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Looking at a Victorian house which would have started life with a slate roof. At some point in time, this has been replaced with concrete tiles. I don't know if the roof timbers were upgraded at the same time to cope with the extra load.
So, assuming the worst case and the timber hasn't been changed, what's the mode of failure? Will it gradually sag and bow with time to catch it or will we wake up one morning to find to roof on the bedroom floor?
How big a deal would it be? Walk away if the price doesn't drop to cover a new roof or is there an acceptable remedial action to consider?
Concrete slabs and you think you'll wake up with them over the bedroom?
It'll sag first. If it's been standing for it a little while I'd be incredibly surprised if you had sudden catastrophic failure.
One option could be to get the seller to take out insurance for future failure, or maybe go 50/50 on a structural evaluation?
Um, isn't this the point of getting a survey done but you can direct them with a specific query so they can't come back with the "it had a roof but we couldn't check anything else"?
That's very common in my Victorian suburb. I've not noticed any houses with their roofs caved in.
Additional collars can be fixed to existing rafters to help prevent more sagging but I would guess that all movement has already occurred and the roof would be quite stable. As has already been mentioned, this happened to lots of roofs and most are still standing.
We replaced old slate roof (house built 1890) with a new dry verge concrete tile. Must admit I was worried for the same reason as op-but when I queried this with the roofer (very well respected in the area/Victorian pocket of Glasgow) he gave me 4 old slates (I.e the same surface area as 1 tile) and I was surprised just how thick/heavy the original slate is. I'd be amazed if the new tiled roof is significantly heavier at all. I'd get into the loft and check the rafters/sarking etc aren't suffering from any damp/rot, if not then I expect you've nothing to worry about.
Well, surveyor said there was no sign of trouble in the loft other than a bit of woodworm, but has recommended we get a structural engineering to inspect and calculate.
I hate house buying and selling.
Coverage of concrete pan tiles is bigger and slate is heavy.
Slate is circa 100kg/m2 and concrete inter-locking tiles are 75kg/ m2
this can vary with different laps etc, but you shouldn't have a problem
Interesting comments guys. I'd assumed the concrete tiles would be a greater static load, not because of any knowledge about the materials but because the internet throws it up as a common cause for concern.
I'm currently renovating a victorian school into houses. The roof space is vast, in fact £60k vast to reslate! There has been some deflection caused by the original load but 95% of the roof timbers have been left as there is no cause for concern. The 5% replaced is down to water ingress and subsequent rot. Blurtys stats above seem a little bit over re the slate in my book. As I reckon if I stripped a m2 of roof there is no way the subsequent slates would weigh a 100 kilo.
We had this, all that was required (and we never did) was some vertical bracing under the Purlins (joists that run under the rafters at waist level.
Never bothered as the roof was dead level and mid terrace so fairly small.
We did sort out the purlin that didn't reach the gable wall and was wedged over a block of wood on the chimney breast 🙂
All slate is different though. New Spanish slate will more than likely be lighter than its concrete tile equivalent. However I imagine your existing slate roof is much better quality, probably Welsh so it's going to be heavy. Ours were an inch thick in places!
Most slates are 20" x 10" (400mm x 200mm), and given a typical battening gauge of 8" (200mm) this results in a 3" (75mm) headlap. To cover 1m2 you would need 20-25 slates depending on the gauge. A single slate would weigh around 1-1.5kg, so for 1m2 this would equate to 20-45kg per m2.
This is helpful: