MortarThePoint

Digging through the Structural Engineer's calculations I see he calculated a requirement of 21.5kN, and specified an IG L1/HD110 lintel with a capacity of 22kN. That was when he was using the wrong cavity size and the equivalent lintel is an IG L1/HD100 with 35kN capacity. ? I've gone with a HT/HD100 with capacity 35kN. 35kN is 159% of 22kN and I calculated that if all the load on the lintel is concentrated on the central 50% (so between 0.45 and 1.35 out of 1.80 span) the moment is 150% higher than if UDL across full span. The actual positions are better than mid span bearing, but this provides a useful worst case presuming ~45degrees of load spreading under the truss bearing. Quite relaxed now. I'll await confirmation from the lintel expert and keep the PCC option up my sleeve if needed.

The load/span table has lower figure than the CE cert so looks like 10.14kN/m --> 18kN. Not sure why the figure is lower. https://www.supremeconcrete.co.uk/media/2620/prestressed-lintels.pdf

It is challenging to say the least. Even more so when doing it for the first time

I'm using HiTherm+ lintels which have reduced cold bridging above windows but are mostly made to order, so not quick to get. The Structural Engineer made a note of the drawings which I didn't understand, but now do, which says avoid multi-truss bearings above lintels. This means avoid things like hip trusses and velux trusses/rafters bearing above the lintel. I've asked the lintel manufacturer to check the situation, but they are being a bit slow. In parallel I want to consider my options to add strength. The spans are 1800mm. Lintel specified is HT/HD100 with capacity of 35kN. First floor, 525mm below top of wallplate, 4 brick courses on outside. I'm not a Structural Engineer, but some logic suggests the following options could help: Upgrade to an XHD lintel (50kN capacity), but there are difficult lead times and circumstances there Add a Box lintel either screwed to the rear flange of the cavity lintel or more easily with a course of coursing blocks between (or just a mortar bed between?). A standard box lintel has capacity of 30kN so should add something useful (i.e. somewhere between 0kN and 30kN :-). I am sure if screwed on it would add its full capacity. With a course of coursing blocks I'd hope it would add at least half its capacity giving 50kN total?? Add a precast concrete (PCC) lintel. R15 2100mm has a load bearing capacity of 22.82kN/m so 1.8m should give a total of 41kN. This could be bedded straight onto the flange of the cavity lintel so I'd expect it should add a fair amount of strength. PCC may have a different allowable deflection at capacity so may be complex working out combined strength. Construct a broken lintel using a box lintel (2100mm standard 30kN, HD 50kN) and a single leaf lintel (e.g. IG L10 2100mm has 10kN). Brickwork only weighs 1kN. Options 2 and 3 are the easiest, but in most scenarios involve laying the lintel of a full lenght bed of mortar. Does that worry anyone? Does anyone have any thoughts as to how the lintel strengths may add together? It's crazy times at the moment and particularly for lintels it seems.

Have a long hard think about what lintels you might need and get then on site ASAP

Thanks Peter. Crazy really, so you die falling from the roof rather than roasting to death due to the fire. The rules are the rules.

Yes I wasn't clear, the challenge is on the other side of the <1100mm cill ?

Pretty much spot on, it adds £550 for 4. 1100mm cill may be a challenge as it's a long way down. off the roof

Don't want to annoy the Structural Engineer so will try to observe the 1200 centres, but have a strap immediately either side of openings and if the opening is >1000mm have 500mm long strap(s) in middle. 1.2 is no more than 2 ?

Structural Engineer has specified 30x5mm straps and their c/c distance, but not their length. Any thoughts on what length to use for wall plate? Obviously above any openings they shouldn't be long enough to reach the opening ?. The top of wall plate is 525 above top of frame so I guess that means max. 500mm straps there. What do people recommend for panels? Other perhaps more daft questions: I presume the strap goes down the inside of the inner leaf rather than down the cavity Are the lengths generally stated as plus or including the bend? My inner leaf is 100mm. If the bent bit is 150mm should it be cut or not bother?

