How to join a flat roof
This is something I’ve been asked quite a lot, so I’ve been forced to build a web page to try and help deal with this sometimes tricky problem. Often when considering a new flat roof you want something different to your neighbours roof, they may have a traditional felt roof but you fancy a single layer system like fibreglass or E.P.D.M. ( rubber roof )
The reason this is tricky is because there isn’t a consistent and correct answer for all occasions, normally a roofer would come to look at your flat roof in the flesh so to speak to evaluate and estimate. Although they probably wouldn’t literally be holding a clipboard with a check list on, there is probably one in their head which counterbalances experience and knowledge of what will and won’t work as they look at the roof. Factors like slope ( fall ) , size, expansion and contraction, possible height differentials, laying water ( ponding ) , drainage and available outlets will all factor in the final recommendation and estimate.
Let’s look at a few factors first and what they might mean for a finished flat roof joint.
Flat roof ponding
When new, all flat roofs should have been built with a small fall to a gutter, outlet pipe or hopper head to remove the rain water. Sometimes the builder / roofer gets it wrong and quite literally creates a flat roof. Quite often though the timber joists underneath have settled a bit with age and you either end up with a totally flat roof, or even worse if you’re unlucky a slight dish creating a pond. A join in any roof is a potential leak and roofers will always pay extra attention to joints when installing a flat roof even when working with like for like materials. The potential for a leak always increases when joining one type of flat roof material to another, especially when it will be effectively underwater in wet conditions.
Is you flat roof flat holding water
This is your first consideration, does mine or my neighbours roof hold water where I am proposing the join? If the answer is yes you may need to build a physical separator called an upstand between the two types of roof ( more on this later ) or reconsider your flat roof choice by using the same covering as your neighbour making it like for like and easier to joint. Also if your neighbours roof is old and worn, you could negotiate a re-roof on that too and eliminate the joints altogether. I find often that customers go to crazy lengths to avoid re-felting their flat roofs even though great strides in felt roof technology have been made, often a new torch on felt roof can be a fantastic long life and logical choice, see my article on best flat roof.
When assessing the pitch of a flat roof always get an accurate level onto the roof and carefully study the degree of fall to build up a mental picture of potential snags before you get them, this way plans can be made in advance of problems.
Level flat roof ? – If your level isn’t long enough get some strong straight timber to sit underneath the level to extend the reach and accuracy of your readings. Alternatively look at the roof after a heavy rainfall. Here we have determined the fall to be a 15mm drop into the guttering.
There is also a scenario where jointing a roof in the correct manner will accidentally create a pond, for instance if the joint is horizontal to the fall ( passing across the slope ) and now is the time to give this serious thought. One such situation would be joining EPDM to felt with a termination bar, if the termination bar runs across the fall of the roof instead of with the fall, the result would be that you have built an artificial dam about 6 – 8mm high .
Termination bar – This could also be true of a felt on felt join and others, but they may not have quite the same height impact and therefore water holding potential however.
See this video further down…
If you have found that your roof does hold water and the join may sit underwater, it could be beneficial to consult a roofer on how best to make the joint and whether what you are proposing is practical, especially if you are in any doubt. It may come to light that a fall has to be introduced to your roof via firing strips for instance, or a physical separation built into the roof via an upstand of some sort if it’s viable.
A physical separation of two roofs
This is a very good way to separate two different roofing materials. Here I will give an example of how to separate a dormer roof when the neighbour’s roof is torch-on roofing felt, and yours is a new GRP fibreglass roof.
One of the best ways I have found is to create a physical separation between the two properties with an upstand. For this example let’s assume that if there is any ponding it’s minimal, so I can simply use some good quality roofing batten for the upstand itself.
Stage 2 – Screw the roofing batten directly to the roof deck, this now creates a physical boundary in the shape of a raised upstand to both parties. Picture shows a fully replaced roof deck and ‘ boarded on top ‘ example.
Stage 4 – Make good next doors felt roof with a heavy weight torch on felt. You should be insured to work with hot flame and be competent in the field, If you are not, get a qualified roofer to complete this process for you, do not take chances with hot flames.
This same method will work for joining most types of roof to another roof because each neighbour gets to repair their own side of the join in their own chosen roof covering. If at a later stage your neighbour wishes to change their roof covering to the same as you, the upstand can be left in or removed if no longer needed.
