Verging on poor build standards

A Quote for £3.5k on Potential Roof Repairs Triggers a Roof Inspection

Failed cement roof verges and undercloak

Failed roof verges are a common problem

We were recently called to a property in Stoke on Trent after a client was quoted £3.5k for substantial roof repairs relating to roof verge failure. This is quite a chunk of money considering that the roof was on a property built in the mid 90’s; surely we can expect a new roof to last longer than 20 years can’t we?

What we found on attending was fairly  representative of the issues we find when inspecting fairly modern roofing installations.

We found substantial failure of the cement verge fillets and resultant moisture damage to both the cement board undercloak and the tannelised roof batten ends.

Missing undercloak and wet rot to batten ends

Missing undercloak and wet rot to batten ends
















Unnecessary Recommendations.

We understand that the roofer had suggested works to replace the sarking membrane but since water ingress by wind driven rain or otherwise, had not been a problem then this was clearly an unnecessary recommendation.  Wind driven rain tends to only affect shallow pitch roofing and since  this roof pitch was circa 45 degrees then we saw no need to spend money addressing a problem that didn’t exist.


On inspecting the roof from a ladder we noted a number of key defects:

  1. Cracking, failure and close of cement verge fillets
  2. Wet rot to the roof batten ends
  3. Damaged and missing cement board undercloak
  4. Sarking membrane not draped into gutter and no alternative eaves flashing detail.

Failure of roof verges is incredibly common and often stems from two underlying problems:

  1. The mortar does not contain the minimum 30% required sharp sand.
  2. The verge tiles have not been bedded on the mortar in one operation.
Eaves flashing only visible in limited areas

Eaves flashing only visible in limited areas

It is a mistake to try and point verges after the tiles are already in place and given the depth of remaining cement verge fillets, we feel that this is precisely what happened. Failed verge fillets are top of the NHBC’s hit list because it is one of their most commonly reported defects on relatively new build properties. They also commonly report a failure to add sharp sand to the roof mortar mix.

Thinking the problem through logically, if the cement verges have failed then can we expect similar problems with a failure to ridge tile mortar beds? In almost all cases, the answer is yes… if they failed to add sharp and to the verge mortar then why should we expect to see it under the ridge tiles! A shot with a zoom camera confirmed evidence of failure to the mortar bed joints.

Failed cement mortar bed

Failed cement mortar bed

You may not know that the fixing requirement has changed for roof ridge tiles under BS5534 and it is no longer allowable for ridge tiles to be simply bedded on mortar; they must also be mechanically fixed now so the repair specification we produced covered this point. The repair specification included for stripping off the large format interlocking concrete tiles at the verges, renewing the fibre cement undercloak and cutting out and replacing tantalised roof batten ends affected by wet rot. Critically, the mortar mix was correctly specified and it was further detailed that verge tiles should be replaced on a fresh bed of mortar and nailed in one operation. Similarly it was specified that ridge tiles should be re-bedded on the correct mortar mix and mechanically fixed to the ridge. We would substantially extend the life of this critical roof detail if roofers understood the critical importance of adding sharp sand to their mortar mix.

Roof verge technical detail

Roof verge technical detail



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  • Tom

    Hi, would cement board fillet undercloaks contain asbestos? House was
    Built in 1995

    • Joe Malone

      Highly unlikely Tom, but Chrysotile was used up until about 2000, You have to assume that any house built before that date may contain some. If it does, it’s very low risk, so long as you don’t drill or sand it.

  • Tom

    Did the cement undercloak on this photo and page contain asbestos ?

    • Luke

      I’ve undertaken sampling of properties built in the mid-late 90s and some of these fillet undercloaks have contained asbestos. As Joe says you should always presume it is present until it is tested.

      • Joe Malone

        Thanks for confirming that Luke.

        Regards. Joe Malone

      • Tom

        Is there any way to tell visually or any clues if it’s asbestos or not? Mine has turned white over a short space of time, almost to the point you can barely see the cement. I contacted an asbestos surveyor and he said its not the normal characteristics of an asbestos undercloak?

  • Robert Evans

    I’m having UPVC roofline fitted in place of the old wood from when the house was built in 1991. I’ve noticed that some of the undercloak has dropped down away from the cement and/or has come loose and is now no longer flush with the end of the verge. Is this the fault of the fitters? Is it something to be concerned about?

    Thanks, Rob

    • Joe Malone

      Hi Rob,

      If it’s the fibre cement verge boards then I doubt its really the fitters fault. Hammering may have worked the boards loose, but then they were probably never properly mechanically fixed anyway.



      • Robert Evans

        Thanks Joe

        They’ve put them back now so they’re flush. There’s still a small gap between some of the undercloak and the bottom of the cement.


  • Tom

    The undercloaks have turned white and I can barely see what’s left of the cement. Is this what Asbestos cement looks like?

  • SHD

    Thanks for this useful article and the mention of the importance of 30% sharp sand
    That said, as a DIY’er, i’m still confused as to what ratio should be used and i have seen 4:1, 3:1:1, using plasterers sand etc etc

    Can you confirm what you believe to be the correct ratio of building sand / sharp sand / cement please ?
    Also, your view on using plastiser in the mixture

    Thanks in advance

    • Joe Malone


      For roofing work, use 1 part cement to 3 parts sand, that would normally be builders sand, but ensure that at least 30% of this sand is sharp sand. This is a British Standard requirement, and even the NHBC quote this mix, though we rarely see it. Or use 50% builders sand and 50% sharp sand, even better, though workability will be reduced.
      Plasticiser is to increase workability, which is fine for mortar mixes used for brickwork, blockwork etc, but I can’t imagine you’d need it for roofing work, verge fillets, chimney flaunchings etc.

      Regards. Joe Malone

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