The truss designer is talking about doubling up the whole truss, but I don't understand why the whole truss needs to be doubled rather than just the rafter part. Doubling the rafter can be done on site with the benefit of time whereas doubling the whole truss requires a lot more thought

I was thinking of incorporating Dormer window provisions into a roof truss design to allow for future conversion, but have changed my think on that in large part because it needed too much input at the truss design stage. Another reason is that it made for big gaps between some trusses. I had thought I could simplify matters by using Velux windows instead. It normally seems to be simple to fit Velux windows by doubling up the rafters on each side, adding purlins top and bottom and then cutting out sections of any rafters in the middle. I spoke to the truss designer and he said for attic trusses it wasn't that simple. I don't understand why. Is it because attic trusses have bending moments on their members rather than just the tension and compression loads of a fink truss? Even then I still don't see why it's anything more than doubling up the rafters either side rather than the whole truss.

Looks smart. Is this over a non-enclosed space (e.g. porch)?

I'm considering lead alternatives for the flashing. Perhaps Zinc, but a supplier has quoted to use Wakaflex. Are the rubber type flashings any good? How well do they stand the test of time?

I'm still not convinced I have made the correct decisions with all this, but the roofers have said they can work with it so fingers crossed

@farm boy that looks like a fantastic job.

Surely you have to over bay windows or any enclosed spaces?

Sorry for the delayed responses, I was busy on site achieving nothing ??

In alphabetical order: Donaldson, Pasquill and Truss Form. I've found Pasquill very helpful so am leaning towards them. So 400mm is a thing of the past then?

I've had some truss designs from some suppliers and they have all gone with 600mm centres. As I understand it that's the normal way to go these days. Does anyone go for 400mm centres any more? There is a housing estate near us that has lots of slate roofs and they look bad after 5 - 10 years. Not due to the slate, but looks like the underlying timber has shifted over time. We're going for plain clay tiles, but this has made me think about whether too many savings are being made on trusses. It's interesting to see the sizes of timbers chosen by the suppliers for the top chords (i.e. the actual roof slope). One supplier has designed based on 47x147, another used 47x197 and the third used 47x222. There was more agreement over the bottom chords with the same first one going for 47x197 and the other two going for 47x222. The one with the slighter timber wasn't the cheapest either.

So you end up with a constant with band of lead on the wall at the same slope angle as the roof. What's done with the cavity trays then?

Thanks, do you have a drawing or photo of this as it's difficult for me to get my head around. Do you still do the same thing as in my first post with stepped cavity trays though?

I'm trying to work out where the flashing and cavity trays will sit on a roof abutment. I don't have the rafter to put in place and measure off, but I do have a drawing of the bay window roof exterior surface. I need to work all this out so I can get the positions of the stepped cavity trays correct as they get built in to the wall now. Their corners align with the corners of flashing, so it's lead me to try to understand this (excuse the pun). I can see NHBC recommend at least 85mm (some of their docs have 65mm min) between the roof surface and water line which defines the positions of one set of corners of the flashing. What I can't work out is what sets the position of the other set of corners. Is it just set by the height of courses and drawing a line perpendicular to the roof slope? I've seen something that suggests they sit on a line 150mm from the roof slope. I have attempted to draw this below with the lead shown in grey and the reference lines at 85mm from roof slope and 150mm from roof slope. The red rectangles show coursing and would be the positions of stepped cavity trays. Wider lead flashing or moving it off the roof slope slightly (so up the soakers a bit) would create a tighter angle on the 'saw teeth'. I imagine these must never result in the cut line sloping the other way. Is there a minimum amount of overlap on each step? Finally, can you stop when the lead goes past being vertically above the end of roof slope or is there a minimum amount it has to go past the end of roof slope. In the example below, could I have stopped one brick course higher at the bottom? Lots to understand so thank you if you made it this far.

I'm tempted to go for these which look good. Has anyone used them or have anything to suggest? http://www.glidevale.com/uploads/a241ee6cbc0b69457c89339f2886b778.pdf

I find that often when you are asking merchants about a product and you have to explain what it is, it means you may be trying to do something odd. Neither SIG nor Jewson seem to carry stepped cavity trays which I find strange as don't they get used a lot? We've got various roof abutments that as far as I understand need these: single storey side room that meets main house hipped roof above a bay window porch roof between two returns hipped veranda roof The architect has drawn them on all of these (example below), but I was wondering if they are actually needed for [3] and [4]. The cavity trays make sense to me when the wall below the roof is going to be internal and therefore you can't have weeps lower down. The porch and veranda are open to the outside world below the roof so the cavity trays seem pointless.