Understanding Roof Underlay and Why It Matters

Q: Does breathable underlay mean vents are not needed?

No. Vapour-permeable membranes assist moisture movement but do not replace ventilation. We design eaves, ridge or tile vent solutions to current guidance.

Q: Can you replace underlay without changing tiles?

Yes. If tiles or slates are sound we can strip, replace underlay and battens, then refit coverings and swap only damaged units.

Q: Will I need Building Control approval?

If more than 25% of the roof is replaced Building Control is typically required and insulation upgrades may apply. We manage the process for you.

Maxwells Roofing is a trusted, family-owned roofing company in Milton Keynes, led by owner Luke Maxwell. We are highly reviewed with 80+ 5-star ratings on Checkatrade and a 5-star record on Google Business. From urgent leak repairs to full re-roofs, we install, replace and maintain roofs across Milton Keynes, Newport Pagnell, Bletchley, Stony Stratford, Great Linford, Bradwell and the wider Bedford, Luton and Northampton areas.

In this practical guide, we explain roof underlay in plain English – what it does, why it is critical for keeping your home dry and efficient, and how we install it to modern British Standards. If you are seeing leaks, damp patches, or planning a re-roof, this page will help you choose the right solution.

Call 07311091804 for a free quote or email sales@miltonkeynesroofing.uk.

What is roof underlay?

Roof underlay is the continuous layer laid over your rafters and beneath the external covering. On pitched roofs it sits under tiles or slates. On flat roofs it forms part of the waterproofing build-up, often alongside a vapour control layer. Underlay is not the main weathering surface, but it is a vital secondary barrier that:

Helps prevent wind-driven rain and snow reaching the loft
Reduces draughts and improves thermal performance
Works with ventilation to control condensation risk
Improves overall airtightness and wind uplift resistance

Most modern pitched roofs use synthetic membranes rather than old bituminous felt. On flat roofs, underlays are installed within built-up systems such as reinforced bitumen membranes, or beneath single-ply coverings like EPDM. Underlay needs to be correctly draped, lapped and detailed to do its job.

Important: underlay does not replace proper ventilation. Condensation control is a system – insulation, airtightness, vapour control and ventilation must work together.

Need advice now? Call 07311091804 and speak to Luke or a member of the team.

Why roof underlay matters in Milton Keynes

Local homes face a specific mix of conditions: open estates with higher wind exposure, frequent driving rain, winter frosts and warm summers. Correctly specified and fitted underlay protects your home through all seasons by:

Keeping water out when tiles lift under gusts or during heavy rain
Reducing condensation risk when paired with the right ventilation strategy
Improving energy efficiency by cutting cold air movement into loft spaces
Protecting structure by limiting moisture in timbers and insulation
Extending roof life via consistent drainage at eaves, valleys and verges

We regularly see 20-40 year-old roofs around Milton Keynes still using brittle felt. When felt cracks at the eaves or over rafters, water tracks into soffits and lofts even where tiles look fine. Upgrading the underlay during repairs or a re-roof is often the difference between a roof that just about copes and one that is ready for the next 20+ years.

Types of underlay and when to choose each

Breathable (LR) membranes for pitched roofs

Low resistance, vapour-permeable membranes allow moisture vapour to escape while resisting rain penetration. These lightweight synthetic sheets are now the default choice on most re-roofs.

Best for: Most tiled and slated roofs where we also provide ridge, eaves or tile vents to meet BS 5250 and Approved Document F.
Pros: Assists with surface condensation control, lighter than felt, good tear resistance when correctly installed.
Consider: Breathable underlay is not ventilation. A coordinated ventilation plan is still required.

Non-breathable (HR) felt-type underlays

High resistance underlays include traditional bituminous felts. They are robust but not vapour-permeable.

Best for: Heritage projects, certain slate specifications or where a dedicated vent strategy already exists.
Pros: Heavy-duty and time-tested.
Consider: Needs increased attention to ventilation to prevent condensation.

Vapour Control Layers for flat roofs

In warm flat roofs, a vapour control layer above the deck limits internal moisture reaching insulation and the waterproof layer. Above this, we install either a built-up underlay and cap sheet or a single-ply system such as EPDM over insulation boards.

Best for: Kitchens, bathrooms, living spaces and extensions where vapour control is critical.
Pros: Reduces interstitial condensation risk and protects insulation performance.
Consider: Detailing at edges and penetrations must be precise, including fire performance requirements such as BROOF(t4) where applicable.

Accessory components that make a difference

Eaves support trays: Stop membranes sagging into gutters and prevent ponding at the eaves.
Sealing tapes: Improve airtightness and wind uplift resistance at laps where specified.
Counter-battens: Create ventilation paths on complex roofs or where coverings require it.
Eaves comb fillers and bird guards: Keep wildlife out while allowing ventilation.

Not sure which system suits your roof and budget? Call 07311091804 for friendly advice and a free survey.

How we install underlay

Excellent materials still fail without excellent installation. Our team follows current British Standards and manufacturer instructions throughout.

