A guide to Cob and Lime Maintenance

Cob and Lime Maintenance

We talk a lot about restoring cob and lime buildings on this site – probably because by the time Earthouse has received a phone call, there is already a problem! In this blog we are going to talk about maintaining your building so that you do not get to this point.

Unfortunately, if you can see the problem, it generally means there is something underlying. In the worst cases we have seen, this has been severe damage due to complete deterioration of the cob structure. The most common reason customers contact us is to say that there is a crack in their plaster, or a damp patch has appeared. When we hear this, we know that the issue will be that a cob or stone wall has been rendered using modern cementitious render. Modern cementitious renders have their place; on modern buildings with a damp course and effective air circulation systems. They were never meant to be used on cob or stone walls and consequently cause problems.

Water damage to decorative plaster

Water damage to decorative plaster

The damage is almost always caused by water ingress. Modern cementitious renders are waterproof; this means that if water somehow gets in it cannot escape, so it remains within the structure and moisture builds up over time. The frost and thaw process aggravates this deterioration, resulting in the cob degrading into a topsoil-like substance.

However, cob buildings if treated correctly can last for hundreds or even thousands of years; as long as you are prepared to spend money putting right what previous generations have got wrong, there is no reason your building cannot last for a similar timespan. In addition to this, conservation officers will no longer allow you to render cob in sand and cement, as this does not conserve the integrity of the building.

Cob and Lime Specialists

As a cob and lime specialist, if we can see cracking or damp on your walls, we will assume that there could be issues with the underlying cob. They may also say that they cannot diagnose the extent of any damage without being able to see beneath the plaster. As a guide, if you have excessive or large visible cracks to the render, you should expect a number of cob repairs.

Whilst it is straightforward to give a cost for lime rendering per square metre, it can be difficult to provide a quotation for remedial cob work. Generally Earthouse advises customers to allow a 15-20% contingency for cob repairs, although this may differ depending on the situation. There is no steadfast way to deduce the state of the cob without taking the render off; we have worked on properties where the render looks fine, but the cob is really badly affected underneath. Conversely we have taken off badly damaged render expecting the worst, only to find that minimal cob repairs were required.

Structural repair to cob wall

This shows an extreme example where the cob structure had been undermined at the base of the wall and was subsequently repaired by Earthouse

How to avoid cob deterioration

If you have a cob property there are things that you can do to maintain the integrity of your building.

Cob cottage with cementitious render removed

Cob cottage with cementitious render removed

  • Prior to buying a cob property, you might consider asking a cob and lime specialist to accompany you on a site visit. If the property has been rendered in anything other than lime it may be that you can negotiate on the price, as ideally you would re-render the house.
  • If you suspect a cob property has been plastered in cementitious materials, ask the surveyor to confirm the render material and the paint used.
  • If your house is cob, always consult a builder that has specialist knowledge of cob and lime render.
  • Remove aggressive creeping plants at the earliest opportunity as they can damage structures. Be aware that any plant or tree that touches the building can affect the render negatively, particularly if it is windy.
  • Damaged or blocked guttering is probably the most common cause of water ingress; check your gutters and ensure they are cleared regularly.
  • Thatched houses tend not to have guttering; ensure there is an alternative way to drain excess water away from the house, for example via a French drain soakaway.
  • If you have anything other than lime render on your walls we would suggest that you remove this internally and externally. If you have a cob property, this will then need to be re-rendered in lime based products. Stone or brick properties without a damp course will need to be treated with lime based mortars throughout.
  • If removing the render internally and externally throughout the house is not an affordable option, it may be that there are key areas that you could restore to prevent further damage.
  • Use the correct decoration on walls that have already been lime rendered. We recommend lime wash or you could choose one of the excellent silica based paints internally. Externally you also have the option of using a casein distemper or a clay based paint. Please see our blog Lime Wash vs Silica Paint for more detail.
  • If you decide to use lime wash, bear in mind that you will need to re-decorate within 3-5 years to maintain your aesthetics.

Finally if you are concerned about your property, please contact us. We are able to provide free advice about the best way to approach any remedial or preventative work.

Threshing Barn Conversion – Central Devon

The Threshing Barn

Cob Barn Conversion Project

In May 2016 we commenced work on a cob barn conversion. The old threshing barn is located rurally, in the heart of Devon’s countryside. When we began work the building was derelict. The tin roof was failing, thereby allowing water to seep into the cob walls, causing more damage with each soaking; if our customer had not rescued the barn, it would have slowly deteriorated until it crumbled completely.

