Preparing A Flat Roof Timber Deck For GRP

If you are a builder, and are in need of a GRP flat roof to meet the architects spec, this guide will help you to prepare the timber deck, ready for a professional GRP roofer to install the covering.

Use OSB Boards Only

The first requirement, is that the top deck (on which the GRP will be laid onto) is 18mm OSB. Do not use any other type of sheet timber. Do not compromise with thinner sheets or ply backed insulation. The deck must be 18mm OSB.

T&G Boards Are Preferable

Tongue and groove boards are preferable because they do not require expansion gaps to be left in between each board... although the edges that meet walls will still need expansion gaps (more explained later).

If you are using 8x4' sheets, leave approx 5mm between each board to allow for expansion resulting from changes in climate. Failure to allow for expansion will result in rucking & lifting of the GRP system. 

The boards must be completely dry. Otherwise the GRP system will not bond to the timber deck, resulting in bubbles or lifting.

Expansion Joints

1. Leave approx 40mm to all abutments (where boards meet walls). The pre formed GRP trims will easily cover these expansion joints. 

If you have an edge on your flat roof that doesn't butt against a wall, cut the board flush with the fascia boards.

2. If using 8x4' OSB sheets, leave 5mm between boards.

Fixing The Boards

Fix the boards with 31/75 ring shanked galvanised nails.. Preferably with a Paslode gun (or similar) to avoid internal damage to ceiling/plastering. Using nails as an alternative to screws, minimises screw-heads, which would be visible once the final top coat has been applied. If you insist on using screws, use screws with sunken heads.

Keep The Newly Installed Deck Dry

The newly installed 18mm OSB deck must be kept completely dry. We recommend using a visqueen sheet, as opposed to a standard tarpaulin.

When Installing Rigid Board Insulation

If you are installing rigid board insulation, you have two options:

Cold Roof:

Install celotex (or similar) insulation between joists. The new deck will be installed on the joists as usual. Consult your architect regarding adequate ventilation. If installing vents, inform the roofing contractor before obtaining a quote. The detailing around vents is complex and will add to the GRP installation price. This should be ascertained prior to quotation.

Warm Roof:

Lay a deck (and possibly a vapour layer, subject to architects spec) over the joists. You can use any form of decking for this (plywood etc is fine). Lay rigid board insulation over the previously mentioned deck. Your architect may insist that you bond the insulation with an adhesive. The 18mm OSB deck is then fitted over the insulation boards. Screw through into the joists with 175mm screws, fastening the whole deck, insulation and top deck. Use screws that do not show proud on the top deck (18mm OSB deck). The flatter the better.

Other Points Of Consideration

- Many new build extensions (as of 2013) are installed with box gutters and outlets passing through a parapet wall. In this instance, board the box gutter bottom, and the bottom of the outlet with 18mm OSB. There is no need to board box gutter or outlet sides.

- You do not need to board the walls of your flat roof. The pre formed trims (which can be viewed here) are used to finish edges and wall detail.

- Flashings to walls etc on GRP installations will be pre-formed GRP simulated lead flashings as standard. Please prepare your brickwork in advance by chasing out any brickwork to take the flashings. This will avoid dust on the newly installed GRP surface.

- When installing copings to a wall, the GRP trims will be sufficient to enable you to bed the copings without the need for lead. Please inform the roofing contractor of any parapet walls that will be finished with copings, as additional trims will be required.

Roof Windows

Please prepare the window frames, and do not install the window top until after the GRP system has been applied. GRP installations are performed with liquid & can flick onto windows easily.

The wall fillet trim is used to seal window frames to the timber deck. This is performed by the roofing contractor.

Critical Measurements For Roof Windows

If you are working with a window that has its own frame/drip, you MUST inform the roofing contractor of critical measurements. Failure to do so, may result in the wall fillet trim installation expanding the window frame to a point where the window frame no longer fits.