BS 5534 compliant setup: Correct batten gauges, lap sizes and fixings matched to your wind zone and exposure.
Correct drape: Membranes are laid with a slight drape between rafters to shed water and avoid abrasion – never drum-tight.
Laps and seals: Horizontal and vertical laps to spec, with sealed joints where required for wind uplift resistance.
Eaves and ridge detailing: Eaves support trays as standard. Ridge and hips detailed to maintain airflow where continuous ridge ventilation is specified.
Perimeters and penetrations: Extra care at chimneys, skylights and abutments using compatible flashings and collars.
Ventilation strategy: Underlay choice aligned with eaves, ridge or tile vents to meet condensation guidance in BS 5250 and Approved Document F.
Flat roofs: Correct sequencing of vapour control layer, insulation and underlay or single-ply membrane, including fire performance where required.
Quality control: Photo records at key stages and a final check for drainage paths, ventilation continuity and neat workmanship.

The details make the difference. We take the extra time to get them right.

UK Building Regulations and standards

We work to UK Building Regulations and the relevant British Standards. We will also advise when Building Control needs to be notified.

Part A – Structure: Changing to heavier coverings may require a structural check.
Part B – Fire safety: Materials must meet fire performance requirements. Flat roofs may need BROOF(t4) classification.
Part C – Moisture: Roofs must resist rain and condensation. Underlay and ventilation choices are key.
Part F – Ventilation: Minimum airflow rates are required to prevent damp and mould.
Part L – Energy efficiency: Replacing more than 25% of roof area usually triggers insulation upgrades to current U-values.
BS 5534: Code of practice for slating and tiling including wind uplift resistance.
BS 5250: Management of moisture in buildings and condensation control.

Do I need Building Control? If you replace or refurbish more than 25% of the roof, alter the structure, add solar panels or convert a loft, you must notify Building Control or use an eligible Competent Person Scheme. We will guide you and coordinate inspections where needed, with costs and timings clearly shown in your quote.

Have a compliance question? Call 07311091804.

Signs your underlay has failed

Water staining in the loft even when tiles look intact
Damp patches at ceilings or eaves after wind-driven rain
Visible tears or sagging membrane at the eaves or seen from the loft
Daylight visible through the underlay or debris blown into the loft
Persistent loft condensation or mildew despite good insulation
Birds or insects entering at the eaves or under tiles

These symptoms can overlap with cracked tiles, failed flashing or blocked gutters. Our inspection pinpoints the root cause so you only pay for the repair you actually need.

Repair vs replacement

The right option depends on roof age, extent of damage, your budget and long-term plans.

Localised repairs: If damage is confined, for example around a penetration, targeted underlay replacement with correct detailing can be cost-effective.
Partial re-cover: Where deterioration affects one slope, we can strip and re-lay underlay and battens on that elevation, reusing sound tiles to reduce waste.
Full re-roof: Best where underlay and battens are at end-of-life across the roof or you are upgrading insulation and ventilation. This delivers the most robust, compliant result.

Timelines range from a day or two for small repairs to a week or more for full re-roofs, depending on weather and complexity. We always present clear options with transparent pricing, pros and cons.

Every property is different – let us take a look. Call 07311091804 to book your free survey.

Our survey-to-completion process

Free survey: Visual inspection and drone if required to assess coverings, underlay, ventilation and details.
Practical advice: Owner Luke Maxwell talks you through options from like-for-like fixes to future-proof upgrades.
Written quote: Transparent costs and timelines. If Building Control applies, we include it in the plan.
Scheduling: We agree a start date and protect your home and garden before work begins.
Installation: Underlay and associated roof works completed to BS 5534, BS 5250 and manufacturer guidance.
Clean and handover: Site left tidy, reuse of sound tiles where possible, photos provided, and relevant guarantees issued.
Aftercare: We are local and responsive if you ever need us again.

You get specialist workmanship with the friendly service you only get from a local, owner-led company.

Roof underlay FAQs

Does breathable underlay mean vents are not needed?
No. Vapour-permeable membranes assist movement of moisture but do not replace ventilation. We design the right combination of eaves, ridge or tile vents to meet current guidance.

How long does roof underlay last?
Quality membranes can perform for decades when installed with correct drape, laps and ventilation. Older bituminous felts often become brittle at eaves and over rafters sooner.

Can you replace underlay without changing tiles?
Yes. If your tiles or slates are sound, we can carefully strip, replace underlay and battens, then refit the existing coverings and swap only damaged units.

Will I need Building Control approval?
If more than 25% of the roof area is replaced, Building Control is typically required and insulation upgrades may apply. We will manage the process for you.

Do you work on flat roofs too?
Yes. We install and maintain EPDM, reinforced bitumen and GRP systems with correct vapour control, insulation and fire performance where required.

Can you stop a leak immediately?
For urgent ingress we aim for same-day or next-day temporary weathering, followed by a permanent repair or replacement as soon as practical.

Areas we cover

Milton Keynes – all estates and nearby villages
Newport Pagnell, Bletchley, Stony Stratford, Great Linford, Bradwell
Bedford, Luton, Northampton and surrounding towns

Ready for expert help?

Your roof deserves the right underlay, installed the right way. Join local homeowners who trust Maxwells Roofing for honest advice, careful workmanship and long-lasting results.