Crumbling exterior of the cob barn prior to restoration - Earthouse Building Conservation Ltd

Crumbling exterior of the cob barn prior to restoration

So commenced the project to convert the barn into a traditional living space with a modern twist. The plans allowed for a three bedroom barn conversion of the main barn with a modern extension and an attached thatched linhay.

The end result will encompass a mixture of old and new, combining traditional materials such as cob, and specialist lime rendering techniques. The use of glass and metal will create a modern contemporary feel with careful consideration to the surrounding environment.

With a project such as this one there are many elements involved. We work closely with the customer, architect, and various subcontractors on each part of the project.

The threshing barn covered by a temporary roof.

Threshing Barn covered by temporary roof

Removal and repair

The first job was to remove the superfluous tin roof. This in itself an easy job; the repair to the damage its inefficacy had caused, more demanding! The cob walls required extensive repairs. We commenced work on the south facing wall and worked our way clockwise around the building, removing any inadequate cob and replacing and rebuilding with cob blocks and earth mortar. Overall the work took around six months to complete.

The commencement of cob repairs to the walls in the main barn

The beginning of cob repairs to the walls in the main barn

External wall repair using cob blocks

External wall repair using cob blocks

The roof

After we completed the main barn’s cob walls, the oak wall plate was fitted. Four oak trusses, hips and purlins then soft wood rafters were put in place, with the insulation installed between. The insulated felt was overlaid and battens made ready for the first slates to be laid.

The slate used for this project was a natural slate called Glendyne. We used a local company, Kilbride Roofing Ltd, for this element of the project.

Oak trusses in the roof of the main barn

Oak trusses in the roof of the main barn

Partially laid slate roof

Partially laid slate roof

Following the completion of the slate roof, the soffits, fascias, guttering and downpipe details were added.

Slate roof with guttering in place

Slate roof with guttering in place

External Lime rendering

Whilst there was activity on the roof, Earthouse was busy rendering the exterior with a traditional lime render. Lime render allows cob to breathe, allowing moisture to travel in and out of the cob without damaging it. An insulated lime render was used for the first three coats.

External lime render on the barn conversion

External lime render on the barn conversion

Windows

The main barn has two windows of note. Both are floor to roof, a height of 4.5 metres each. The window frame is made from Douglas pine, sandwiched between 80mm of oak either side. Aluminium windows will be fitted.

Window being installed in the main barn

Window being installed in the main barn

Floors

Next was the excavation of the interior floors. Then came the laying of insulation (glapor) and the installation of underfloor heating pipe. The main barn has a cut back pumice limecrete screed, not dissimilar to polished concrete, but with different shades of grey and white tones and a matt finish giving a more subtle look.

Limecrete pour finished

Limecrete pour finished

A closer look at the limecrete floor in the main barn

A closer look at the limecrete floor in the main barn

Modern Extension

The floor in the extension is a polished concrete, which has a beautiful shine when sealed and is more in keeping with the modern element of this part of the building. Again, the concrete is warmed by underfloor heating.

Concrete floor just laid in the extension

Concrete floor just laid in the extension

The extension itself is a single storey lean-to attached to the north and east elevations of the main barn. The external walls and roof are ultra-modern dark grey crimped zinc. We are seeing so many buildings using crimped zinc now.

Commencement of the contemporary extension

Commencement of the contemporary extension

The extension with a crimped zinc roof and cladding and large glass sliding doors

The extension with a crimped zinc roof and cladding and large glass sliding doors

The extension spans two walls of the main barn conversion. It is 50% glass, comprising both doors and windows supplied by Aspect Windows Ltd in Exeter. The photo below shows the far end of the barn conversion, with the extension; these rooms are ensuite bedrooms.

Inside the Threshing Barn

The interior of the barn on completion is large and light. The main living space is vaulted double height, overlooked by a timber mezzanine floor.

Work in progress in the interior of the barn conversion

Work in progress in the interior of  the barn conversion

Upon the mezzanine is the office. Below the mezzanine floor lays a hallway leading to bedroom 3 and the staircase leading up to the floor above. There is also a passageway leading to the ensuite bedrooms 1 and 2 in the extension.

The mezzanine wall in place in the main barn

The mezzanine wall in place in the main barn

Upon completion, the mezzanine wall and timber was painted with Farrow & Ball paint in many different shades of grey. The majority of the lighting is from a fantastic shop in Exeter, Amos Lighting, which we highly recommend.

