Replacing a flat roof
You are probably wondering where this fits in the grand scheme of my blog. Well a while ago now I said I would write posts about energy and money saving measures I’ve done to my home; I have written one very short piece about how we reduced our electricity bill from £55 to £28 a month and I hope this is the first in a series of posts on how we reduced or gas bill (whilst this is an older piece of work I’m hoping to do some stuff this Autumn/ Winter to cut our crippling bill).
So this post is about insulation and draughts, but I will describe in detail how we replaced our kitchen roof with a mind to improving each. Now I’ve said that I should mention a few things; during this post I am going to mention JJA Carpentry and Jeff Anderson a lot. Jeff is my father-in-law, he runs JJA and gave his time freely to help me, so the least I can do is give him a little free advertising.
Also these are old photo’s from a few years ago; Building Regulations have now changed and the method of insulation (and probably a few other things) have also changed now, so please do check with your local building control office before undertaking any work.
Also during this post please note while I will describe everything we did I am not a professional tradesman (only an engineer), I had the help of a professional when doing this work and that any mistakes are my own (Jeff did not help to write this – and it happened a while ago so my recollection might not be perfect).
So to start what was our roof like? Well it was a 2.5m by 3.5m (approx) flat roof covered with felt; when we moved in we noticed
that at two places in the kitchen someone had drilled holes in the ceiling and they were next to areas that looked like they had water damage. We immediately (in fact the first job we did) put lots of roof repair gunge on top of the felt near to where the leaks were and then didn’t think about it again for a few years. Then one January it started leaking again, and when I went up there I was shocked to find a small lake running the length of the front of the roof; a whole section of the roof had caved and formed a large depression.
After fixing the leak with more gunge (I lined our ‘pond’) I took a closer look at the roof; in one corner the felt at the front had lifted/ disintegrated and exposed the wood underneath…. which was chipboard, soaking wet and crumbling, plus there was an odd smell of cat wee coming from the gap.
So in brief we had a very serious problem and the roof would probably not last another winter.
I wish I had taken some photos because it really did look in an awful state, and just shows how far a roof can crumble without you being aware. Even worse I had no idea that my neighbours had already replaced theirs; when asking permission from them to work next to and on their roofs one of them described how her kitchen ceiling had collapsed the summer before.
Anyway the moral of that story is inspect your flat roof.
We had hoped to start the job in August, I had the time booked off, all the materials ordered, Jeff could help…. but it decided to rain for most of the week.
We finally did it the work in early September, against the advice of Jeff and Kelly (my wife) but we got lucky with the weather (I do mean lucky as I will explain later; I did take an awful risk).
Jeff started by removing the old roof (I was on the ground bagging it up) which came up in very small chunks as it was so rotten and sodden. Underneath there was a small amount of insulation and lots of other fun things, like a wasp’s nest, mice droppings and even a few areas where the mice had chewed on the electrical cabling.
We also discovered what had caused the death of the roof (other than being made of chipboard); the firings ended before and an inch below the last roof timber. The chipboard was unsupported and sagged for the length of the front of the roof, leading to the dip and eventual ponding/ small lake.
We removed all the old insulation and waste and took a look at what sat below it. Above the partition wall with our neighbours
we found several holes; the wall was made from a single breeze block section covered with plasterboard at a spacing of approx 1 inch (plasterboard was mounted to a frame sat on the breezeblock partition wall). The holes were supposed to be covered with tiles but some of them had come loose; this was allowing cold air from the roof space to run behind the wall. As this is a warm wall (heated kitchens on both sides) I decided to plug the holes up with expanding foam.
We also exposed the top of the cavity wall, which as can be seen is not insulated. I was kicking myself as I had thought to buy Warmcel insulation which could be poured into the gap, but decided against it as we might not have been able to get access to the top of the void (also for this work to count on a HIP you must have it undertaken in line with part L, at least that is true today, I don’t know what the regulations were when we did the work).
