Fine capillary gaps where moisture can become trapped need to be avoided as these can allow uncontrolled moisture ingress leading to possible coatings issues and potential wood rot.
All edges (arises) should be rounded to a radius of 3 mm as recommended in the British standard. Cill extensions must have a maximum protrusion of 75mm and machined at such an angle that promotes good water shedding.

All glazing should be to a high standard where the glazing sealant or gasket forms a water tight contact with the glass and timber. It should also have a sufficient angle to shed water away from glazing lines. Internally beaded systems are preferable. Where external beading is used it should be constructed from highly stable timber or using wood plastic composite beading material. We recommend that glazing systems should be drained and vented to reduce the risk of condensation.

Joint adhesive must be applied to all parts of the joint and there must be no gap where moisture ingress can occur. Adhesives with good gap filling qualities are highly recommended. These need to be flexible enough to cope with the natural movement characteristics of the timber. Where a fully filled joint is not achievable the adhesive must conform to the minimum D4 specification as described in BS EN 204.

BS 644 details the requirements for window construction. Refer to this for more information.