Problems with roof structure

[xdrive]

From survey report

Roof structure
thorough examination of roof timbers was hampered by insulating
materials.
The roof structure is of traditional timber, purlin and rafter construction.
The roof is underfelted. The underfelting appears to be in adequate
condition where visible; however, it may be the case that once the roof has
been cleared, repairs become more apparent.
Constructional detailing and materials utilised throughout, where
accessible, were noted as free from serious defects.
While constructional detailing to the roof structure generally appears
satisfactory, it is advised that long term stability be improved by the
introduction of ties between rafter feet and adjacent ceiling joists on an
alternate basis.
Roof triangulation generally appears to be inadequate and you are advised
that this should be upgraded in accordance with a schedule prepared by a
suitably qualified specialist contractor.
Some splinter/shrinkage cracking was noted to roof timbers, although this
is not considered to be of structural significance.

Roof
The main roof is of traditional pitched and hipped construction, on a timber
frame, faced in interlocking concrete tiles.
The roof coverings do not appear to be original and are imposing an
additional load on the structure which has led to “dishing”. There does not
appear to be any immediate cause for concern, however we cannot rule out
the possible need for strengthening works in the future.
The roof generally appears to be in adequate condition, based on our
ground level inspection only. It is likely that a closer inspection will reveal
some minor maintenance matters. As such you are advised, in the course
of normal maintenance, that a more detailed inspection of the roof be
undertaken, and any evident repairs should be carried out at that stage.

 

[Harry Bloomfield]

Tieing rafters to ceiling joists is always done where a triangle frame is used for the roof. The ceiling joists will be under tension. Some house roofs were built with an long and very large timber horizontally, side to side, halfway down the roof's slope, with an extra support halfway forming a V terminating at the base on a supporting wall. My semi is like that, the large timber is around 12" x 8" in cross-section (one front and one rear), they sit on the end wall and the party wall.

If the surveyor guy can see 'dishing' in the roof, you should also be able to see it too.

 

[Notch7]

I have done a bit of reading and what he suggested to tie rafters to ceiling joists would be to prevent roof spread which could potentially happen because of the extra load of concrete tiles

I thought ceiling joists were normally tied to rafters anyway to resist spreading, at least in a cut n pitch jobbie.

Is it possible to add a bit of additional strengthening to prevent any further sag? Even if the existing sag cant be corrected with tiles in situ -at least I guess thats what you want to know.

I guess it all depends on the existing structure, but Id be surprised if it cant be done.

 

[noseall]

Joists are spiked to the rafter ends and wall plate to prevent spread. However the suggestion I thought was to connect the rafter to the joist (somewhere mid span-ish) to help counter sag?

Silly idea if this was put forward.

...unless the roof was converted and there are some beefy joists or some midspan support walls.

 

[^woody^]

That surveyor is arse-covering, and not giving advice based on actual evidence.

It's a purlin roof, his statements about strengthening rafters makes no sense. Transferring roof loads to ceiling joists tends to cause continual cracked ceilings too.

 

[blup]

If you are going to continue with the purchase, you will need a structural engineer or other specialist to carry out a more detailed survey - with estimated costings for any works needed. If you are buying with a mortgage it is likely the lender would want that assurance in any event.

It is then a question of negotiating the price down - or walking away.

Blup

 

[datarebal]

pictures suggest original roof was concrete plain tiles, new roof marley modern or redland mini stonewolds
therefore the new roof per m2 will weigh less than the original.

 

[blup]

The survey extract says:

"...it is advised that long term stability be improved by the introduction of ties between rafter feet and adjacent ceiling joists on an
alternate basis".

That suggests the suggests the rafter feet are not tied to the ends of the joist. So what are the implications for the integrity of the roof structure, if the timbers are otherwise properly sized and fixed to the wall plate?

Blup

 

[Harry Bloomfield]

Unless the attempts of the roof weight to spread at the walls is restricted by the loft floor timbers - the base of the triangle, then it will push the walls outwards. I don't believe for one minute that could possibly be the case.

 

[xdrive]

some better quality photos. The sagging is mainly noticeable on the rear of the roof.

The bow is more apparent in this photo. It is very noticeable in person.

noticed this crack on the side top window. Is this because of roof spread?

the downpipe joining the gutter is straight at the top. Don't know if this is normal. Same on the side also.

roof gutter on the side is wonky. Also the plastic soffits are coming away from the property in various positions mainly side and back. Front seems ok.

What do you guys think? Cheers

 

[Harry Bloomfield]

Those are later replacement gutters, fall pipes and soffits - that is just poor workmanship by whoever installed them. Both fall pipes appear to have simply been forced to spring into place, but difficult to do such a short dog-leg.

I can see the apex of each side of the roof seeming to droop down at either side from the shared chimney. I can also see what appears to be some dishing of the flat slope of the roof of both sides of the semi, worst below the Velux. The centre of the roof coming down from the chimney (over the party wall) seems straight, as if that part is very well supported. It all looks straight enough at the eaves.

I'm not sure what to make of the crack to the left of the window, but I doubt it is due to roof spread - it seems the crack is in the wrong direction for spread, unless the hipped part is spreading out.

Best get a professional opinion on it I think!

 

[23vc]

I used to have a house with a similar roof, ie hipped with purlins, and a structural engineer who was round there for something else once remarked that I should put some vertical posts connecting the purlin to an internal wall directly below it, as “these type of roofs have a habit of spreading/moving”. I never bothered tho. The size of the nails and timbers those guys used in the 30s looks OK to me.

 

[Leofric]

Don't see how anybody on here can guess what needs doing without knowing the details of the roof construction . Only a proper 'structural ' survey and report would tell you what needs doing to make the roof structurally stable.

 

[xdrive]

Would it be worth calling out local roofer to assess bow and roof shape and cost remedial work?

Will be calling surveyor tomorrow to see if he can clear a few things up.

 

[Harry Bloomfield]

Would it be worth calling out local roofer to assess bow and roof shape and cost remedial work?

Probably not, they will not know much about actual roof structure. Your surveyor will likely just repeat what he has already said.

Could you perhaps take some photos of the internal roof structure?