In the end we used some of the old insulation to pack the top of the gap; this is to allow us to get the wall cavity filled later without the blown insulation pushing up into the roof space. It’s also a sort-of eco-friendly way of insulating the wall as the rest of the old fibreglass went straight to land-fill. I have yet to get the wall filled but there is an offer on/ grant available in our local area.
After replacing the mouse damaged sections of electrical cable we put in the new insulation.
We considered the insulation system for our home very carefully. Today you will have to meet a U rating of 0.18 to comply with Building Regulations, but this work was done before those regs came into force and so the following discussion ignores them (so this is of more use to people outside the UK).
The best (in our opinion) system would have been a warm deck, with the insulation on top of the plywood, the 2nd best (taking lifetime costs into account) would have been a green roof (Sedum system). The very worst would have been a cold deck; basically replacing what’s there. In the end though we went for the cold deck replacement, and all due to cost.
The cost of the insulation for the cold deck (thanks to a government insulation subsidy at the time) was £15 ish, all in. The cost of a warm deck or green roof (as well as new timber etc) ran at £250-300+ (assuming you grow Sedum from scratch). We were unlikely to save £300 off our heating bill quickly just by upgrading the kitchen roof alone, especially allowing for the fact that there are no radiators in the kitchen, but there was a good chance we would make back the cost of the cold deck during the winter.
In the end I don’t know if we did get the money back; our heating bill still went up but since then I have realised that there are
many other things we should have done to cut our heating bill (such as stopping draughts – which I will cover in later posts). I expect it did make a difference but it was probably small overall.
The insulation we chose was 150mm deep by 370mm wide Space Blanket which we got from B&Q with an R value of 3.4 (I kept the packaging so we could have it included in a HIP assessment). I determined the required depth by pushing a probe (well, a chop-stick) up through one of the holes in the ceiling and measuring off of it the depth of the void, which was 200mm. Allowing for a 50mm air gap as per Building Regs this gives 150mm insulation. I determined the width of the insulation with a screwdriver handle by tapping it against the ceiling, listening for the change in pitch and then dividing the area equally by the number of ‘beams’.
The other reason for chosing the Space Blanket was that it came encapsulated in a mylar and plastic tube, which helps with moisture transmission (important as our ceiling plaster board is from the 1970′s and has no vapour-barrier). Today you would have to use a plasterboard with a vapour-barrier for this type of installation. The Space Blanket also has a reflective mylar covering, but I don’t see how this actually does anything in this type of installation (I of course made sure I fitted it silver side up as the instructions say – on the right of the page, after the advert, divorced from the actual installation instructions….).
You will notice that there are gaps in the insulation, these are where electrical cables pass through the ceiling/ walls and the insulation is kept back and where the ceiling light fitting and ceiling rose are.
So to recap (other than none of this applies today) we sealed up all the cracks and gaps letting cold air down into the walls, stopped up the top of the cavity wall with old fibreglass (more to stop any blown insulation from the planned later job forcing its way into the ceiling, and replaced the existing fibreglass with new encapsulated fibreglass insulation.
Next we had to put the roof covering on. As mentioned the firings were too short and low, so Jeff put new firings over the old and supported them flush with the first timber. This raised the height of the roof slightly and in theory we could have added a little more insulation, however the roof was not ventilated and we kept with the original design (the void from our roof through two neighbours down is connected so we did not want to significantly change the existing ventilation arrangement), so the slightly bigger gap aids air movement over the joists (above the insulation).
is moisture/ water resistant (it can be used for external structures untreated and will resist te weather for a some time before deteriorating) and so is the perfect choice for a new roof (I think it is now a requirement to use WBP). It’s also important for this type of roof due to the ventilation arrangement (see above); whilst we had no evidence (and still don’t) of condensation build-up within the void (if there had been the chipboard would have completely disintegrated) and we have increased the ventilation gap this type of plywood will resist the effects of condensation, which will build-up against the cold surface of the deck material (or hopefully it doesn’t make it past the mylar and insulation).
Next I sealed the edges (between it and the walls/ neighbours roof only) of the plywood with expanding foam. This was partly to improve the weather proofing and to help reduce any draughts getting in (even if the rain doesn’t get in air can run up from under the edge of the roof). To do this I squirted the foam in there and then cut off the excess whilst still expanding with a scraper (that way the edge forms a solid surface – if you wait until it has dried and break the excess off you expose the inner air-pockets).
Next Jeff put down arris rails along the sides (over the foam). Arris rails are normally installed when using felt roofing to reduce the angle the felt is bent through at the edges when forming upstands (it also ensures there isn’t a void under the fold in the felt). I was using a liquid roofing product so didn’t need necessarily need these, however I didn’t (and still don’t entirely) trust the liquid roof and so wanted the option to put felt on later, and (discovered after applying the roof covering) it is easier to apply the reinforcing fleece (explained later) to a slightly angled surface rather than at a right angle.
I then went over all the seams and screw-holes with mastic. I mentioned earlier how much of a chance we had taken with the roof; the weather had been overcast for most of the morning and now (lunchtime), just as the timber work was done it chucked it down. On the plus side it proved that the new roof was weather proof/ resistant but if it had fallen n hour earlier we would have lost the kitchen ceiling, and probably had some electrical work to repair too (all the kitchen electrics are routed through the ceiling). Whilst it was chucking it down I kept going with the mastic; luckily I had decided to get a mastic that could be used in all conditions (even under-water), it was much more expensive than the alternatives but it was worth it in this case. I don’t remember what I used on the day but recently I have used CT1 which also works underwater.
The roof was now weather proof, but needed a permanent roof covering. There were lots of choices for this but in the end it came down to money and help available. We went for a liquid polymer roof covering (I think we used Rubbaroof, if not the stuff we used was identical) because it could be applied by a person working alone and because it was the cheapest option at the time.
Sadly it was also the most labour intensive; first it required that all the joints be sealed with mastic (as done earlier), then a slow curing primer had to be applied. This was ready pretty quickly, but made working on the roof horrible. I had gone for applying the roof coating with a squeegee, which meant crouching/ kneeling/ sitting on the roof, and pretty soon the very tacky primer made the surface like human-sized flypaper; I lost some skin on my hands a few times pulling them off (the first time it happened it was ok, but by the 5th or 6th it was very painful) and the knees of my jeans became solid lumps within 15 minutes. The coating was hard to apply, you trowel it out and then spread it with the squeegee, but it didn’t half look good, like someone had poured oil over the roof. It was very sticky and easy to apply to the walls (upstands) without dripping, but when it came to applying the reinforcing fleece (needed at all joints and on the upstands) the fleece did not lie completely flush to a 90 degree bend easily, so it was lucky that we had the arris rail installed around the perimeter of the roof. Sadly on two sides of the roof this meant that we also had to use flashing tape (not shown in the photo’s here, I shall either have to find them or take some more).
I completed the last bit of the first days work at night with my wife holding a torch (I couldn’t use a head torch; my hands arms and hair were filthy from mastic and roof covering). The last bit was applying the coating to the leading edge of the roof (where the rain runs off into the gutter). The fleece still looked white as I had only done one side of it, and the lead flashing was still lifted up where it came off the roof .
The next day I finished off the roof. We ran out of liquid roof coating just as I completed covering the fleece on all sides except the back, where it was protected by the lead flashing/ curtain. I was a tad peeved as I’d spoken to the supplier who said that I’d have enough for the roof; I’d bought enough for 10m², and he said that would be enough for two coats plus the fleece, it was just enough for one coat and I didn’t like the way it covered the fleece, it seemed a shame to have this look of poured oil over the whole roof with sections of grubby face flannel running across it. In the end to complete the work I used another much cheaper coating product (again I have no photos here but will find some).
So there you are, two days work to complete our kitchen roof with better insulation and fewer draughts pushing cold air into the
house! Hopefully with the other work I intend to do this winter we will have a lower gas bill come the spring.
the whole void from our roof through to two neighbours down is open so we did not want to change the existing ventilation arrangement)
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