water ingress? Cover it with some nice trim coping cap that hides the membrane.

So many schools have this problem. Some think a topping stone will do, its not a dam wall outside a house

Agreed Rodger as a golden rule I always try to have all insulation over lapping onto each other, mitigating the cold spots where the ceiling meets the external wall insulation.

My old man hated modern flat roofs fifty years ago. They always leak, he'd say - and he had charge of maintaining a big new hospital at the time. The roofs took up loads of his work time for the first few years. We're a wet country, it rains all the time, then it snows, why the hell do they spec flat roofs? His words. We didn't agree on much, but he was army trained and he knew what worked. In Japan they'd call Roger a living national treasure (this'll annoy him ; ) ) - but he's talks so much knowledge and makes so much sense that every day's a school day. Brilliant.

I had exactly that problem but persuaded the builder to replace the coping stones with lead, 40mm outer overhanging and approximately 300mm on the inside, with the grp tucked under. Two years ago, all dry and me and the missus very happy!

I'm a brickie who hates parapets but some clients/architects/building inspectors love them so here's what I've done in the past. For a flat roof, build your parapet no more than 150mm above the highest point of the roof. Get some insulation board. (25mm should do) and no nails (Or similar) it to some 18mm ply. Then no nails it to the inside of the parapet (Obviously cut it to shape), ply side out. Then when your roofer lays the roof finish material (Felt or whatever), tell him to take it up the 18mm ply and right across the top of the wall (Over the cavity, ensuring it does not sag) and onto the outside brick/block skin. This will make the standard wall width approximately 350mm. Bed a suitable DPC and coping stone on top. The DPC won't sag and allow water to build up because of the roof material covering the cavity. (The roof material does not usually have the same lifespan as a DPC and it can break down slowly over time, that's why its important to use a DPC). Job done. It's possible to repeat this method on a pitch roof build but it takes a little figuring out and planning.

Roger confirms that I am a fully certified fashion victim! I've done an extension virtually identical to your descripton....... no gutters.... minimal (aluminium) coping...... with a parapet wall... I was relieved to see that my approach mirrored your recommendations with a flat warm roof with the parapet added after the fact. I'm a big fan of external insulation in that it allows a complete blanket of insulation with no thermal bridging other than the fixings themselves. The added bonus with a warm roof is that it can sail over the wall structure to meet the external wall insulation, forming a warm junction with no gaps. I have to say if you get the details right, I've had no problems with water staining of the walls, so I'm still a fan of the very clean looks.....

The problem isn't the parapet, it's all in how you handle flashing/insulating it. Your solution will work, but is really only an option for new construction. You seem to be talking more about existing buildings and reworking the deck and wals like that is pretty impractical. The best solution in my opinion is to add rigid insulation board to the inside of the wall. I see this even in New construction. To build on that, encapsulate the wall completely with a thermoplastic single ply, ditch the stone cap and terminate on the outside wall. This will leave your wall water tight alot easier.

Superb explanation, thanks for sharing 👍🏿

I've really learned something today Rodger and will be using this if I take on an extension like this, keep up the great vids😁👍

Great taught me something about the improved design but its always old parapet walls on OLD BUILDINGS

Parapets can also be detailed as balloon-type timber frame to help reduce cold bridging. Keeps it rigid.

Im well experienced builder but you have my thumbs up!!!

Yes. The grubby, stained wall effect. Loathsome. In cold wet areas the bricks can get wet and overnight freezing gives a beautiful spalling effect.

I think you are spot on - avoid parapet walls if you can. And double height splay bay windows too but that is a whole other story....

Yes I think the term is thermal bridging. Best done by connecting all walls, ceiling, roof with external insulation, physically and thermally connected outside the main structure behind the cladding, etc. Water condenses on cold surfaces. Remove cold spots, no condensation using thermal bridging.

Our 30 year old house has a separate garage with parapet style roof, it's uninsulated but that hasn't stopped the ongoing problem of leaks. Parapet incorporated into roof design is bad because it's so easy to get a faulty, problematic result. We have just replaced the roof and now must find a carpenter that can address the parapet because as described in video the roofing isn't even the real problem. Even though we aren't dealing with insulation and condensation, addressing the coping stone or top of parapet in video was very helpful. I can only fantasize about ripping out roof and redesigning - I hate our parapet roof!

Something I noticed about the UK is they don't use a metal capping that caps the top of the Parapet wall and runs down the wall to cover the upstand of the roof membrane. You could still fasten a stone capping on top of this metal capping if you want. No chance of a leaking parapet, it's all down to the roof membrane.

Roger, the drip groove on the coping stone (and external window chills) works due to surface tension, not capillarity. Good video.

