resource hub | Using oak in construction

Using oak in construction

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Introduction

 

For the purpose of our warranty there are three types of oak we are considering:

 

  • Green oak – recently felled oak with a moisture content typically between 60%-80%
  • Air dried (seasoned) oak – naturally stored oak with a natural seasoning process moisture content up to 30%
  • Certified kiln dried oak – processed seasoned timber with a moisture content of 12% or less.

 

Identifying green, air dried (seasoned) or certified kiln dried oak

 

Where timber has been dried and has a moisture content less than 12%, it will usually be marked with a stamp saying “DRY” or “KD” (for kiln dried), however, if timber has been cut or machined, it is likely these stamps are not visible, in such circumstances, proof of purchase confirming that timber is kiln dried should be provided.

 

The purpose of this article

 

This article identifies in the following tables, various ‘environments’ where ‘oak’ could be used on project covered by our warranty. The tables identify the ‘oak type’ and the applicable insurance policy. A simple ‘yes’ or ‘no’ indicates whether oak can be accepted for our warranty purposes. Additional guidance notes (where applicable) help clarify our insurance stance.

 

Whilst the Technical Manual is clear on the use of green oak and nay new orders for project are received with the understanding oak will not be acceptable in most circumstances, there may be a situation where a change in specification occurs post offer of warranty or it may not have been disclosed at the time of application that oak is intended. For these situations, surveyors and developers must be aware of the following ‘environments’ where oak is not acceptable.

 

Abbreviations:   NH – New Homes policy    SH – Social Housing policy   PRS – Private Rental Scheme

 

Environment 1: 'Through wall'* oak framed construction forming part of the waterproof envelope 

 

Oak environment 1

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

Note: *‘Through wall’ timber construction, of ANY species is NOT acceptable for warranty purposes

 

 

Environment 2: Internal frames not forming part of the waterproof envelope including. Floor beams, floor joists, trusses and purlins, where any movement in the oak WILL NOT affect the waterproof envelope** 

 

Oak beams

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried (max 12% moisture content)

Yes

Yes

Yes

Yes

Yes

 

Note: **Providing the structural engineer proves that any shrinkage/twisting in the oak will be catered for and be isolated from the waterproof envelope.

 

 

Environment 3: 'Open' canopies

 

Open canopies - yes

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

Yes1,2

Yes1,2

Yes1,2

Yes1,2

Yes1,2

Kiln dried

Yes1,2

Yes1,2

Yes1,2

Yes1,2

Yes1,2

 

Open canopies - no

Note: 
1 The canopy must not exceed 3m2 and should be structurally independent from the dwelling with no timber element built into the external walls, see top image.
2 The canopy roof must not link with the roof over the main property as shown in bottom image.

 

 

Environment 4: 'Enclosed' porches 

 

Oak: enclosed porch

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

Note: This applies to a porch being built prior to completion of the house (i.e. part of the housing unit)

 

 

Environment 5: Structural oak embedded into the ground

 

Buried post in concrete

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

 

Environment 6: External windows and doors

 

Oak window

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

Yes1

Yes1

Yes1

Yes1

Yes1

 

Note: 
1 The requirements in Section 8 of the Technical Manual must be met (including that of weathertightness)

 

 

Environment 7: Second fix joinery (internal) including floor boarding, skirting boards, internal doors, stairs

 

Wood floor

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

Yes

Yes

Yes

Yes

Yes

 

Note: 
Reclaimed oak will be acceptable where it is proven to have a moisture content below 20%

 

 

Environment 8: ‘Attached garages’ constructed of ‘oak’

 

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

 

Environment 9: Any detached garages

 

Garage in oak

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

Subject to approval

Subject to approval

Subject to approval

Subject to approval

Subject to approval

Air dried

Subject to approval

Subject to approval

Subject to approval

Subject to approval

Subject to approval

Kiln dried

Subject to approval

Subject to approval

Subject to approval

Subject to approval

Subject to approval

 

Note: 
Oak is not acceptable for use outside of the guidance provided in this document.

An exclusion may be applied to the policy, where oak is incorporated into a detached garage, subject to our agreement. Such an exclusion may only be applicable to the detached garage and cannot be applied to the home.

If an exclusion is approved:

A. The developer must be made aware, from the offset of our concerns over the use of oak and the potential impact on the sales and future sales prices that this exclusion of the detached garage from our warranty cover this may have

B. We will require written confirmation of the policyholder’s acceptance of the exclusion of the detached garage from the policy, prior to sign off.

C. The developer will need to understand, if a purchaser is unwilling to accept the exclusion being applied to the policy for the detached garage which contains oak, we will be unable to offer warranty cover for the property irrespective of the stage of build.

D. The warranty surveyor will not sign off a property containing an oak detached garage until we have received the developer’s/policyholder’s acceptance and we have confirmed in writing that the policy has been endorsed to exclude the oak detached garage

 

 

Environment 10: External cladding

 

Oak cladding

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried (max 16% moisture content)

Yes1

Yes1

Yes1

Yes1

Yes1

Kiln dried

Yes1

Yes1

Yes1

Yes1

Yes1

 

Note: 
1 For cladding purposes:

- The oak must be installed with a moisture content around 16%.
- The developer must take into consideration movement and potential shrinkage; ensure fixings are compatible with oak: i.e. stainless steel. Substrates and drained and vented cavities must comply with the Technical Manual.
- A second line of defence must be provided by the provision of a suitable breather membrane to protect the inner structural frame (as well as the drained and vented cavity).
- Further guidance can be found in the TRADA third edition ‘External Timber Cladding’.

 

 

Environment 11: Mock Tudor cladding (decorative cladding)

 

Mock Tudor cladding

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried (max 16% moisture content)

Yes2

Yes2

Yes2

Yes2

Yes2

Kiln dried

Yes1

Yes1

Yes1

Yes1

Yes1

 

Note: 
1 The developer should:
• fix cladding onto a continuous render substrate (not render infill between timbers)
• timber should be fixed using stainless steel screws that are at least 3 times the depth of the timber and appropriate for the substrate.
• Substrates should comply with Section 6 of the warranty Technical Manual.

NB. A further Technical Update is available on mock Tudor cladding.

2 Kiln dried is the preferred option in order to reduce the potential for curling.

 

 

Environment 12: External embedded oak lintels (structural or non-structural. With or without relieving lintels)

 

Lintel

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried 

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

Note: 
- Concrete or steel lintels are appropriate for use in masonry walls. Support for masonry should not be provided by timber lintels, regardless of the timber species.
- Decorative timber cladding may be acceptable, to provide a mock timber lintel/beam effect. The mock timber must be planted onto the face of the wall and not embedded into the wall. In this situation please refer to the guidance above in Environment 11.

 

 

Environment 13: External timber balconies (including oak) more than 2.4m above the external ground level

 

Oak balcony

 

NH & SH

PRS

Self-build

Completed

Commercial

Green

No

No

No

No

No

Air dried 

No

No

No

No

No

Kiln dried

No

No

No

No

No

 

Note: 
* Balcony construction, of ANY timber species is NOT acceptable for warranty purposes where the balcony is more than 2.4m above the external ground level.

 


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Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.