If you have any further questions, please contact us via www.roofingsouthlondon.co.uk

By Nicky Morris

10 Tips In Choosing a Domestic Roofing Contractor

With all of the recent media coverage in the UK (Cowboy Builders etc), choosing a roofing contractor to carry out work on your property can be a real dilemma. In this blog, we have tried to highlight some of the points to consider when choosing tradesmen for your project, with these 10 tips.

1. Keep It Local

It sounds obvious, but keeping your search local will undoubtedly offer the most competitive prices. You can also ask for references of previous completed work, and perhaps drive by to view completed projects. Roofing contractors further than an hour or so away will tend to 'make the job worth their while' in the quote.

2. Stick To 'Roofing Contractors'

Try not to be tempted by a general building contractor, or other multi-trade company. Roofing is a specialist field, and companies that operate solely in the roofing sector will have the most knowledge and experience in your projects required execution techniques.

3. Shortlist Three Companies/Businesses

Choose the three companies that stand out for you. You should obtain at least 2 quotes for projects over £500, ideally three. But before you jump into arranging an appointment...

4. Look Out For Accreditations

The two main accreditations you want to look out for are the NFRC & Trustmark. Both of these accreditations are achieved by strict vetting in work standards, health and safety, company policy and financial security. You can search for contractors in your area on the NFRC website by clicking here. I can personally vouch, that obtaining membership in these organisations is by no means easy. Many other accreditations are very easy to obtain... They all involve monthly subscriptions, so certain (although by no means all) trade organisations may want more monthly subscriptions, and prioritise member numbers over vetting process.

As a bonus, check if your contractors hold any roofing qualifications. NVQ's are available in many fields of roofing including Flat Roofing, Slating & Tiling and Heritage Roofing.

5. Check Previous Work Photographs & Reviews

It has become standard practice for serious roofing contractors to collect a portfolio of previously completed projects. Browse the contractors website. You may not know what you are looking at, but poor workmanship will be obvious to you. Compare photos with your three chosen companies.

Reviews are an excellent way to hear about previous customer experiences with your chosen contractor (so long as they are from a reputable source). Checkatrade is a brilliant review source, and is near impossible for companies to scam their own home-made reviews.

6. Obtain Your Quote 

Contact the company, arrange an appointment for the contractor to view the property, and obtain a quote. If the company produces an estimate, ask for a firm quote to avoid an price-bumping. Most roofing contractors will offer an estimate as standard (due to possible findings revealed when your existing covering is stripped back or partially stripped). Most of these companies will also be more than willing to issue a firm quote at your request. Payment terms for roofing works should be included somewhere on the quote. If they are not, ask for them to be added. Factor in VAT & credit card charges, if they are applicable.

7. If In Doubt, Call And Ask

If you have a query with the estimate/quote, email the contractor with your questions. Any reputable contractor will be happy to answer your questions, and will understand that you want to be clear on what is included in terms of method and materials. Common areas for query are:

- Materials being used

- Method being used

- Process for unexpected findings (for example, how much will it cost to top the insulation to the government required level, if it were found that no insulation was present) 

- Guarantee Lengths

8. Check That Scaffolding / Health and Safety Equipment is Included

Make sure that any necessary access or Health and Safety equipment has been included, and ask to have it in writing on the quote. Whilst on the subject of scaffolding, this would be the time to contact the roofing contractor with any specific time restrictions in erecting and 'striking down' the scaffolding.

9. Pencil In A Date To Commence Works

Arrange a date with your chosen contractor to commence works. The roofing trade is vulnerable to delays due to weather conditions, so this date should be treated as a target date, and not necessarily as a firm date. With that said, if the weather conditions were fine on the target date, you would be within your rights to question the contractor if they didn't show.

10. Obtain Your Guarantee Before Payment

Insist on receipt of your guarantee before you settle the invoice. It sounds obvious, but you've gone to all the trouble of the previous 9 steps, so you don't want to fall at the last hurdle.

Good Luck!

By Nicky Morris

Barris Roofing Ltd

www.roofingsouthlondon.co.uk