A great topic Roger, there is certainly a lot to discuss when it comes to parapet walls. You didn't mention anything about cavity trays, if you can put a cavity tray in above the roof junction this ensures that any water that gets in above is directed out and they are certainly prone to failure. I suspect that many instances of condensation are caused by the wall being a little damp and reducing it's insulation value leading to condensation. I liked your idea of continuous insulation but I don't think it is good to build the structure off the insulation deck, it would be more likely to move and has no strength in tension not being anchored down. It might be better to fix the studwork to the roof structure and then insulate between the timbers, this would introduce only minimal thermal bridging but would be a lot stronger. Another issue is that a stone capping on a timber and masonry construction would move and likely crack the joints so this detail would work much better with a dry fix capping system

Great explanation Roger.

I have an extension on a bungalow being built, planning insisted on a parapet wall, they objected to a pitched roof to match the rest of the bungalow as it would impact on the light linked to the neighbouring semi detached house.

What if you have railings on top of the coping? how do you prevent ingress through the bolts?

“If you don’t know what it is, why are you watching?” That’s an easy question. I figured you wouldn’t talk about archers and medieval architecture - or WWI trench construction. So I was curious to learn a bit about it! Thanks for your description and construction alternatives.

Brilliant episode and great info. Just about to build a flat roof with a parapet funnily enough 😄

What is the specific building regulation and/ or british standard that dictates that one MUST use a cavity wall closer on a parapet, under the DPC and coping?

Good demonstration as why one should try to avoid parapet walls and where these cant be avoided make sure you eliminate cold bridging. As you pointed out people do not realise that condensation often gets worse locally on areas that remain cold after the house generally has been thermally up[graded. The denser the internal masonry leaf the colder it is therefore the more condensation forms. Parapets and just not a good idea as sooner or later the rainwater outlets become blocked which can lead to formation of a pool on the roof, and leakage to the interior.

As a architectural metal fabrication/installation master id say they are a must for ef1 or higher rating.

Really, really interesting and well explained, nice one!

Architects love them. Important to get into the grove.

Great, educational stuf to consider before getting to that point. Watched it twice

My extension has a parapet wall. Fine when the weather is nice, but when it rains it drips on to the window sill below and is especially irritating at night.

Great video. Does part explain a problem I and many are having with a open air Deck access & wall of is above our dwellings. Two years and the factor owned by Housing Association refuses to act and the Council, cannot even offer to inspect and declare below tolerable property. And not one RICS local Building Surveyor accept the job, either citing conflict of interest or they just don’t want the job. Sooner covid is a non news story the better.

i am a roofer for over 30 years and i have converted quite a few flat roofs to walkways and garden space, play areas etc. lots of customers ask can they have a brick/ block parapet because it look s nicer ( and it does ) , i allways refuse and get them to have RAILINGS instead otherwise i will not do it because your name is mud after condensation like you have shown . PROBLEM SOLVED !

Excellent illustration.

Thank you Roger (Chathura Kulasinghe)

I got an art deco house with a 44sq roof terrace with parapet walls, whole house is solid with no cavity and got to say ive never had any issues with what your saying. Needs a re render and am weary of silicone based renders staining i must say due to little overhang like you say.

Moncouche render and no roof overhang is the design trend that needs to FOAD.

You are the man.

Thank you for your insight! Interesting topic! You are awesome Roger!

Excellent case study

Is there a cross section detail on this and detail of how the inner leaf is then fixed and secured? by brickie hasnt seen it done this way before and doesnt seem super keen!

Good video, seen loads of these roofs and always thought they looked good. Didn’t know about the issues with them though, again just comes down to that state of builders in the UK these days. Hard to find good ones now :(

Thermal bridging is absolutely a problem and your solution is a good one. There are however various accredited details from insulation suppliers that can make a masonry scenario comply. Xrtatherm for example use full full cavity and permitter upstand which has been thermally modelled to show SAP compliance. Sarnafil take perimeter insulation up over and beneath a metal coping to comply!

Nice one Roger. The height of the parapet can also create problems for the structure due to wind and snow drift loadings. If its too high you may need to add buttressing to give the wall some lateral stability. As you say, better with an overhang.

nice solution Roger

Always make absolutely sure that the parapet top has a definite slope inwards and form the internal skin with insulated stud work

Great video Roger!We have 2 parapet walls, nothing but hassle and complication!They came about years ago when the second owner put up a ground floor extension on the rear of the property and then decided to add another one next to it about 5years later.Really I would like to do away with them and make it a single roof instead of 2.Just don't know if I can handle all the upheaval involved.🤔I just wish people would think more before they do these things.

Spot on

I have an old house with solid walls and a parapet wall.... How can I insulate the parapet wall and will it have the same effect as would a cavity wall ?