The woodburner and flue was supplied and installed by Woodwarm near Cullompton, who offer a wide range of contemporary and highly economic woodburners.

The almost completed mezzanine in the main barn conversion

The almost completed mezzanine in the main barn conversion

Wild complications

No project is ever without its own unique set of complications and whilst at Earthouse we are lucky to work in some beautifully remote locations and we regularly see fantastic wildlife, our barn owl invasion was by far the most spectacular.

Midway through the cob repair process of the walls in the main barn, we came across a nest in a hole in the cob containing two young barn owls. We were first alerted to their presence by the malodourous smell, presumably from the owls eating and digesting numerous small creatures. (We had a good rummage through some of the furry grey pellets they had discarded at a much later date with a vet friend of ours who identified any number of tiny mouse and bird skulls and bones!)

The young owlets were snuggled up in the wall which was in the worst state and without attention this wall could crumble, crushing the birds.

Barn Owl chicks nesting in a cob wall

Barn Owl chicks nesting in a cob wall

Barn Owls are a protected species which we were well aware of and we sought immediate advice from The Barn Owl Trust. In order not to disturb the creatures we stopped work until the Barn Owl Trust had visited site and specified where and what we could continue. A temporary owl box was erected closely in the vicinity of their original nest and they quickly took to this, the droppings making their use of it evident.

Many people would be disgruntled by owls stopping work on their site and to a degree it was frustrating. However, the owls enchanted us and the owner, who now has a fantastic extension of a large owl box which the owls began using almost immediately, obviously not concerned by the continuing building works going on around them.

Permanent owl nesting box built into the roof of the house

Permanent owl nesting box built into the roof of the house

Another view of the barn owl nesting box at the top of the threshing barn

Another view of the barn owl nesting box at the top of the threshing barn

Before and after – a drastic restoration project!

The final parts of the project are always vital; the construction is obviously key, but these are the bits that the customer living in their home will see. As I write, the site is once again a building site as work commences on The Linhay, so I don’t have the beautiful landscaped photographs of the finished barn quite yet. Below you can see the stark contrast of the barn when we first started working on it, and the newly lime rendered conversion and extension in the wonderful Devon sunshine.

Interior of cob barn prior to restoration - Earthouse Building Conservation Ltd

Interior of cob barn prior to restoration

Crumbling exterior of the cob barn prior to restoration - Earthouse Building Conservation Ltd

Crumbling exterior of the cob barn prior to restoration

The barn looking stunning in the sunshine with a tree-lined backdrop

The barn looking stunning in the sunshine with a tree-lined backdrop

View of the barn conversion and extension

View of the barn conversion and extension

The owners spent Christmas in their beautiful barn conversion and we hope to have more interior and exterior photos to show you very soon.

If you would like to know more about the work we do, or to discuss your barn conversion project, please feel free to contact us.

To be continued…The Thatched Linhay

Brithem Bottom – Lime Render

Lime rendering process

Brithem had substantial cob repairs and a large number of structural timbers replaced including the inglenook beam. The final touches were the re-rendering and lime washing of the property.

Lime render removal at Brithem Bottom

Lime render removal at Brithem Bottom

In this photo, we have begun to remove the old sand cement render.

Scaffold erected ready for lime rendering

Scaffold erected ready for lime rendering

This shows the property with old sand cement render removed, ready for repair to the exposed cob and once this has been completed, for lime rendering.

Brithem Bottom - completed lime render

Brithem Bottom – completed lime render

The completed external works on the property with a lime render finish.

Internal timber renovation and lime rendering

This job also included the replacement of windows and extensive internal repair work.

Fire place prior to restoration. New oak lintel recently installed.

Fire place prior to restoration. New oak lintel recently installed.

Fireplace at Brithem Bottom after restoration.

Fireplace at Brithem Bottom after restoration.

We were really pleased to receive the following testimonial form this customer:

‘Earthouse do what they say they are going to do, in the time scale and do a great job. Sean Parker, who heads it up is straightforward, and knows what he’s doing with a wealth of knowledge and experience. He really understands what is required, especially with period buildings, listings etc. and how to do the work without the faff so often found amongst so called experts. I’ve been involved in doing up old buildings for years and Sean really is up there with skill and plain hard work. There is a great team around him, and all good to be around – watching them work was a real learning curve for me around lime render, lime wash, and the cob in our very old thatched cottage which really did need expert hands . I trusted them and always felt confident, and this trust wasn’t in any way misplaced. Highly recommended!’ Steve Jamison, Brithem Bottom