Frequently Asked Questions

Easy Guides & Technical Questions

I’m looking to build a new extension. I would like some technical advice on what I need to do?

Read our Easy Guide Download

I’m looking to convert my garage to a habitable room. What do I need to do?

Read our Easy Guide which may help Download

I am looking to convert my loft to a habitable space what do I need to do?

Read our Easy Guide which may help Download

What can I do if the glazing in my extension exceeds to allowed 25%?

The glazing of any windows, doors and roof lights to an extension should be limited to 25% of the extension floor area (plus and doors/windows being built over)

The 25% rule is set by the government and is aimed at reducing global warming.

You can exceed the 25% rule if you demonstrate the increased glazed area does not increase the energy loss above that of a 25% or less glazed building (notional compliant extension). This can be achieved through compensatory measures such as increasing the insulation elsewhere in your home or even upgrading your boiler to a newer / more efficient model.

There are two ways to do this.

  • Employ the services of an energy assessor to provide a whole dwelling calculation (SAP), this may give you greater flexibility.
  • Providing an area weighted U Value calculation.

How much and what insulation do I need for my extension?

The amount of insulation is dependent upon the particular element (ie roof, wall, floor) and the u-value required.

Click on the LABC u-value guide download which gives you more information Download

What is the difference between a warm roof and a cold roof?

There are two ways to insulate a roof see our Warm / Cold Roof Easy Guide for more detailsDownload


What size timbers do I need for my rafters or joists?

The size of rafters or joists will depend on several factors including the span and spacing of the timbers. The following is a guide for the most common sizes.



Joist Span(m) Floor Joist(mm) Ceiling Joist  (mm) Pitch Rafters 22.5o – 30o (mm) Flat Roof (max 10o pitch) (mm)
2 47×120 @ 600c/c  38×120 @ 400c/c 38×125 @   600c/c 38×120 @ 400c/c
2.5 47×145 @   600c/c 38×145 @ 400c/c 38×150 @   600c/c 38×145 @    400cc
3 47×170 @  400c/c 47×170 @ 600c/c 47×150 @   400c/c 47×170 @    600c/c
3.5 47×195 @   400c/c 47×170 @ 400c/c 47×195 @    600c/c 47×195 @    600c/c
4 47×220 @  400c/c 47×195 @ 400c/c Purlin needed 47×195 @   400c/c
4.5 75×195 @   400c/c Binder Needed Purlin needed 47×220 @  400c/c
5 75×220 @  400c/c Binder Needed Purlin needed 75×220 @ 400c/c

If timber sizes to be used or the pitch of the roof differs to that indicated then further professional advice should be sort.

How do I calculate the size of a soakaway required for my extension?

There are generally two types of soakaway, either a crate system or rubble filled.  Below is information on how to calculate the size required for each.



Where ground has good soakage (such as sand/gravel) then the size/volume of a crate type soakaway can be calculated as follows:

Volume = Roof area being drained x (50mm rainfall rate per hr/3000).

For example, a 50m2 roof area would require a crate soakaway of

50m2x(50/3000) = 0.84m3



These soakaways are normally filled with clean brick/block/concrete pieces no larger than 150mm.  It can be assumed that the voids left between the rubble equates to about 30%.

To calculate the size/volume of a rubble filled soakaway, the same calculation can be used but the result divided by 0.3.

For example, 50m2 roof area requires a rubble filled soakaway of (50m2x(50/3000))/0.3 = 2.8m3



All soakaways must be sited at least 5m from any building or road and preferably at least 2.5m from any boundary.


For larger areas to be drained or where ground soakage is not good, a soakage test and more detailed calculations should be carried out and further guidance can be found in Approved Document H and BRE Digest 365

I need some advice on the size and position of escape windows

Please refer to our technical guide which can be downloaded by clicking this button  Download