As an alternative to a roofing batten when using fibreglass, E35 / E40 or E280 trims can be used to create an upstand with a little expansion and contraction. See our page on fibreglass roofing trims.
Can I join a flat roof
I know you’re probably reading this because you’ve got you’re heart set on a new single ply, fibreglass or rubber roof, but I’m going to tell you now something you may not want to hear… it may not be sensible or practical. If you think you might fall into this category you will have to either make your own mind up or contact a competent roofer for an expert opinion.
Here are some of the situations it may not make sense to change from the norm…
Connected common shared roofs – Let’s say you live in a block of flats all with the same roof covering i.e. roofing felt. Sometimes these have complicated designs with built in gutters or gulley’s, this could be a nightmare for sensible and practical joins. Also if you are situated away from the outside edge of the building you double your neighbours and the joins, and therefore double the potential for problems later.
A ponded roof with the neighbour – As discussed above, a flat roof that holds rain water any larger than merely very small puddles may submerge the joint underneath the water for large periods of time. A physical separation in the form of an upstand may be a practical solution, but if overlooked and visible from bedroom windows by either party it may not be desirable from an aesthetic point of view.
As an added complication, in order to make good the separation on the neighbours side it may mean working on their side of the property with their existing flat roof medium, even if this is only by a small amount. This means any small leak, your fault or not, remotely near this area in the next 100 years will be your fault.
Joining different flat roofs
Lets have a quick look at how to join a roof without an upstand where practical…
Joining onto roofing felt with EPDM – Roofing felt is bitumen based and it can be difficult to get any sort of good bond onto bitumen with anything other than a like for like bitumen product. There is a system of mechanical fixing however with a ‘ Termination bar ‘. This is basically a long strip of metal with a series of holes pre-drilled that clamps the EPDM membrane onto the roofing felt with a dedicated water cut-off mastic sandwiched in-between. The felt must be clean, dry and free from dust before applying the mastic, the roof timbers underneath ( decking ) must also be in good condition so that when the fixing screws tighten up to clamp everything together, there is enough purchase to torque down all the fixings equally.
Because there can be long term problems where EPDM touches bitumen based products it may be prudent to smear water cut of mastic thinly onto the surface of the felt with a trowel where it comes into contact with the felt, and a thick bead near the overlap itself as shown in the video.
Joining felt to EPDM – Long term direct contact and exposure with the oils in bitumen can cause the join and the EPDM to fail as discussed above. If you need to connect a new torch-on felt roof to existing EPDM an upstand separation may be your best bet.
Joining EPDM to fibreglass – Here I would use a termination bar again if no ponding or large puddles are present. At this moment I am 100% unsure as to the long term effect of direct contact between EPDM and cured top coated fibreglass although I believe it to be OK. To be on the safe side I would be tempted to smear water cut off mastic thinly where the rubber and GRP makes contact, and thickly near the actual lap edge of the join and termination bar. ( all surfaces must be clean )
Joining EPDM to Asphalt – Again a termination bar is viable but the direct long contact between bituminous asphalt and EPDM can deteriorate the rubber leading to failure. I have heard that sometimes old weathered asphalt may not damage EPDM or at least not as quickly as new asphalt, but my advice again would be to smear water cut of mastic between the two wherever contact would be made, and again a thick bead near the edge of the lap itself. ( all surfaces must be clean )
Joining Fibreglass to Roofing felt – Fibreglass does not like to bond to bitumen. Very short term it may cure and even look like a bond if you’re lucky, but it wont be long until it delaminates horribly meaning a big leak. When jointing a new fibreglass roof onto any sort of felt or torch-on a physical upstand is best in my opinion.
Joining Fibreglass to EPDM – Although you may not realise it, rubber is an oil based product and therefore just like bitumen or roofing felt I would recommend an upstand separation.
Joining Fiberglass to Asphalt – Again asphalt is a oily bitumen based material, old weathered asphalt may provide a bond for GRP fibreglass for longer than most but it will almost certainly fail sooner rather than later, again a physical separation of the two, though the use of an upstand is best.
A final note
Unless you are working to a plan often there is more than one way of joining one flat roof to another, the problem is as with many situations in roofing and building, there are so many possible variables that it’s impossible to cover them all. Hopefully this article provides you with a solution, maybe you can improve on these ideas, or it gives food for thought.
- Best flat roof replacement – See the most common flat roofs compared
- Flat roof costs – Current flat roof prices
- Find a recommended roofer – How to find a good roofer