Technical roofing consultant John Mercer continues his series of articles focusing on best practice when it comes to pitched roofing projects, this month focusing on the key points and options to consider when constructing bonding gutters.
In my previous article I discussed the potential for disputes to arise when stripping and re-tiling a roof adjoining an adjacent roof. In this article, I will examine the various technical options for successfully constructing bonding gutters.
To recap; what is a bonding gutter and why is it necessary? There are various forms of bonding gutter, but essentially they are all used for the same purpose, i.e. to weather a junction between differing or unlike roof coverings; for example, between double lapped plain tiles and single lap tiles. The most common situation is where an old roof is replaced on a semi-detached or terrace house and the junction between the new roof and an existing, neighbouring roof needs to be weathered – i.e. sealed.
Perhaps the most simple way of weathering the junction between roof coverings is to finish the new tiling close to the neighbouring tiling and then mortar-bed some form of covering over the joint; for example, paving stones, copings or even a row of tiles. Although I see this quite commonly done, it is rarely successful or adequate. The integrity of the joint relies on the mortar bedding, so even if it is waterproof to start with, it will inevitably start to leak as soon as any small cracks appear in the mortar bed.
The correct way to weather the junction is to install some form of gutter, or channel, between the different tiling. This can be formed from lead or be a manufactured, pre-formed product.
Illustration no. 1 (see above) shows a typical bonding gutter detail using a lead-lined drainage channel at the junction between the two roof coverings. In this detail, the lead lining carries over vertical battens, finishing in welts at each side. By limiting the gap between the two roof coverings to no more than 15mm, there is no need to use mortar to close the gaps between the tiles to prevent access to birds and rodents.
Another option is to use a preformed bonding gutter lining with water bars either side to direct water towards the gutter. Some of these have an open channel for the water, with the tiles mortar-bedded onto each side, or as is the case in illustration no. 2, the edge tiles are bedded onto the centre of the gutter.
An increasingly popular form of bonding gutter is a pre-formed one with a central upstand. The advantage of this type is that there is no need for any mortar bedding. The tiles at either side are finished close to the upstand to close the gap between the tiles – see illustration no. 3.
When installing the tiles abutting the bonding gutter it may be necessary to remove tile nibs to enable the tiles to sit close over the bonding gutter upstands. It is also a good idea to use screws to secure the edge tiles, making sure the fixings do not penetrate the gutter lining, of course. Screws make it easier to pull the tiles closer onto the battens, further reducing the amount of possible ‘kicking’ over the gutter upstands.
Remember that if the tiles or slates are laid cross bonded, it will be necessary to either cut half tiles in the neighbouring tiles or slates, or obtain tile and half or slate and half widths.
Assuming that the bonding gutter is to be installed over the party wall, it is important that the fire stopping between the adjoining properties remains uncompromised or be installed if not present. Firestopping material must be installed tightly packed and compressed under the bonding gutter and between the battens directly over the party wall.
The preformed gutter linings shown in drawings 1 and 2 can be installed directly over the tile battens. This means that, although the bonding gutter is normally installed centrally over the party wall, it does not have to be. I reiterate the advice I gave in my previous article; bonding gutters can often be the cause of disputes between neighbours, and my strong advice to anyone about to install a new roof adjoining a neighbour’s roof is to firstly talk to the neighbour to explain and agree the installation process.
This is important because half of the bonding gutter will be on the neighbour’s side of the roof. If agreement is not possible, the alternative option is to position the bonding gutter so that it is wholly on the side of the roof being renewed and there is no encroachment onto the neighbour’s side.
This is particularly easy to do for bonding gutters that sit over the tile battens, such as those shown in illustrations 2 and 3, as there is no need for any additional structure support for the bonding gutter lining.
- Bonding gutters form a weathertight junction between differing roof tiles or slates.
- Ensure all tiles or slates fitted directly over the bonding gutter weather channels are securely fixed – ideally use screws, but take care not to penetrate the gutter lining.
- Where installing over a party wall, ensure the firestopping is present and not compromised.
- Try to reach agreement with the neighbours before starting work. If that is not possible, install the bonding gutter wholly on the roof being renewed.
Read John’s previous article on bonding gutters here.